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Hypocenters and focal mechanisms of the Arctic earthquakes
Two possible solutions to the problem of describing the pattern of earthquakes in the Arctic Region are discussed. In previous summaries on seismicity of the Arctic, a number of seismically active zones of different intensity, shape and dimensions were used. The main zone was the Mid-Arctic Earthquake Belt extending from Iceland through the Norwegian-Greenland Basin to the Lappet Sea shelf, and farther penetrating into the Asian continent in the form of a wide band. Lower order zones have been established in some marginal seas, Northeastern Russia, Canadian Arctic Archipelago, etc. (Fig.6). In our view, outlining the data, such as hypocenters and focal mechanism solutions, is convenient and instructive if it is described by subregions. These subregions are based on a number of tectonic and geophysical indicators. In addition, each study region elucidates the tectonic setting and deep structure to the extent necessary for further understanding of its geodynamics and estimation of the tectonic nature of modern seismicity.
4.1. Basic principles of data selection
The Arctic Seismological Data Bank was used as a source of information on earthquakes. A map of Arctic epicenters (Fig.6) was computer generated on the basis of the General Catalogue. The data selection were based on the principle "better less but better". This principle was implemented so that earthquakes occurring since 1970 have been used primarily because the network of Arctic stations was more reliable. Only the strongest were taken from the earliest events. Usually, earthquakes epicenters recorded by at least 10 stations were plotted on the map. However, it should be noted that in two cases in order to reflect the true level of seismic activity in different Arctic regions this condition was modified.
Using the uniform approach the map summarizing all earthquakes including northern Alaska would suggest a predominance of seismic activity, which according to data on strong earthquakes, is not true. The better observational network in Alaska prejudices the data distribution. Therefore, for northern Alaska, the threshold value of the number of recording stations was raised to 15. According to ISC, location error for earthquakes epicenters meeting these conditions (min.10 and 15 stations) does not exceed 15-20 km.
For areas with poor coverage such as Baffin Island, Baffin Bay, the Queen Elizabeth islands, and western Greenland, the fulfillment of the condition of at least 10 recording stations would lead to distorted vision of fairly low seismic activity in these regions or even complete absence thereof. Thus the limit was reduced to 4-5 stations. Location error here reaches 50 km or greater.
It should also be noted that the above map does not contain weak earthquakes from northern Fennoscandia, Yakutia, and southern Laptev Sea. This information is included in the regional not in the General Catalogue. Earthquakes from the Chukchi Peninsula and adjacent offshore areas recorded only by the Iultin station are also not drawn on our map. All these earthquakes are shown on maps compiled for epicenters of the appropriate regions.
Some 300 focal mechanism solutions are known for Arctic earthquakes. However, the number of events is less than 200 because for some of them up to 5-6 determinations have been made by different scientists. There are several solutions based on application of different wave types.. In our national practice (and abroad until recently) most values were determined by A.V.Vvedenskaya's method (1972) where P-wave first motions are used. It should be noted that this method requires a vast reliable coverage (at least, 30-40 stations) at a wide range of epicentral distances and sufficiently even distribution of stations through all quadrants around the earthquakes. For the Arctic region where the observation network is limited and irregularly distributed, this method has produced many unreliable and frequently opposite solutions for the same earthquake. Therefore, to reduce the ambiguity we have reviewed the information on focal mechanisms. Based on the review we selected the most completely recorded earthquakes. Signs of first motions were checked using national and foreign bulletins with computerized redetermination or determination of the mechanisms through a consistent method designed at the Institute for Physics of the Solid Earth of the Russian Academy of Sciences, and employed for bulk determinations (Aptekman and others,1979). It is noteworthy that for strong earthquakes, dozens of alternative solutions are often possible of which each has different probability. For some events, when the above procedure failed, the best results were taken from various sources based on the following criteria: additional information recorded by the author, particularly on the data obtained from the local observation network and information on signs of first motions directly from seismograms rather than from bulletins. For instance, for earthquakes of northern Yakutia and the Laptev Sea, on which determinations by foreign and Yakutian seismologists are available, the latter were preferable because they had data collected from local stations. All available solutions using the first motions method are shown on the maps.
Since 1982, the ISC Catalogue has been publishing data on focal mechanisms obtained using centroid-moment tensor method (CMT). Similar determinations have been also published for earlier periods beginning 1977 (Dziewonski and others,1981). CMT method is based on conversion of the complete package of waves recorded by a digital network, ranging from compressional to surface waves, so the process in the source is averaged and the major stage in the fracture is characterized whereas the mechanism determined from first motions of compressional waves corresponds to its initial stage. Therefore, the data obtained from both determinations will only coincide if the process is uniformly developed in time, and their comparison without taking into account the above would be incorrect. Thus, all solutions using CMT method have been plotted on separate maps. A similar approach has been employed for outlining the material regionally.
4.2. Norwegian-Greenland Basin and surrounding areas
A specific feature of earthquake distribution in this area is the presence of a narrow epicentral band which is coincident with the mid-oceanic ridge and related faults. The contemporary axis of the ridge is clearly defined by a positive gravity anomaly of more than 50 mGal within which, however, an axial minimum is observed on the sites having distinct rift valley.
Major intersectional faults split the ridge into three basic segments separated from each other either by a distinct lateral offset or by a drastic azimuthal alteration and geomorphologic rearrangement (Fig.7). This causes the separation of each ridge segment and fracture zones dividing them into independent structural geomorphologic units. Each of them has characteristic seismological features and may be regarded as a specific seismically active zones.
Apart from the central axial zone, sites with higher seismicity are reported from the periphery of the basin as well as from its coastal surroundings.
The Iceland-Jan Mayen (Kolbeinsey) Ridge is the southernmost fragment of the mid-oceanic ridge restricted by the Tjornes and Jan Mayen Fracture Zones. The lateral offset with respect to its southern and northern extensions is 150-200 km westward. Bathymetry of the ridge is complicated and in general, is very poorly known. It is made of a series of highs and lows widening from 40 km in the southern part between Iceland and 69°N (Spar Fracture Zone) to 100-110 km in its northern part. Within the ridge, the most notable axial offset is observed along the Spar Fracture Zone; indications of other less notable offsets are established from detailed aeromagnetic data (Geophysical characteristics.., 1985). The axial rift valley is missing from the southern part of the rift whereas in the north it is represented by several en-echelon valleys with depth reaching 500-700 m with respect to surrounding mountain ridges (Udintsev,1987).
The pattern of earthquake epicentral distribution near the ridge changes laterally. North of the Spar Fracture Zone where the ridge is widest, the axial band of epicenters is evidently linear, while in the narrower south section, a considerable dissemination of epicenters is obvious with offsets from the axial line up to 100 km. In the Spar Zone the line of epicenters is clearly broken. Depths of hypocenters are typical of mid-oceanic ridges and do not exceed 25 to 30 km.
In this area, focal mechanisms solutions have been obtained through the first motions method for 10 events (Table 12, Fig.8a) of which 3 (2, 4, and 9) took place in the Tjornes Fracture Zone.
According to the data compilation on the ridge, the pattern of tectonic movement corresponds mainly to the normal fault with the fault fissure plane dipping around 60° to the horizon, which is typical of rift mid-oceanic ridges. Noteworthy are considerable variations in subhorizontal extension axis strike azimuths, which permitted A.V.Drumya (VNIIOkeangeologia, unpublished reports, 1988) to conclude the non-orthogonality of this type of stress in the ridge axis and suggest the presence of geodymamic factors complicating the geodynamic process. However the seismological data indicate azimuth relationships of stress axes with the epicentral line which characterizes only averaged, generalized strike of the structural axis which is not necessary coincident with the strike of its separate
Focal mechanisms of earthquakes of the Iceland-Jan Mayen Ridge (First motions method)
* - in, or close to one of the fault zones + - CMT method solution available
Axes of principal stresses: Nodal plane:
T - tension; P - compression STK - strike azimuth; DP - dip; SLIP - slip
PL - plunge; AZM - azimuth Dislocations:
Quality N - normal fault; S - strike slip; T - thrust
G - good; F - fair; P - poor; U - mechanism type only
N (n) -general quantity of values available(including inconsistent)
Focal mechanisms of earthquakes of the Iceland - Jan-Mayen Ridge (CMT method)
Axes of principal stresses: Nodal plane:
T - tension; P - compression STK - strike azimuth; DP - dip; SLIP - slip
VAL - value; PL - plunge; AZM - azimuth Dislocations:
N - normal fault; S - strike slip; T - thrust
fragments. Therefore, it is possible that an essential contribution to the axial strike azimuth oscillations was made by the difference between the strike of those sites of the ridge where the earthquake occurred.
The only focal mechanism solution that dramatically differed from the others was that for the earthquake 7 which provided a thrust mechanism. Estimating the accuracy of this solution it should be noted that the above event occurred within an anomalous site of the ridge, namely in the area of its intersection with the Spar Fracture Zone. The nearest earthquake 1 provided a strike-slip mechanism.
The strongest events in this region (2 and 9) occurred in the Tjornes Fracture Zone. Various scientists are rather confident about the dextral strike-slip along subvertical planes. NP 1 strike is practically coincident with that of the fracture zone. A similar mechanism has been established for earthquake 4.
The solutions through CMT method (see Table 13, Fig. 8b) have provided a normal fault and strike-slip mechanisms. For event 2 strike-slip could have been expected.
The Jan Mayen Fracture Zone consists of two separate segments: northwestern where the shear of mid-oceanic ridge and epicentral belt occurred to 200-210 km and southeastern intersecting deep sea part of the Norwegian Basin and penetrating into the Norwegian shelf. (Fig.7). Of these two segments only the northwestern one is seismically active only in the part located between the offsets of the ridge. In its active area the Jan Mayen Fracture Zone is represented by a downwarp of 10-15 km wide and up to 2.2 km deep.
Focal mechanism solutions obtained through the first motions method (Fig.8a, Table 14) show most often sinistral strike-slip along the plane coincident with the proper zone (NP2). The fact of incidence of the plane which can be regarded as a fault plane to the south-west under Jan Mayen Island and the presence of a certain thrust component at the hanging wall have permitted L.A.Savostin and A.M.Karasik (1981) to conclude that the uplift of the Jan Mayen Island block was caused by the lighter weight of its continental crust in comparison with surrounding oceanic crust.
Considering that seven determinations using the CMT method (Fig.8b, Table 15) have also been shown the sinistral strike-slip, the aforesaid tectonic model can be adopted for the Jan Mayen Fracture Zone.
The Mohns Ridge is a fragment of a mid-oceanic ridge of the Norwegian-Greenland Basin north-east of the Jan Mayen Fracture Zone to the Knipovich Ridge (Fig.7). This is the only segment of the ridge symmetrical with respect to the surrounding continents. It has the most typical characteristics of mid-oceanic ridges throughout its entire length, such as sharp topography, distinct rift valley with a depth of 3 km, i.e. 1-2 km lower than the surrounding axial ridge and the general absence of sediments near the axis that thicken toward the periphery.
The typical pattern of the Mohns Ridge is verified by the seismological data analysis showing the linearity and continuity of the epicentral belts and their clear association with the axial zone of the ridge.
Fairly unexpected are results of focal mechanism solutions through the use of first motion method. Presently on the Mohns Ridge, nine events had been processed. As Table 16 shows the data obtained are rather ambiguous and provide the full range
Focal mechanisms of earthquakes of the Jan-Mayen Fracture Zone (First motions method)
See Table12 for the legend
Focal mechanisms of earthquakes of the Jan-Mayen Fracture Zone (CMT method)
Focal mechanisms of earthquakes of the Mohn Ridge (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Mohn Ridge (CMT method)
of mechanisms from normal fault to thrust. The thrust mechanisms are inexplicable in the extensional structure. The analyses we have carried out indicate this is due to insufficient reference data that did not allow reliable solutions to be obtained. For 6 of the above earthquakes the overall number of signs of first motions is less than 15-20, some of them being unconfident and non-compliant; two solutions have 35 and 40 signs, respectively with more non-compliant ones. Besides, in all cases irregular distribution of signs by quadrant is observed. Even the most confident determination (4) has yielded two solutions by two authors where the position of one of the nodal planes differ by 80 degrees. These determinations were admissible 20-25 years ago due to the shortage of seismological information on the Arctic, and even necessary for initial understanding of the Mid-Arctic Earthquake Belt geodynamics. However, in the second half of 1980s, the solutions like those for 3, 6-9 are inadequate. It should be noted that similar solutions are available for other Arctic regions.
To enhance the data base on focal mechanism solutions on the earthquakes on the Mohns Ridge we have carried out more reliable determinations (10-13) which provided fairly uniform normal fault and normal fault - strike-slip mechanism with the extension axis suborthogonal both to epicentral line and the axis of the ridge.
All eleven determinations made by the CMT method have provided solely normal fault mechanism (Fig.8b, Table 17). Data compared from the three latest earthquakes processed using both methods show that they are nearly identical in the orientation of the extension axis, but showing enhanced dipping in the compression axis according to CMT. Further acquisition of similar information is the only way to judge whether this is caused by determination errors or it makes any geological sense related to change in fault plane orientation throughout the time.
The Knipovich Ridge is the northernmost part of the mid-oceanic ridge in the Norwegian-Greenland Basin changing its strike by nearly 90 degrees and extending almost meridionally to the Spitsbergen Fracture Zone (Fig.7). In contrast to the Mohns Ridge it occupies an asymmetrical position in the Greenland Sea clearly shifting eastward. Change in the strike of the ridge is accompanied by a similar radical rearrangement of its geomorphology. It is represented by a series of mountainous topography divided by depressions of which the most prominent is regarded as a rift valley (Sykes,1965). The cross section of the ridge shows definite asymmetry: its western flank with respect to the rift valley is made of 6 to 7 topographic highs and is much wider than the eastern one consisting of 3 peaks and gradually merging with the continental margin of Western Spitsbergen Island and the Barents Sea.(Desimon and Karasik,1979). The rift valley is usually narrow and 1 to 2 km across with its bottom contour at 3 km, and steep walls up to 1 km high. Seismic investigations (Vogt and others,1979) have reported thick sedimentary units both in the rift valley (up to 1000 m) and along both sides, particularly eastward (up to 3000 m).
Linearity of the seismic belt above the Knipovich Ridge is sharply disturbed. A more adequate suggestion might be made about the three higher seismicity zones between which weaker seismic activity is observed (Fig.7). One of these zones is located at 74-75°N at the junction with the Mohns Ridge. In the second one, located at 76-77°N fewer epicenters are observed and these show an isometric swarm that is denser in its center. A large number of earthquakes occur east and west of the nominal rift valley. North of 77° the epicentral belt after a break becomes more linear up to the Spitsbergen Fracture Zone.
Focal mechanism solutions have been found for 16 earthquakes at the Knipovich Ridge: 10 solutions were made through first motions method ( Fig.8a, Table 18) and 8 by CMT (Fig.8b, Table 19). Solutions 3, 4 and 6 made by first motions method are irratic .
Data on focal mechanisms indicate peculiarities of the tectonic process in this segment of the Mid-Arctic Ridge. In contrast to the normal fault mode, a strike-slip component is essential and sometimes predominates despite a distinct normal fault component. Almost complete coincidence of the solutions made by both methods with respect to the earthquakes date April 25, 1988 is noteworthy giving evidence on the uniform nature of the fault development in the focus at different stages of the seismic process.
Profound en-echelon pattern of the ridge frequently divided by faults, lateral dissemination of the epicenters provides no evidence to associate each particular earthquake to any element of the ridge, thus making difficult the selection of a nodal plane which is the fault plane. Complication of the tectonic setting at the Knipovich Ridge is frequently manifested in notable oscillation of subhorizontal extension axis strike in strike-slip solutions, and available reliable solutions showing no remarkable strike-slip component. This normal fault mechanism obtained through CMT method for an earthquake in close vicinity to the Mohns Ridge ( 2), thrust mechanism (first motions method) on the earthquake dated 1981 (7) which occurred near the earthquake of 1983 with strike-slip mechanism, and also thrust mechanism (CMT) on the earthquake dated 1995 (7).
The Spitsbergen Fracture Zone is a link between mid-oceanic ridges of the Norwegian-Greenland basin and Eurasian Subbasin (Gakkel Ridge) in the Arctic Ocean. It covers a fairly large area between Svalbard and Eastern Greenland, and is represented by a number of en-echelon segments resulting in stepwise displacement of the zone 70-80 km northward to the Gakkel Ridge (Fig.7). Near its junction with the Knipovich Ridge (78-79°N) the seismic belt displaced 60-70 km northward is traced up to 80° N where it sharply displaced again 70-80 km westward with farther north-northwesterly strike up to 83° N to the junction with the Gakkel Ridge. It should be noted that scattering of epicenters within the Spitsbergen Fracture Zone is much less than at the Knipovich Ridge.
At present, about 20 focal mechanism solutions with allowance for those made in this paper, are known for this fragment of the Mid-Arctic Earthquake Belt as obtained through first motions method (Fig.8a, Table 20). However, some of them, for instance 12-14 are barely reliable.
In general, great uniformity of the vast majority of the solutions showing the presence of subvertical nodal planes should be emphasized. One of them (NP2) that has northwesterly strike coincident with the general strike of the fracture zone and adopted for the fault plane allows dextral strike-slip mechanism to be confidently
Focal mechanisms of earthquakes of the Knipovich Ridge (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Knipovich Ridge (CMT method)
Focal mechanisms of earthquakes of the Spitsbergen Fracture Zone (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Spitsbergen Fracture Zone (CMT method)
established. At the same time, the presence of cross-cutting faults making the Spitsbergen Fracture Zone en-echelon-like predetermines the existence of other mechanism types. This is likely to be supported by significant normal fault and thrust components occurring in a few determinations. Earthquake 11 has provided the only thrust mechanism with subhorizontal compression axis of northwesterly strike, i.e. along the general line of the zone.
Obvious predominance of strike-slip mechanism with the same planar and axial orientation has been also supported by 10 CMT solutions (Table 21). Comparison between data on the earthquakes dated October 8, 1986 and October 7, 1989 with both solutions available shows an indisputable similarity of the extension axis parameters, whereas the compression axis is much steeper for first motions method. The good quality of both solutions gave a good indication about the faulting process throughout the time. On the other hand, highly scattered parameters of the focal mechanisms obtained through the first motions method which had been observed in all aforesaid areas and will be shown in the following one is evidence of greater sensitivity of this method to actual environmental heterogeneity. This should have been expected keeping in mind the higher frequency of the first motions in comparison with the subsequent records.
The Lofoten Basin is located within the Norwegian Sea south-south-east of the junction zone of the Mohns and Knipovich ridges (Fig.7). Its deep-sea part may be classified as an abyssal basin. The eastern border="0" of the Lofoten Basin is the Senja Fracture Zone traced from the Norwegian Shelf and the mid-oceanic ridge. This zone is a boundary between the continental crust of the Barents Sea shelf and oceanic crust of the Norwegian Sea. It is buried under a thick unit of Cenozoic sedimentary rocks derived from shelf. The basement of the Lofoten Basin is 1.5-2 km deeper along the fault, based on the free-air anomaly with an amplitude of up to 100 mGal.
Epicenters of earthquakes in the Lofoten Basin with magnitudes reaching 5-5.5 are extended submeridionally in a scattered band of up to 250 km wide from the mid-oceanic ridge to the weak seismicity zone in northern Norway and adjacent shelf. More distinct is the eastern boundary of this band marked along the Senja Fracture Zone. Interestingly, the strike of the epicentral band nearly coincides with that of the seismic belt of the Knipovich Ridge, and the map presents both zones as an unit.
At present, for earthquakes of the Lofoten Basin 3 of focal mechanisms solutions available are obtained through first motions method and one through CMT (Table 22, 23). On the eastern border="0" of the epicentral band in the Senja Fracture Zone, two first motions determinations have provided a normal fault - strike-slip mechanism with extension axes orthogonal to the strike of the zone; at the same time CMT method has established a thrust movement with the compression axis orthogonal with respect to the fracture zone. Near the western flank of the epicentral band a strike-slip mechanism has been established with a negligible thrust component. (Fig.8a.) In general, instability of tension axes is obvious, however, the few determinations and the insufficient reliability of at least two of the three first motions solutions prevents understanding the tectonic nature of this event. In our view, the only substantiated conclusion from the data on focal mechanisms is the
Focal mechanisms of earthquakes of the Lofoten Basin (First motions method)
See Table 12 for the legend
Focal mechanism of earthquake dated July 2, 1991 of the Lofoten Basin (CMT method)
statement that, in contrast with the adjacent seismically active zone of the Mohns Ridge, horizontal compression in the Lofoten Basin plays a significant part.
The Svalbard Archipelago is the northwesternmost exposure of Eurasian continental crust exposed many kilometers offshore. These islands with their rocks that span a large age range are key role in understanding the structure of adjacent offshore shelves and the transition to the ocean.
A number of national and foreign investigators (Harland,1979; Orvin,1940; Semevsky,1967; Sokolov and others,1972, etc.) indicated that the archipelago was formed during the Caledonian folding that was completed between the Silurian and Devonian. This event produced a structural fabric similar to the modern one showing meridional and submeridional strike of linear folded structures (Fig.9). During the Devonian in the course of the subsequent postgeosynclinal orogenic period intensive movements from different directions took place which rejuvenated major faults emplaced at the geosynclinal evolutionary stage. Concurrently, superimposed graben-like depressions were formed also inheriting the strike of Caledonian structures. The largest Devonian graben is located on West Spitsbergen Island north of Isfjord between the Billefjorden and an Bockfjorden fault zones.
Intensive differential movements began in the Middle Paleogene concurrently with the opening of the Norwegian-Greenland Basin and Eurasian Subbasin and continued during the entire late Cenozoic period and resulted in the revival of ancient faults and formation of new weakened zones. Where most intensely manifested these movements lead to the formation of an orogenic belt traced along the west coast of West Spitsbergen Island, and a system of fjords with sublatitudinal strike. The product of the deformation of Svalbard is the distinctive block arrangement, clear linearity of major structures extended in a submeridional direction and divided by regional faults primarily of the same strike, and narrow fjords showing sublatitudinal strike on the west coast and submeridional on the north.
Initial information on the higher seismicity on Svalbard and adjacent areas was obtained from the beginning of observations on Arctic earthquakes. In the vicinity of the archipelago is the Mid-Arctic seismically active zone. (Fig.7) The nearest epicenters of earthquakes from the Knipovich Ridge and Spitsbergen Fracture Zone are located maximum 25-30 kilometers westward. A few epicenters of fairly strong earthquakes with magnitude 4-4.5 are located in the Barents Sea immediately south and southeast of the archipelago. Within Svalbard itself two localized seismically active zones were evident even from teleseismic data: in the Heer Land on the west coast of the Storfjord Strait and on Nordaustlandet. Field observations by foreign investigators conducted during field seasons of 1977-1982 and in the summer of 1986 (Bungum and others,1982; Chan and Mitchell,1985; Mitchell and others,1979; 1990), and by national seismologists conducted in 1983 -1984 and later (Panasenko and others,1987) have confirmed and clarified these assumptions. On the archipelago no earthquakes have been found save for a few on the Heer Land mainly related to meridional faults.
The epicentral zone of the Heer land shows a distinctive latitudinal strike. Every day several earthquakes occur there and it is there the strongest earthquake on the archipelago with an intensity of 5 MKS was recorded on January 18, 1976. As field
observations have shown, most earthquakes of the Heer Land are focused west of the shoreline of Storfjord. No direct links are established between the main cluster earthquakes and meridional Billefjorden and Lomfjorden faults, because it is located definitely south of the visible end of these faults and shows as it has been mentioned above orthogonal strike in respect of the these faults. However, one cannot eliminate the possibility that these faults extend farther south buried under sedimentary cover and reach the zone of earthquake concentration
Information on the depth of Svalbard earthquakes has been very approximate and often contradictory. Thus, for the strongest event, depths of hypocenters from the data of different agencies range between 47 km (ISC) and 10 km (CSEM) According to the assessment of the above parameter obtained through the use of different methods, Svalbard earthquakes seem to occur near the surface ( not deeper than 10-15 km) (Panasenko and others,1987).
The event of January 18, 1976 is the only one where sufficient date for fairly reliable mechanism solution was obtained (Table 24). Among known solutions that made by (Gutenberg and Richter,1954) should be acknowledged as the most reliable because the authors used not only catalogues for information on the first motions but also reference seismic material. It should be noted that nearly the same solution was published by (Panasenko and others,1987). The first of the two subvertical nodal planes with the strike similar to that of the epicentral zone is recognized as the fault plane where a sinistral strike-slip took place. Data on first motions signs of a few earthquakes from the Heer Land with magnitudes 1.5-2.4 are consistent with the solution obtained from teleseismic data thus showing the similarity of stress field of strong and weak earthquakes.
On Nordaustlandet in the course of field observations in 1982 and 1986 totaling 50 days (Mitchell and others,1990) over 100 microearthquakes with magnitudes ranging from 0.6 to 3.9 were recorded. Within the wide epicentral zone previously established from teleseismic data detailed investigations have allowed two main clusters to be identified with north-northwestern strike coinciding with that of faults mapped by geologists in this area and spaced at 15 kilometers from each other. A notable scarcity of epicenters within the clusters in association with a fairly significant density of the faults especially in the area of eastern concentration reduces the confidence of relating higher seismicity to a particular fault.
By combining the observations of 1982 and 1986 and using the group method for each of the concentrations a general focal mechanism solution has been obtained. In both cases the preferable solution is the strike-slip mechanism, but due to lack of data one may suggest a thrust movement, especially in the western group. However, latitudinal compression is distinctly determined, and one of the nodal planes is located along the epicentral zone.
The Greenland Sea is a part of the Norwegian-Greenland Basin opposite to the Lofoten Basin in respect of the Mohns Ridge axial line. A specific feature of the Greenland Sea is large shelf of up to 300 km at 76-77°N. Based on geophysical, primarily, aeromagnetic data (Talvani and others,1977), the Greenland shelf may be divided into the southern (up to 75°N.) zone adjacent to the Jan Mayen Fracture Zone and characterized by the section saturated to a greater extent with Tertiary Magmatic
Focal mechanisms of earthquakes of Svalbard (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Greenland Sea and Eastern Greenland (First motions method)
See Table 12 for the legend
Focal mechanisms of the earthquakes of the Eastern Greenland (CMT method)
products, and the widest northern zone with insignificant occurrences of volcanism. From the data, a system of extended uplifts of the Caledonian basement subparallel to the shoreline and depressions filled with sediments up to 8-10 km deep is traceable across the entire shelf, especially its northern part.
In general, the Greenland Sea offshore areas should be assigned to areas of low seismic activity, although certain events may reach magnitudes of 5-5.5. All earthquakes known from here are located within the oceanic part of the offshore area, and trend towards the Greenland Fracture Zone (Fig.7). This circumstance also suggests that comparison should be drawn between the Greenland Sea and Lofoten Basin where as it has been mentioned above epicenters primarily are concentrated near the Senja Fracture Zone. However, it should be noted that because of the distance from recording stations the local error for epicenters often exceed 50 km. In the shelf region of the Greenland Sea earthquakes are not observed, in contrast, higher seismicity is obvious at sites on the shoreline and the adjacent narrow band of the offshore and onshore areas. This band is coincident with the zone of exposed Caledonian basement. Within the line of single epicenters along the eastern and farther northern coast of Greenland, distinctive concentrations are observed. The most prominent is located at 79-82°N where the mid-oceanic seismic belt is nearest the coast. The strongest earthquake in the area occurred here with magnitude 5.1 and depth of hypocenter of 19 km was recorded in 1971 (Sykes and Sbar,1974).
This earthquake is the only one providing focal mechanism solutions through the first motions method (Table 25). A normal fault regime with the extension axis suborthogonal to the shoreline has been obtained (Fig.8a).
4.3. Eurasian Subbasin and surrounding areas.
In contrast to the Norwegian-Greenland Basin the Eurasian Subbasin is bounded by geomorphological structures of the Arctic Ocean floor rather than by its coastal features. Boundaries of the Eurasian Subbasin on the south and north are, respectively, the Eurasian continental slope and the Lomonosov Ridge (Fig.10). The subbasin has an oval shape and an edge at the continental slope of the Laptev Sea in the south-east. In the north-west it is separated from the Norwegian-Greenland Basin by the Spitsbergen Fracture Zone.
In this region the earthquake epicenters are concentrated along the central axis even more than in the Norwegian-Greenland Basin. This band shows no obvious breaks along its length from the Spitsbergen Fracture Zone to the Laptev Sea shelf tracing the axial and near axial zones of the mid-oceanic Gakkel Ridge. In contrast to the Norwegian-Greenland Basin where the axial seismically active zone is divided into several fragments different in parameters, axial seismicity in the Eurasian Basin may be regarded as continuous.
In addition, some epicenters of fairly strong earthquakes have been determined along the continental slope and frontal Eurasian shelf.
The Gakkel Ridge is a subsea mountain range, symmetrical about the Eurasian continental slope and Lomonosov Ridge. Along its considerable length it has a distinct axial rift valley up to 5 km deep, which is 1-2 km deeper than the level of surrounding ridge crests. Geomorphologically, the ridge is fades 200-250 km prior to the Laptev Sea continental slope (Naryshkin, 1987).
The seismic belt tracing the Gakkel Ridge has a nearly persistent strike and an average width not exceeding 20-30 km. The most remarkable offset of 100-120 km of epicenters is located at the site between 40 and 80° E where nearly linear fragment some 300 km long is shifted to the north it its western part, and farther east it fairly smoothly approaches the general axial line. Based on bathymetry, the rift valley in this area gets less distinctive and then completely disappears. It is noteworthy that the disturbed mid-oceanic seismic belt is colinear with Eurasian submeridional Franz-Victoria and Saint Anna offshore troughs. These troughs show a few earthquakes magnitude up to 6.5 on their intersection with the continental slope.
Twenty events have been processed through the use of the first motions method, some of them by different authors (Table 26). For the sake of objectivity earthquakes providing less than 20 or even 30 signs of first motions should not be employed for earthquake mechanism solution when the amount of data collected is fairly large. Computer data processing experience has shown that even having better information one cannot avoid a number of alternative solutions which are nearly equal in statistical criteria and providing different mechanism solutions which may prove any viewpoint.
Based on reasonable determinations one can state that the Gakkel Ridge is dominated by focal mechanisms typical of a mid-oceanic ridges. These are focal mechanisms of normal fault and strike-slip with extension axis subhorizontal to the epicentral line (Fig.11a) . The strike of the line practically coincides with that of the ridge. It should be noted that two of the three rather confidently processed earthquakes provided a strike-slip mechanism solution (2, 3 Table 26) through the use of first motions method. One of the two is located in that part of the ridge where a stronger influence of transforms might be expected that is in the western part of the ridge near its junction with the Spitsbergen Fracture Zone. The other occurs in the area of displacement of the seismic belt about 40°E.
CMT method data provide even more uniformity in the results proving predominance of the transverse extension with respect to the axis of the ridge. A strike-slip mechanism is only achieved in one case. Surprisingly, this occurred on the site of the ridge where obvious transforms should not happen with the nature of the epicentral distribution. (Fig.11b). Nearly full coincidence of the source parameters determined by both methods is observed for the earthquake dated March 19, 1980, June 12, 1982 and June 11, 1991.
The Laptev Sea is located in the area of continental extension of the Mid-Arctic Oceanic Earthquake Belt. This makes the sea nearly unique on the Earth (similar are East Africa and western North America). Due to its poor accessibility, this sea is largely unknown.
Its tectonic classification is a marginal continental platform, with an Upper Cretaceous - Cenozoic sedimentary cover (Geological structure..,1984).
The structure of region was altered during the Paleogene stage of the stretching which resulted in the system of grabens (downwarps) on the shelf. These are now filled by sedimentary strata and divided by marginal and inner highs. In the
Focal mechanisms of earthquakes of the Gakkel Ridge (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Gakkel Ridge (CMT method)
See Table 13 for the legend
gravity field, the system of valleys is clearly defined by lower values. Additionally, contrasting tectonic movements caused by contemporary activity of the Laptev Sea lithosphere resulted in fragmentation of the region by subvertical and vertical faults which are identified by refraction (Avetisov and others,1994; Vinogradov and others,1987; Reports of the Polar Expedition of Sevmorgeologia Association) and CDP reflection methods (Data by L.A.Savostin).
By mid-1960s indirect assessments based on geological data on continental and insular surrounding areas suggested structural and tectonic heterogeneity of the basement. The exposed outcrops of the coastal zone of the Laptev Sea are rocks of different ages. In the west there are the arched-block uplifts in the Taymyr-Severnaya Zemlya System with folded complexes at the base which consolidated during the Precambrian to the Kimmerian. The southern fringe of the Laptev Sea is the ancient Siberian platform whereas in the Lena River delta and farther east it is the folded mesozoid mountainous structures of the Northeastern Eurasia. In the east it is restricted by arched-blocks made by Kimmerian folded complexes.
The modern understanding of the structural and tectonic features of the Laptev Sea region has been based of the analysis of geophysical data in the offshore areas. Initially the interpretations were determined on the basis on ice gravity prospecting (Reports of Polar Expedition NIIGA-PGO Sevmorgeologia, 1964-1988). The magnetic field offshore area shows no distinctive anomalies. Distinctive features of the gravity field are evident for two areas: western and central with broad isometric anomalies and the eastern characterized by frequent alternation of linear fairly narrow high-gradient anomalies of northwestern less frequently meridional strike with a higher background of the gravity field values. In comparison with the western and central zones, in the eastern zone an increased dislocation of the principal gravity-governing object and its shallower depth are evident. The seismic refraction results in the western offshore area at the outlet from the Khatanga Bay (Geological structure,1984) and deep seismic sounding in the southeastern part near the Buor-Khaya Bay (Kogan,1974) were in good agreement with the gravimetric data. Comparison of the seismic sections indicates that the Laptev sedimentary basin was emplaced on the heterogeneous basement (Geological structure,1984). In the west the sedimentary cover of 3-8 km with its underlying crystalline basement in general corresponds to the stratigraphic units of the Siberian Platform while in the east immediately beneath the basement there are high-velocity deposits identified with the Mesozoic basement.
The most convincing argument for heterogeneity of the basement of the Laptev Sea Region has been the information from marine seismic reflection -CDP survey (Ivanova and others,1989) interpreted in conjunction with the geology and refraction data. The CDP data presents the most realistic cross-section of basic tectonic units in the Laptev Sea (Fig.12). According to these data, the Mesozoic cover is confidently identified across the entire offshore area, however the underlying part of the section in the western and eastern Laptev Sea are radically different. The former contains another five reflectors beneath the Mesozoic cover which is identified on the basis of all geological and geophysical information with the epi-Karelian platform stage as the basement of the Laptev Platform. The lowest horizon traceable in fragments is
identified as the top of the crystalline basement consolidated during the Archean to Early Proterozoic. The western Laptev Sea area is limited in the west by the Taymyr Folded System while in the southwest by Lena-Taymyr conterminous uplifts zone separating the proper Laptev Sea Platform from marginal depressions of the Siberian Platform. In the eastern Laptev Sea area between the Yana Bay and Belkovskiy Island beneath the reflection from the base of the Mesozoic cover the seismic record is erratic and non-correlative. This horizon has a rugged relief mirrored in the gravity field where even its minor details with horizontal and vertical scales of 5 km and 300 m, respectively, are recorded. The thickness of the Mesozoic cover is 1-1.5 km on average, thinning in the arches of the uplifts down to 0.5-1 km. Based on the available geological and geophysical information on the Yana coast and Stolbovoy Island where folded structures of Northeast Eurasian mesozoids have been identified the authors (Ivanova and others,1989) conclude that the Mesozoic cover in the eastern Laptev Sea area lies immediately upon the Mesozoic basement, and consequently, it should be regarded as the northern shelf extension of the Verkhoyansk Folded System. The eastern Laptev Sea area is limited by the Belkovskiy-Svyatonosskiy fault traceable from an intensive band-shaped negative anomaly and dividing mio- and eugeosynclinal mesozoid areas.
The suture zone between the paraplatform Western Laptev Sea and folded Eastern Laptev Sea areas according to reflection-CDP data, has north-northwestern direction within the superimposed Omoloy graben near 130-131°E.
Information on seismicity of the Laptev Sea and surrounding areas until recently has been based primarily on data collected by distant stations which only approximately determine the epicenters. All the data showed was the disappearance of the linear trend of the Mid-Arctic Earthquake Belt in transition from the oceanic offshore area to the shelf. The existence of an axial seismically active rifting zone has been assumed to be associated with the Ust-Lena trough passing through the central shelf and reaching the continent east of the Lena River delta through the Buor-Khaya Bay. As well as two lateral, eastern and western zones of induced ("passive") seismicity are caused by discharge in weakened lithosphere of stresses generated in the axial zone (Avetisov,1975; 1983b).
Since the mid-80s after installation by the Yakutia Geoscience Research Institute Siberian Branch AS USSR several new stations on the south coast of the Laptev Sea, the energy level of confidently recorded earthquakes was reduced. At present, instead of previous K=12 (M=4.5) it is 10-11 and 7-8 for the northern, and for the southern offshore area and Lena delta, respectively. In the course of field seismological observations conducted by Sevmorgeologia using 12 "Cherepakha" recorders during spring and summer field seasons of 1985-1988 weaker events were also confidently recorded in the Lena delta and along the coast of the Buor-Khaya Bay (Avetisov,1993c).
According to cumulative data (Fig.13) the pattern of earthquake epicenter distribution in the Laptev Sea and surrounding areas are presented as follows.
Linearity of the Mid-Arctic Belt is preserved at the intersection of the continental slope within 200-250 km of the northern part of the shelf up to 76-76.5°N. The line of the epicenters is directed not to the central part of the shelf, Northern and
Ust-Lena troughs but to the south-east through the northern termination of the Omoloy graben towards the New Siberian Islands. Some 50-100 km northwest of Belkovskiy Island it becomes considerably wider and changes its strike submeridionally extending up to Stolbovoy Island south of which it degenerates. In the southeastern part of the offshore the density of epicenters is much lower and their distribution is scarce. Latitudinally-oriented chains of epicenters in the Eterikan and Dmitriy Laptev Straits are identified. Within the eastern fringe of the Laptev Sea epicenters are assumed to bend round an aseismic block of the lithosphere incorporating Belkovskiy, Kotelnyy, Faddeyevskiy and M.Lyakhovskiy Islands. At any rate, inferred seismological observations conducted by NPO Sevmorgeologia during field seasons of 1972-1976 on the New Siberian Islands did not reveal any epicenter within the aforesaid area.
West of 130-132° E the distribution of epicenters is quite different. Two sublinear zones are traced here radially diverging northwestward off the Buor-Khaya Bay. The most prominent one passes along the west coast of the Buor-Khaya Bay, intersects the delta and farther extends with certain breaks through the Olenek and Khatanga Bays up to the Taymyr coast. Within its confines, events with magnitude 5.5 are identified, macroseismic information being available for some of them. In particular, for the earthquake of February 1, 1980, the southeastern Olenek Bay, oval-shaped zones 6-7 MCS with a long northeastern axis coincident in strike with the line of epicenters.
The southern part of the second zone is traced by a thick band of axial and off-axial areas of the Buor-Khaya Bay and farther north, thinning possibly related to the distance from recording stations, passes northwestward along the Ust-Lena trough towards the central offshore area where drastic attenuation occurs near 120-123°E without reaching 76°N. Earthquakes with magnitude up to 5.5 are also known from here. Outside these zones single epicenters are only identified in the offshore area and northern part of the delta.
South of the Buor-Khaya Bay on the continent, a wide band of epicenters passes southeastward to the area of the mesozoids of Northeast Eurasia. In Western Yakutia single epicenters are identified off the northeastern margin of the Siberian platform.
It should be noted that we did not establish any link between the depth of the hypocenter and its lateral position. Hypocenters are mainly located between depths of 4-28 km with a clustering at a level of 13-14 km. Some hypocenters occur within a range of 35-42 km and one at a depth of 55 km. In the Lena delta and Buor-Khaya Bay based on numerous weak earthquakes, foci tending to identify seismic boundaries of the earth crust including the basement (Avetisov,1993c). Recent seismological data on the area of the Buor-Khaya Bay have been collected in the course of short-term (July 28 to August 22, 1989) observations conducted by Okeangeologia Research Institute RAN staff using bottom stations (Kovachev and others,1994). These observations have in general supported the distribution of hypocenters established by onshore stationary and field stations. They also include information on the existence of upper mantle earthquakes.
At present, for the Laptev Sea and surrounding areas 18 and 12 focal mechanisms solutions are known made through first motions and CMT methods, respectively, 8 of them using both methods. (Table 28, 29), (Fig.14a, b).
In the northern part, in the epicentral zone, directly related to the oceanic part of the seismic belt, a normal fault mechanism with is extension axis suborthogonal to the general strike of the epicenters definitely prevails. This is especially seen from CMT method determination and is quite similar to that obtained at the Gakkel Ridge. According to the first motions method, the discrepancy in the orientation of the axes is more significant (some solutions even provide a normal fault- strike-slip mechanism), which seems to be more geologically realistic. The same response to effective stresses of the thin oceanic crust and thick heterogeneous ancient continental crust is not expected.
Predominate subhorizontal extension is also observed in the most southwestern seismically active zone. The strike of the stress axis although suffering considerable deviations probably due to of the ancient faults and older geological features on which modern processes are superimposed, nevertheless tends to suborthogonally strike with respect to that of the epicentral zone. Individual solutions regarding strong earthquakes from this zone are heavily supported by the results of group determinations obtained through the use of numerous weak impulses recorded by Sevmorgeologia field stations and regional network of Yakutian stations (Avetisov,1993c).
At the same time, in the central Buor-Khaya Bay, based on the earthquake of July 21, 1964 (Parfenov and others,1987) and a group of weak earthquakes (Avetisov,1993c), subhorizontal compression nearly orthogonal to the line of epicenters is identified as predominant. It should be noted that in paper Cook and others,1986) by using surface waves, a normal fault mechanism for the same earthquake was obtained. Our determination has shown a solution with low confidence levels, however the most realistic solution seems to be that of (Parfenov and others,1987).
In the northwestern fragment of this zone ( earthquake of 1983) both methods provides a normal fault mechanism.
For the earthquake of December 12, 1973 near the New Siberian Islands ambiguous results have been obtained from several solutions of fair quality. Noteworthy is a good fitness of azimuths of all focal mechanism components with a considerable discrepancy of plunges. Our first manual determination (Avetisov,1979) was supported by an impartial computer solution which provides convincing arguments for adoption of the strike-slip mechanism for this earthquake. In the continental part of the seismic belt southeast of the Buor-Khaya Bay stress mechanisms are steadily replaced by thrust and strike-slip - thrust.
Eurasian continental slope has rare epicenters, however earthquakes are known with magnitude excessing 5, one of which dated February 18, 1948 having magnitude 6-6.5 (Fig.10).
Understanding of seismicity of this region is obscured due to the distant location of the recording stations. The Kheis station on the Franz Joseph Land was the only one to be installed (closed in January 1992). This station annually recorded single
Focal mechanisms of earthquakes of the Laptev Sea and surrounding areas (First motions method)
Table 28 (Continuation)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Laptev Sea and surrounding areas (CMT method)
weak impulses near the archipelago, while between late August 1983 and early January 1984 a local burst of activity was marked by 8 events of energy class 8 to 11 in the Austrian Straight, some 10-20 km east of the station (Kochetov and Lazareva,1986). A few weak impulses near the archipelago were also identified by a field station operating on the westernmost island, the Alexandra Land (Avetisov,1971).
Within the continental slope distribution of epicenters is irregular: they form a separate group tending towards marginal shelf depressions and faults. Following along the slope from the west to the east one can find epicenters in Franz-Victoria, Saint Anna, and Voronin troughs, and in straits of the Severnaya Zemlya Archipelago.
Focal mechanisms were only determined for three earthquakes from the Franz- Victoria trough (Table 30). This event of 1948 shows that both solutions available provide strike-slip mechanism, but with good coincidence of nodal planes azimuths of corresponding stress axes differ nearly by 90°, which suggests a mistake made by one of the authors. A similar pattern was obtained for the second, less strong earthquake of 1948. Based on three determinations for the earthquake of 1967 the coincidence of the results is satisfactory. Strike-slip or normal fault - strike-slip mechanism with latitudinally oriented extension axis has been obtained. More dipping compression axis is suborthogonal to the strike of the Mid-Arctic Earthquake Belt.
The Lomonosov Ridge is a subsea boundary of the Eurasian Subbasin opposite to the continental slope of Eurasia. Evidence of earthquake epicenters are missing from the Ridge along its length. Data are only known from its junction with the shelves of Lincoln and East Siberian seas. Events with magnitude up to 5 are known from the former, while in the latter weak earthquakes were recorded by field stations on the New Siberian Islands (Avetisov,1975).
A focal mechanism solution is available for one fairly strong earthquake dated January 4, 1978 (Fig.11a, b, Tables 31, 32). Both CMT and best solutions made by first motions method have provided a strike-slip mechanism with a sublatitudinal extension and submeridional compression axes.
The Barents Sea is located in the northwesternmost part of the West-Arctic continental margin. Its borders with the Lofoten Basin of the Norwegian Sea and Eurasian Subbasin of the Arctic ocean is the edge of the continental slope.
The shelf of the Barents Sea is part of the Barents-Kara marginal continental platforms (Geological structure..,1984). Modern understanding of the basement is based on the geological information onshore and from islands where its exposed or offshore locally by stratigraphic wells (Gramberg and others,1985), Seismic observations by refraction, DSS, WASP (wide-angle seismic profiling) (Daragan-Sushcheva,1991; Litvinenko,1968; Pavlenkin,1981; Tulina,1985, etc.) and regional gravity surveys (Malyavkin,1974; Shimaraev and others,1970; Verba and others,1985, etc.) also contribute to understanding its geological formation.
Fig.15 shows that the age range of folded metamorphic complexes of the basement is great. The oldest deeply metamorphosed Archean to Lower Proterozoic formations of the Baltic Shield are found, as well as the Riphean metamorphosed and folded primarily terrigenous complexes of the Pechora Syneclise. In addition
Focal mechanisms of earthquakes of the Eurasian continental slope (First motions method)
See Table 12 for the legend
Focal mechanism of earthquake dated January 4, 1978 of the Lomonosov Ridge (First motions method)
Focal mechanism of earthquake dated January 4, 1978 of the Lomonosov Ridge (CMT method)
See Table 13 for the legend
the Svalbard Anticlise, Scandinavian caledonid complexes, and kimmerides of the Novaya Zemlya folded area are observed. Offshore of West Spitsbergen there are folded Phanerozoic units disturbed during the epoch of the Alpine orogeny.
In the Barents shelf based on geological and geophysical data, is a northeasterly-southwesterly trough system expanding towards Fennoscandia. This is not well developed on the adjacent onshore area. This zone is characterized by quiet isotropic almost zero-valued magnetic and low gradient gravity fields. Discordant behavior of gravity anomaly contours with respect to the Central Barents zone, as well as adjacent Timan-Pechora and North Kara areas indicate that folded structures of periphery platform zones drop here.
In contrast to the marginal shelf areas they are characterized by a 10-12 km greater thickness of the sedimentary cover and by 8-10 km thinner overall thickness of the earth crust. Verba's (1977; 1985, etc.) presently adopted explanation implies rifting of the trough system generated during the late Devonian-Carboniferous and called the Barents-North Kara Megatrough (BNKM), and its further fast filling with volcanic terrigenous sediments of the Permian -Triassic age. This interpretation is supported by deep seismic investigations showing that the granite metamorphic layer in the BNKM thins to 0-5 km with concurrent earth crustal thinning from 35-40 down to 28-32 km.
In accordance with a different deep structure of the earth crust within the confines of the Barents shelf, three different blocks have been identified (Barents Shelf Plate,1988; Verba,1993). A three-layered crustal model typical of ancient platforms is adopted for the Pechora Syneclise, most of the Svalbard Anticlise and Central Barents Rise. A two-layered "granite-free" crust is established in the rifting BNKM structure. A similar but younger crust is located on the periphery of the Barents Sea shelf on continental slopes of the Norwegian-Greenland Basin and Eurasian Subbasin. A two-layered sedimentary cover-free model is proposed for the shelf adjacent orogenic areas of Novaya Zemlya, West Spitsbergen folded area, offshore Norway and Kanin Peninsula, and the shields of Nortdaustlandet and Kola Peninsula.
Analysis of geological and geophysical data on the structure of the Barents Platform basement complexes allows a conclusion to be drawn on a disjunctive nature of the connection between these blocks.
Fig.15 reflecting the all regional information available on earthquake epicenters from the Arctic Seismological Data Bank demonstrates that the offshore Barents Sea and surrounding areas shows fairly low seismic activity. However some highly active sites occur within its confines. Earthquake epicenters are irregularly distributed and definitely tend towards marginal parts of the basin. Noteworthy are the seismically active zones of the subsea Knipovich Ridge, Lofoten Basis and Svalbard Archipelago on the western and northwestern fringing of the Barents shelf , and higher seismicity sites in the marginal shelf Franz-Victoria and Saint Anna troughs on the northern margins.
In the south fairly weak seismic activity is reported from the junction between the Barents Sea Platform and Baltic Shield - the Murman-Finnmark zone from which, however earthquakes with magnitude 4-4.8 have been known. Epicenters forming a fairly scattered swarm showing however, sites of scarcity and concentration, cover the northern part of the Baltic shield. Quite obvious is a distinct correlation of the northern border="0" of the swarm of epicenters with the junction of the Baltic Shield with Norwegian Caledonids and the Barents North Kara Megatrough supporting faulted nature of these connections. It is here that most of strongest events are concentrated. The distinctive feature of the epicentral distribution is complete aseismicity of the of Kola Peninsula located east of the Konozersky graben colinear with the offshore faults restricting the south flank of BNKM and the contact zones between the Baltic Shield and Pechora Syneclise. A lower seismicity is shown in the area of Norwegian caledonid: single epicenters are reported .in the coastal fjord area.
The central part of the Barents Sea offshore area is almost aseismic. This is support by numerous installations (lasting up to one year) on the sea floor of highly sensitive seismographs and no earthquakes have been recorded (Soloviev,1986).
In the east recent observations have identified the Novaya Zemlya seismically active zone (Assinovskaya1990; 1994). The epicenter of the earthquake dated August 1, 1986 (magnitude 4.6) located on the east coast of Novaya Zemlya near the Matochkin Shar Strait is associated with the intersection of submeridional and sublatitudinal faults shown in the topography with an amplitude up to 700 m and transition from lowlands to the coastal marine plain.
A local zone of weak earthquakes is identified by the most recent seismological observations at the southesternmost flank of the Barents Sea immediately south of Amderma (Assinovskaya,1990;1994; Assinovskaya and Soloviev,1993).
Apart from the above (Knipovich Ridge, Lofoten Basin, Svalbard, continental shelf of the Eurasian Subbasin) another 4 focal mechanism solutions made through first motions method are known for the Barents Sea and surrounding areas (Table 33). Because of the low intensity of the earthquakes, data coverage is poor and, so the solutions cannot be regarded as reliable. In view of the above and based on the data collected we suggest only one conclusion: in all four cases subhorizontal compression axes and thrust or strike-slip - thrust mechanism have been obtained.
4.4.The Chukchi Sea and surrounding areas.
In contrast to other shelves of Eurasia, the western (with the East Siberian Sea) and eastern (with the Beaufort Sea) boundaries of the Chukchi Sea are to a large extent tentative and marked along the meridian of Wrangel Island and 155-156° W, respectively, The northern termination is conventionally recognized as the edge of the continental slope.
According to V.A. Vinogradov et al. (1974), based on the structure of the sedimentary cover, the offshore Chukchi Sea is located within the eastern part of the Chukchi and western part of the Alaska megablocks. The region is divided by the Chukchi-Bering deep fault traced from geophysical data from Bering Strait northwesterly east of Wrangel Island. Therefore the shelf of the Chukchi Sea is included in the vast East Siberian-Chukchi sedimentary basin covering tectonic structures consolidated in different times with distinct flexure fault restrictions (Geological structure..,1984).
Focal mechanisms of earthquakes of the Barents Sea (First motions method)
See Table 12 for the legend
The southern part of the Chukchi Sea is covered by the South Chukchi Trough traced from Kotzebue Bay on Alaska to the Shalaurov Cape in the East Siberian Sea (Fig.16). Folded basement of the trough is represented by mesozoids of the New Siberian-Chukchi miogeosyncline. In the basement relief of the trough according to geophysical data a series of extended arches and depressions is identified. The basement in the southern part has a non-typical northwesterly strike sublatitudinal orientation. It is assumed that the discordant orientation of these structural elements and, particularly, the major one of them the Kotzebue Arch extended almost to Alaska and is related to the existence of sublatitudinal faults. Block movements of the basement units along these faults caused the formation of these structures (Geological structure..,1984).
The northeastern part of the Chukchi Sea is assigned to the Beaufort-Chukchi marginal continental platform due to the older Caledonian basement. And, finally, the north and northeastern parts of the offshore area are occupied by the structures of the East Siberian marginal continental platform the age of which has not been determined due to lack of data. Based on the analyses of airborne survey data on the shelf of the East Siberian Sea the basement of its northern part is presumably Archean - early Proterozoic while in the south it is likely to be represented by Baikalides.
Within the Eurasian continent the structures of the Chukchi Sea are restricted by mesozoids of Koryakian-Anyuyian eugeosynclinal system.
Higher seismicity is known from the southern and southeastern part of the offshore area, and on Chukchi Peninsula (Fig.17). It should be noted that only the strongest events are recorded by the teleseismic network. Immediate seismological observations on Chukchi Peninsula were started as late as in 1964 after installation of the Iultin station which recorded the majority of earthquakes in this region. Prior to 1964 (beginning 1908) only 8 events with magnitude ranging from 4.7 to 6.9 had been recorded while since 1966 after installation of short-period instruments over 7 years 88 earthquakes with epicentral distance up to 500 km have been located. High frequency of earthquakes in the region is suggested from the fact that according to data collected from the only station it is possible to locate only 7% of all recorded events (Lazareva,1977).
Within the offshore area the four strongest earthquakes occurred in 1928 (M=6.2-6.9) some 250 km northeast of the Kolyuchinskaya Bay. Later, two earthquakes with magnitude 5.5 were reported from coastal part of the offshore area in 1962 and 1971. The latter with intensity of 5 MCS was felt in Neshkan settlement and had about 50 aftershocks.
All these events are related to the western part of the Kotzebue Arch, while its central part does not show any activity. Concentrations of epicenters are marked in the eastern part of the Arch reaching to Alaska in the junction zone with the seismically active Brooks anticlinorium, and farther north in the zone of approximation to the Alaska of the Wrangel-Herald Range and suture restriction of the Beaufort-Chukchi platform. From the western part of Kotzebue single epicenters of weak earthquakes are traced along the off-axis zone of the South Chukchi Trough. A
definite increase has been reported from the Long Strait. Here epicenters mainly form a linear group tracing the junction of the trough with the Chukchi Folded Zone.
A few fairly strong earthquakes (M>4) have been reported from the eastern offshore zone, that is in the deep Colville trough where no weakened zones have been revealed by other geological and geophysical data.
Onshore within Chukchi Peninsula and its western surrounding areas the general pattern of epicenters are dispersed presumably caused by insufficiently precise localization. There is a tendency towards linearity along the longitudinal off-axial area generally concordant with the strike of major tectonic elements. At the same time, within this band, obliquely or cross-striked concentrations may be identified. While the easternmost of these concentrations is confidently associated with the Kolyuchinsk-Mechigmen suture zone separating the Anadyr Median Mass from the adjacent geostructures the western is associated with the weakened area crossing the structures of the Chukchi Folded Zone. The topography of the Kuekvun River valley corresponds to this weakened area.
More seismically active are certain regions of Western Alaska where earthquakes with magnitude over 5 frequently occur. Interestingly, the earthquake epicentral distribution suggests no links between seismically active zones of Chukchi Peninsula and Alaska. The dividing Bering Strait is almost aseismic. In the Western Alaska the most seismically active is the Brooks anticlinorium zone. A dense epicentral band is extended across the entire anticlinorium from the Bering Strait eastward to the Central and Southern Alaska where the Aleutian-Alaskian zone of the highest seismic activity which is a northern fragment of the Pacific Seismic Belt is located (Fig.17). Seward Peninsula also shows higher seismic activity with epicenters tending towards periphery of the Seward Median Mass.
Information on focal earthquake mechanisms immediately from offshore Chukchi Sea is limited to solutions on the earthquake dated 1971 made in different years by four different authors. Table 34 shows that in each case essentially different results were obtained, which is evidence of the unreliability of the solution.
For two earthquakes of the Western Alaska in the Brooks anticlinorium zone solutions made by CMT and first motions methods (Tables 35, 36) are known. For the earthquake dated 1981 in accordance with the well fitted results of both methods a strike-slip-normal fault mechanism is obtained in the focus with two steep dipping nodal planes,(DP>70°), one being oriented close to the strike of the Brooks anticlinorium. The results obtained for the earthquake dated 1989 show that the process in its focus was unsteady and the character of the movements changed from initial purely thrust to normal fault in the major phase. It should be noted that similar results are obtained for earthquakes of this zone located eastward outside fig.17 (Avetisov,1995). It is evident that the focus model of earthquakes of the Western Alaska differs from that known for earthquakes of the Pacific Seismic Belt.
The area referred to in the title does not have strict geographic limits. In this work, in view of particular distribution of earthquake epicenters, the region includes
Focal mechanisms of earthquakes of the Chukchi Sea and surrounding areas (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Western Alaska (First motions method)
See Table 12 for the legend
Focal mechanisms of earthquakes of the Western Alaska (CMT method)
the Canadian Arctic Archipelago with adjacent northern and eastern offshore areas as well as the Beaufort Sea and surrounding areas partially including Northern Alaska.
Certain areas of higher seismicity have fairly long been known to exist in this part of the Earth, but more or less complete information on earthquakes became available beginning from the 1960s, after local seismic networks had been created. Now uncertainty of epicenter localization generally does not exceed 50 km, and the minimal energy level of earthquakes recorded without gap is no higher than magnitude 3.5-3.6 when averaged over the region, but in certain zones drops to 2.5-3.0.
The main feature of the distribution of earthquake epicenters in this sector of the Arctic is that they are rather sparse and concentrated in irregularly distributed zones rarely showing sublinear shape. The fact that transregional seismically active zones similar to the Mid-Arctic Earthquake Belt are absent from here, has determined the geographic principle of outlining the material. The only exception is the joint discussion of data on the Baffin Bay and Baffin Land incorporated in the Canadian Arctic Archipelago.
The Baffin Bay and Baffin Island constitute the southeastern part if the study area immediately adjacent to Greenland.
In terms of geology, nearly the whole Baffin Island is located within one of the northernmost parts of the vast (about 1.2 million square kilometers) ancient Canadian Shield where metamorphic and intrusion rocks of the Archean to Proterozoic age outcrop on the day surface (Fig.18). The contact between the shield and the area of development of the younger Innuit folded system is oriented here northwestward along the boundary between the Baffin Island and Fox Basin. Similar ancient rocks also fill up the opposite, the Greenland coast of the Baffin Bay.
The Baffin Bay, according to comprehensive geophysical data, including seismic (Jackson and others,1977; McMillan,1973; Shih and others,1988; Srivastava,1978 and others) is a major sedimentary basin with the thickness of sediments up to 6 km and more. In the deepsea part it is underlain by the oceanic crust of about 10 km thick. Based upon the same data, the boundary between areas with different type of crust is marked along contour 10000 m (Hood and Bower,1975; Orvin,1940).
The Baffin Bay and Baffin Island are characterized by fairly high seismic activity. The strongest Arctic earthquake with magnitude 7.3 and its aftershocks with magnitudes up to 6.5 were recorded in the Baffin Bay in 1933. Strong earthquakes (M=6.0) occurred in 1945, 1947 and 1955.
On the basis of magnetic data (Jackson and others,1977; Kristofferson and Talwani,1977; The Arctic Ocean Region,1990 and others) plate tectonic reconstructions in the Arctic relate the region under study with the former boundary between the Greenland and North America plates, so particular emphasis has always been placed here on the study of local seismicity (Hashizume,1973; Qamar,1974; Reid and Falconer,1982; The Arctic Ocean Region,1990). It is in this region, though difficult to access and carry out observations, that filed seismological studies were conducted using 6 portable (including 3 sea bottom) station (Reid and Falconer,1982). Notwithstanding the short duration of these observations ( from a week to a month) and considerable distances between observational points (100 to 300 km), the authors
managed to reach fairly high accuracy in localizing 9 epicenters in well-known onshore and offshore areas of epicenter concentration and thus confirmed validity of the stationary seismic network data. As is seen from fig.19 which reflects all available information on earthquakes through 1990, a main feature of the earthquake distribution on the Baffin Bay is a fairly wide (up to 200 km) zone of epicenters, their density being clearly variable. The zone is apparently offset from the bay axis toward the east coast of the Baffin Island. Only a few epicenters lie within the deepsea area of the bay and even this is probably caused by spread due to localization errors.
The beginning of this zone can be marked by a few epicenters of fairly strong earthquakes near the bay axis at the latitude 68-69°N, south of which no epicenters are detected. With some interruptions, this zone extends northwestward toward the Baffin Island coast, becomes distinct at 71° N and continues as far as the traverse of northern Lancaster Sound, generally following the zone of bottom depths from 500 to 1000 m. Further epicenters evidently branch off as three chains of different orientation and distinctness: the northern one runs along the northern coastal zone of the Lancaster Sound, dying out at the longitude of central Devon Island; the northwestern one extends into the Jones Sound north of Devon Island; and the meridional one extends as far as 100-150 km toward the Nares Strait and quickly dies out. The virtually complete aseismicity of the northern and southern continuations of the Baffin Bay (Nares and Davis straits, respectively), noted long ago (Basham and others,1977; Reid and Falconer,1982; Wetmiller and Forsyth,1982 and others) is completely substantiated. Thus the data available clearly indicate that the seismically active zone near the axis of the Baffin Bay bears a somewhat local character being closed and non-associated with any modern global seismicity belt.
Apart from the zone mentioned above, one may state a zone of epicenters with magnitudes up to 4.5 exists along the eastern part of the bay; the density of its epicenters is irregular and on the whole smaller than that of the first zone. Nearly all earthquakes of this eastern zone are located in offshore areas with depths of 100-200 m at the most or in the deglaciated coastal zone of Greenland. However, scattered but fairly intensive epicenters are reliably recorded far off the coast line, in the zone of thick ice cover.
By and large, available numerical determinations of hypocentral depths as well as qualitative estimates based on waveforms point to shallow seismicity in the bay. For instance, the value 24 and 20 km were observed from pP waves of strong earthquakes of 1976 and 1983 , respectively (Kroeger,1987). A wider range with maximum values up to 50 km has been obtained from near earthquakes (Reid and Falconer,1982). Prior to 1960, only one very strong earthquake with magnitude 5.5-6.0 was recorded on Baffin Island in 1935. A wider seismic network and field studies now have established higher seismicity in a considerable part of the northeastern coast of
the island, with epicenters clearly concentrated in the Buchan Bay and Scott Inlet north of the latitude 70° and the Home Bay south of it. The very strong earthquake of 1963 with magnitude 6.0 and several events with magnitude 5.0 were recorded here beginning 1960. Hypocentral depths when determined do not exceed 6-9 km. Most of the epicenters do not penetrate inland farther than fjord closure although the number
of epicenters within the central part of the island is rather considerable. Scattered though fairly strong events are recorded outside the zone considered. In particular, a few of them are located in the northwesternmost part, on Brother Peninsula. Thus the seismically active zone of Baffin Island also has a clearly limited, local character and is not associated with global seismic belts.
The first arrival fault plane solutions have been obtained for two earthquakes in the Baffin Bay and three earthquakes on Baffin Island (Fig.20a, Table 37) All authors obtained a thrust mechanism for both earthquakes (1933 and 1976) in the Baffin Bay, although marked discrepancies are obvious in orientation determinations of the compression axis, particularly for the second of the them. These discrepancies are associated with different values used for the hypocentral depth. However, the northwest orientation of the axis should be adopted according to a more reliable solution for the 1933 earthquake (Stein and others,1979) and solution for the 1976 earthquake (Avetisov,1993b) where the hypocentral depth was set equal to the real value of 23 km.
Normal-fault or strike-slip mechanisms were obtained for all earthquakes on Baffin Island, with the subhorizontal extension axis being fairly stable and transverse to the shoreline.
Canadian Arctic Archipelago is a continental bridge between rock masses of Greenland and North America, and separates the Atlantic and Arctic oceans. Because of this geographic position, it is quite soundly considered a geological object whose structure cannot fail to reflect processes of formation of adjacent water areas, which defines its key role in solving the problem stated above.
The whole course of geological evolution of this region, studied in numerous works (Balkwill,1978; Douglas and others,1963; Kerr,1981; Trettin and others,1972; Trettin,1989 and many other) consists of two major stages, partially overlapping in time.
The main features of the present geological structure of the Canadian Arctic Archipelago (Fig.18) were developed at the first stage referred to as a constructive one and began in the Archean with the formation of the ancient crystalline basement. The basement within its confines outcrops on the Baffin Island, southeastern Ellesmere Island, eastern Devon Island, Boothia Uplift in western Somerset Island and a narrow surface band in the land part of the Peary geanticline in northernmost Ellesmere Island. The rest of the archipelago is occupied by structures of the so-called Innuit continental-margin mobile belt represented by deposits practically of the entire Phanerozoic. Most developed are passive Cambrian-Devonian strata in the relatively stable Arctic platform that were folded within the Franklin geosyncline. The relief of the underlying ancient crystalline basement exhibits uplifted zones, the largest of which are the above mentioned Peary geanticline and Boothia Uplift. Geological and geophysical data (Kerr,1977; 1981 and others) show that these structures formed as a result of multiple uplifts of lithospheric blocks along a series of vertical deep faults detected in peripheral areas of the structures. There is a noteworthy wide range of uplift strikes (from submeridional to latitude), which appears to be related to characteristic features of the inner structure of the crystalline basement. The Boothia
Focal mechanisms of earthquakes of the Baffin Land and Baffin Bay (First motions method)
See Table 12 for the legend
Uplift plunges northward under strata of the Innuit belt and sedimentary Sverdrup Basin and quite probably reaches the Peary geanticline The sedimentary Sverdrup Basin lies in the central zone of the northern part of the archipelago and is of triangular shape opening northward, towards the Arctic Basin and closing southward. It is filled up with Early Carboniferous and younger rocks with net thickness up to 13 km and connected in the southwest with the sedimentary Banks Basin via a narrow bridge. The formation of the above basins is genetically related to the Lower Carboniferous initial phase of the second evolutionary stage of the Canadian Arctic Archipelago: the formation of adjacent oceanic basins and their continental branches which in the long run controlled the modern sea and land configuration of the Arctic Canada (Kerr,1981 and others).
The first event of this stage was active faulting (rifting) directed southeastward from the Arctic Basin and concurrent sagging (extension) of the lithosphere. These processes resulted in development of channels and basins in the northern half of the continental structure of the Innuit belt, which have been formed during the first constructive stage. On the whole, this process called the Boreal rifting episode (Kerr,1981) reached only the middle of the Queen Elizabeth Islands and it is along the modern Parry channel that it advanced further to the east, as far as Boothia Uplift. The episode died away in the latest Cretaceous.
Continuation of the second stage is related to Late Cretaceous-Miocene tectonic activity (Geological structure..,1984; Pogrebitsky,1976) that propagated at this time of the North Atlantic Basin. It is associated with formation of rift structures of the Labrador Sea, Davis Strait and Baffin Bay (Eureka rifting according to Kerr), as well as straits and channels controlled by the southeastern part of the archipelago. Rifting was connected genetically with development of a belt of compressional structures (Eureka orogen) at its front; the belt extends from the northwestern Greenland coast southwestward, through the Nares Strait and Ellesmere Island to Ellef-Ringnes Island, and then turns southward and reaches the Lancaster Sound. This belt formed due to the superimposed action of two tectonic phenomena: horizontal motions of diverging lithospheric blocks and gaps in rift fault propagation when it encountered ancient transverse structure such as, for instance, the Franklin geosyncline fold belt or Boothia Uplift (Balkwill,1978; Kerr,1967;1981, and others). In its conclusive phase, Eureka rifting overcome resistance of transverse structures and faults propagated in two direction: westward and northward, along the modern straits of Lancaster and Nares, respectively; thus they extended into the Arctic Basin and triangle-shaped subplate of the Queen Elizabeth Islands was formed. This event in fact completed the formation of the region in its present appearance.
A fairly large number of publications by North American scientists has been devoted to the seismicity of Northern Canada, and the Canadian Arctic Archipelago, in particular. The work by P.Basham et al. (1977) should be unconditionally acknowledged as the most complete for its period of time (before 1974).
Distribution of earthquake epicenters within the study area of the archipelago, although largely scattered, allows making a confident statement on the existence of areas with fairly high seismicity (magnitude 4-5 and higher) and completely aseismic (Fig.19).
First of all one should note a quite distinct high-density epicenter zone extending virtually all along the Arctic coast of the Queen Elizabeth Islands, from Prince Patrick Island in the southwest to northwestern Ellesmere Island in the northeast. In the aforementioned work of the Canadian scientists only the western part of the zone was established on the basis of data available at that time. Numerous local events were detected at station Mould-Bay here, although coordinates of epicenters were not determined. For instance, a cluster of 2000 earthquakes occurring in 30-day period between March and April, 1965 was recorded on the southwest coast of Prince Patrick Island (The Arctic Ocean Region..,1990). One may suggest that this zone would be still more narrow if the accuracy of epicenter localization were improved, with most of epicenters being fixed by 3-4 stations. However, even this accuracy leaves no doubt that, on the whole, epicenters are confined to land areas, follow the shoreline in a number of places, and virtually do not occur in water areas with depths more than 200 m. It is particularly notable from sharp and clear southwestern and northwestern limits of the band in the transition to the open sea. In addition, scarceness of the band is apparent near Ellef-Rignes Island confined with deepwater straits, and disruption of the band between Ellesmere and Axel-Heiberg islands separated by a wide and deep strait.
Another sublinear, though less distinct zone can be traced from the eastern half of Melville Island and the offshore area northeast of it to Bathurst Island and the northwestern end of Devon Island (Grinell Peninsula), and further nearly strictly southward, through Somerset Island and Boothia Peninsula to the continent. On the north this zone and the aforesaid coast line of epicenters appear to merge. Epicenters of the zone are much more scattered and irregularly distributed. The most prominent concentration of epicenters is observed in the shallow part of the Byam-Martin Sound north and northeast of Melville Island. Some ten earthquakes per year with magnitude above 3-3.5 occurred here before 1972, which was believed to be typical these areas of the Canadian Archipelago. An abrupt activity outburst began in late 1972 when 8 strong earthquakes with magnitudes up to 5.6 occurred during November and December, with the total number of events (foreshocks and aftershocks included) amounting to 52. A rather high activity level persisted in the first quarter of 1973 when 38 earthquakes were recorded including one with magnitude 4.9; then began to drop and in 1975 reached the former level (existing prior to 1972). The latter has lasted until now.
A fairly distinct local cluster of epicenters in this zone is observed at its intersection with the shallow zone of the Barrow Sound (continuing Lancaster Sound) where earthquakes were recorded in an area of 100 x 50 km. Two of them occurring in 1974 and 1987, had magnitudes of 4.9 and 5.2, respectively.
A high seismicity area exists in the Arctic ocean, northwest of the archipelago. In spite of large epicentral distances, 57 earthquakes were recorded here, of which 13 had magnitudes 4.0 to 4.9 and two above 5.0. The cluster of epicenters in its densest part is of sublinear shape and NE strike, lies in an offshore area with bottom depths ranging between 200 and 1000 m, and traces the zone of abyssal shelf transition. Epicenters are more sparse in its northern part, which might be related to decrease in accuracy of localization. Noteworthy is an obvious concentration of epicenters in the region bounded by 79-80° N and 107 -108° W, where 14 earthquakes were recorded in a very small area; six of them had magnitudes above 4.0 and two were above 5.0. It is interesting that here, just as in the Byam-Martin sound, a pronounced outburst of seismic activity (7 of 14 events, including one with magnitude 5.0) occurred in the second half of 1972.
The southwestern part of the archipelago including Victoria and Banks islands and adjacent sounds, is distinguished by very low seismicity, as is evident from mapping of epicenters. This is particularly conspicuous against the fairly high seismic activity of surrounding areas, especially eastern and northern ones. Throughout the entire history of observations, no more than 10 earthquakes with magnitudes 3.0-3.2 or less were recorded in this area, about 600000 square kilometers in size.
Three focal mechanisms were derived by the method of centroid moment tensor for this part of the archipelago. Also six mechanisms were obtained from first arrivals for four earthquakes in the Byam-Martin Sound (Hasegawa,1977), one in the Lancaster Sound and one on the west coast of Boothia Peninsula (Avetisov,1993b). Solution by both methods are available for the latter two earthquakes (Fig.20a,b, Tables 38, 39).
A main feature of nearly all solutions is the predominant role of strike-slip motions with thrust-slip component present too. Noteworthy is the general uniformity of the azimuths of stress axes: NNW extension and ENE compression. Taking into account the considerable separation of observational points, one may suggest a regional character of the stress field.
The Beaufort Sea covers the area from the coast of Banks Island in the east to the tentative boundary with the Chukchi Sea at 155-156°W in the west. Unlike other seas of the Arctic Basin of the Arctic Ocean, which are purely shelf seas, the Beaufort Sea partially includes the deepsea area of the basin tentatively up to contour 300 m (Fig.21). Within the Beaufort Sea the outer boundary of the continental shelf is marked fairly tentatively, since in many cases it has no, unlike elsewhere in the Arctic seas, a distinct physiographic step near the contour 200 m. The shelf of the Beaufort Sea is the narrowest among all other Arctic seas and varies between 60 km in the west and 150 km in the east.
In the context of modern tectonic ideas within the Beaufort Sea shelf and surrounding continental areas three major structural tectonic elements are
distinguished: geosynclinal folded system of the Cimmerian age, represented here by northeastern termination of the Brooks Range, ancient pre-Riphean platform and the system of marginal continental troughs (Fig.21). The latter covers the entire shelf and coastal plain area of several dozens kilometers wide west of the Mackenzie Delta, and up to 150-200 kilometers wide in the southeast and east (Banks Basin), and 250-270 km wide in the Mackenzie Delta and Northern Alaska where it includes such geological units as the Colville Trough and Arctic Platform. Within the framework of well-known part of the troughs, namely near the Mackenzie Delta and shelf zone northward, a complex fault tectonics has been established showing essentially different types and spatial orientation of the faults. Directly in the delta and northeastward a system of parallel near vertical faults of northeastern strike is
Focal mechanisms of earthquakes of the Canadian Arctic Archipelago (First motions method)
Focal mechanisms of earthquakes of the Canadian Arctic Archipelago (CMT method)
See Table 13 for the legend
developed with normal fault movements decreasing in amplitude toward the offshore area. The major faults are traced far northeastward up to Banks Island. The region northwest of the delta partially covering a zone outside the continental slope is characterized by intensive development of diapiric anticlines of predominantly northwestern strike and essential role of the compressing component (The Arctic Ocean Region..,1990). Recently the Mackenzie Delta area has experienced subsidence, the cumulative amplitude in the axial part reaching 600 m (Arctic Atlas,1985). A similar situation, judging from the pattern of subsidence isolines amplitudes, is at least over the entire eastern Beaufort Sea Shelf.
Southerly and southwesterly located area of geosynclinal folded structures is represented by the system of alternating rises and depression, being on the whole a zone of intensive modern elevation with the maximum cumulative amplitudes of rises up to 1000 m and over. The apparent block pattern of the structure predetermines under intensive elevation an obligatory differential motions along weakened zones.
Also, a zone of modern elevation, however less intensive (cumulative amplitudes of rise not exceeding 600 m) is the area of ancient platform (Canadian Shield) located in the east and southeast.
The Beaufort Sea is the most seismically active area of the Amerasian part of the Arctic Ocean. Earthquakes in the Beaufort Sea and surrounding areas are grouped in three major zones (Fig.21): on the southeastern coast in the Mackenzie mouth and valley (135-137° W); in the southwestern part along 144-145°N with an evident cluster in immediate offshore zone; in the offshore area near the junction of the shelf and oceanic zone.
As is seen from the above, the first of the aforesaid zones showing earthquakes of magnitude 4.5 is associated with the area of complex fault tectonics, where a subsiding wedge of the lithosphere is restricted by an intensely elevated block. This zone actually is the northern termination of the broken line of epicenters stretched southward along the west rim of the Mackenzie Valley and near 66°N turning southeastward along the east slope of the Mackenzie Mountains (Fig.6). Even more drastical than the northern, the southern termination of this zone, where in 1985-1988 earthquakes were extraordinarily intensive (9 earthquakes of magnitude over 5, with 3 of them having magnitudes over 6) takes place between Big Bear and Big Slave Lakes and coincides with the southern termination of the Mackenzie Mountains.
The southwestern cluster of epicenters with earthquakes of M=5 within the offshore area mainly does not goes beyond the contour 200 m. This cluster tends to the northernmost protrusion of the contact between the folded area and the marginal continental trough. At the same time, it is apparent that it is the northern termination of the meridionally oriented near the coast the line of epicenters which is generally cross-striking the major folded structures, however tracing the zone of their submeridional displacement along the Canning Fracture System, and near the Arctic Circle is connected with the sublateral seismically active zones passing along the southern rimming of the Brooks Range (Fig.6).
Epicenters of the offshore group are located between contours 200 and 2500 m, and form a fairly isometric cluster tending towards the northeastern zone of intensive diapirism and its contact with the marginal continental trough.
No temporal correlation are observed between the seismic regimes of the aforesaid seismically active zones of the Beaufort Sea. In the Mackenzie Delta a drastic burst of seismical activity was observed in 1965 when 11 earthquakes occurred showing magnitudes over 4 (with 6 occurring in January) at an average level of not exceeding 1-2 throughout the subsequent decades. A similar picture was observed in the southwestern zone, but in 1968. Here, 11 earthquakes also occurred showing magnitudes over 4 (with 4 occurring in January) at an average number of less than 1- 2. In the offshore area no temporally localized fluctuations have been observed. Here, a greater intensity reaching up to 2-3 earthquakes per year on average showing magnitudes not exceeding 4, was observed during 1975-1986.
Immediately in the offshore area of the Beaufort Sea, focal mechanisms are obtained for two earthquakes (Tables 40, 41). As to the event of 1975, similar results obtained by two authors through the first motions method showing strike-slip mechanism with the submeridional compression axis. One of the nodal planes coincidents with distinguished deep fault of northeastern strike traceable from the Mackenzie Delta to Queen Elizabeth Islands. For earthquake of 1987, which occurred 250-300 km north-northeastward near the continental slope where it has submeridional strike, well correlated data of CMT and first motions methods provided a mechanism similar to the thrust one for one of the nodal planes submeridionally oriented. For both of the aforesaid earthquakes the movements along strike of the nodal planes, which are presumably fault planes, correspond to sinistral slip.
Determination carried out by two authors regarding earthquakes of 1968 in the southwestern cluster of epicenters in immediate vicinity to the shoreline have provided a strike-slip mechanism, however, the positions of the stress axes are essentially different, probably due to lack of instrumental materials.
Focal mechanisms of earthquakes of the Beaufort Sea area (First motions method)
Focal mechanism of earthquake of the Beaufort Sea March 30, 1987 (CMT method)