Conclusions

Based on the presently available most complete seismological information on earthquakes obtained over the whole period of instrumental observations in this country and abroad, and using a wide range of other geological and geophysical materials hypocenters and focal mechanisms of earthquakes have been studied, new relations between seismicity and tectonics and neotectonics have been specified and identified, geodynamic models of seismically active zones of the Arctic and Subarctic have been designed. In additions, possibilities for unification of earthquakes magnitudes have been analyses for the first time, and a number of regression formulas connecting magnitude determination types, the most frequently used for assessment of Arctic earthquakes are presented. The information applied in this paper was obtained from the Arctic Seismological Data Bank designed and developed by the author.

The conducted investigation has allowed the following conclusions to be drawn:

- both types of seismically active zones, interplate and intraplate, .exist in the Arctic:

- interplate seismicity is represented by the Mid-Arctic Earthquake Belt tracing the spreading boundary of the Eurasian and North American lithosphere plates and extending from Iceland through the Norwegian-Greenland Basin, Eurasian Subbasin, and Laptev Sea shelf to the Northeast of Eurasia.

- the aforesaid interplate boundary which generally followed a single scenario yet consists of a number of conjugated segments, evolution of each having particular features of their own. The leading role in the formation of these features is played by the difference in the pre-spreading structure of the lithosphere. It is shown that on the basis of this point, three types of boundaries may be principally identified; each having relevant analogy in the Mid-Arctic Belt: homogenous and isotropic lithosphere without predominant tectonic elements within which the orientation of a possible fault plane will be primarily defined by rules of mechanics and depend on the orientation of applied forces (Gakkel and Mohns ridges); the environment with a pre-spreading weakened zone where the emerging splitting line rotates from the theoretical position towards the weakened zone (the Knipovich Ridge), and the environment with a rigid monolithic block. In the latter case the split which abuts this block under a right or close to a right angle bends round it along a fracture or a series of en-echelon fractures (Tjornes, Jan Mayen or Spitsbergen Fracture zones).

- existing ideas on the persistence on the Laptev Sea shelf of the Eurasian and North American plates boundary may by only regarded true at first approximation, that is if the entire shelf is considered as a part of the boundary. This is sufficient to gain an understanding of the general kinematics of plate tectonic movements, but does not meet requirements of regional studies. It should be admitted that there are two "blind" sites of this boundary, one of which in the eastern half of the shelf is the termination of the oceanic part of the boundary extended from the Eurasian Subbasin, and the other is that of the continental part of the boundary extended from the eastern Yakutia.

- the formation of a single boundary of the Eurasian and North American plates is controlled by two counter movements of its broken fragments: southward through the area of mesozoids in the eastern part of the shelf, and northwestward along fault margins of the Lena-Taymyr zone of conterminous uplifts. At present, one can only express general ideas on the nature and site of possible junction of broken fragments in case of continuing activity of riftogenic extension forces. It is evident that anticipated position of the single boundary will be essentially defined by that of existing weakened zones in the region. The following options seem to be realistic.

1. Southward promotion of the eastern fragment of the split along the Omoloy Graben and southern part of the Ust-Lena Trough and its junction with continental part of the boundary east of the Lena River Delta; north-northwestward promotion of the western fragment along the contact between the Taymyr Folded System and the western part of the Laptev Platform and its exposure to the continental slope. This will lead to the formation of the Laptev microplate and triple junction near the south coast of the Buor-Khaya Bay. According to this, an exposure of the western fragment of the split to the continental slope is possible through the North Graben.

2. Stop in promotion of splits in the aforesaid directions and junction of presently identified fragments of the boundary along the northwesterly strike line coincident of close to the North Graben. In this case, the formation on the Laptev Sea shelf of a transform fault system like Spitsbergen Fracture Zone should be anticipated.

- based on the all available seismological data and, particularly, comparison between seismic activity levels of individual segments of the spreading plate boundary in the Arctic, one can draw a conclusion that the statement on "absolute" rigidity of the plates is admissible at a global approximation level. A considerable part of stresses generated as a result of spreading is relaxed inside the plates. The most common example is the Knipovich Ridge area.

- unconditionally, zones of intraplate seismicity are Fennoscandia, continental margins of the Eurasian Subbasin and Norwegian-Greenland Basin, the Canada Arctic Archipelago and adjacent areas, Northern Alaska and Northeastern Eurasia. A regional background factor responsible for the generation of intraplate seismicity and affecting its level and depth is discharge of stresses generated on the plate boundaries. Seismogenic impact of this factor primarily occurs in ancient weakened zones of the lithosphere and is enhanced under other processes among which the leading are differentiated vertical tectonic movements. The role of glacioisostatic movements and load of abnormally thick sedimentary units is now regarded secondary.

The total amount of information presented in this paper including data on distribution of earthquake epicenters, magnitudes, depths of hypocenters and focal mechanisms as well as an analysis of tectonic nature of earthquakes provide a clear understanding of the location of hazardous Arctic zones, and in association with the Arctic Seismological Data Bank would be a good basis for seismic zonation of the region. Despite that more than a half of its area is offshore while the land is poorly and irregularly populated, there is a certain amount of macroseismic information directly indicating the intensity of seismic events. It is apparent that distribution of this information by region is primarily defined population rate rather than by seismic activity level.

The most of macroseismic information has been collected in Fennoscandia where 173 felt earthquakes have been reported for the entire period of instrumental observations for the area north of 65° N alone. Among these 7 had an intensity of 4 MSK and more, and one dated April 16, 1989 reached 8 MSK and destroyed the upper horizons of the Kirovsk Mine. There is some historical information on strong earthquakes on Kola Peninsula and northern Archangelsk Region during the last centuries.

Six strong earthquakes with intensity 6 MSK and more in the epicenter are known from the Northern Yakutia north of 70° N. This is primarily, a series of the Bulun earthquakes dated 1927. The northernmost of felt earthquakes in this region is the aforesaid earthquake dated February 1, 1980 in the Gulf of Olenek. Its intensity was 7 MSK in the settlement of Olenek (100 km away from the epicenter) and 3-4 MSK in the town of Tiksi (300 km away from the epicenter).

Felt earthquakes are fairly frequent on the Chukchi Peninsula with the strongest one dated October 5, 1971 which occurred in the coastal zone of the Chukchi Sea and had an intensity of 5 MSK near Neshkan Settlement (100 km away from the epicenter), and 3-4 MSK in Uelen Settlement (250-300 km away from the epicenter).

A great number of felt earthquakes has been reported from the Arctic regions outside Russia: Western and Northern Alaska (59, including 18 with intensity 4 MM and more), Jan Mayen (15), Iceland (10). Two felt earthquakes dated January 18, 1976 and July 17, 1977 are known from Svalbard. Information on 8 felt earthquakes is available even on nearly uninhabited Canadian Arctic Archipelago.

According to the acting map of seismic zonation of the USSR published in 1978, 7-8 MSK areas include the Lena River Delta and adjacent areas located in the area of exposure of the Mid-Arctic earthquake Belt to the continent., while Chukchi Peninsula is the area of 6-7 MSK intensity.

Based on our 1971 tectonoseismic zonation of the Eurasian Subbasin, Norwegian-Greenland Basin and adjacent shelf offshore areas (Avetisov and Golubkov,1971) two seismically active zones have been identified in the region:

- intensity of 8 MSK - mid-oceanic ridges. Within this area earthquakes of intensity of 8, 7, 6 MSK may be expected every 70-80 years, 15-20 years, and 5-8 years, respectively.

- Intensity of 6-7 MSK is the area of the continental slope and frontal shelf. Within the latter a special notice has been taken of transverse troughs such as Bjornoja, Franz -Victoria, Saint Anna, Voronin.

Abyssal Nansen and Amundsen basins having no own earthquakes are yet assigned to 6 MSK intensive areas in view of possible shocks transferred from adjacent seismic zones.

The principal correctness of zonation conducted a quarter of a century ago does not cause doubts now either, which has been supported by similar papers by I.P.Kuzin on the Laptev Sea (1989) and B.A.Assinovskaya (1990;1994) and B.A.Assinovskaya and S.L.Soloviev (1993) on the Barents Sea. Yet, a new geological information made available to the above authors allowed detailed specification of our 20 years old estimations. In turn, data presented in this paper on hypocenters of earthquakes, generally supporting the correct approach to identification of hazardous zones, show a necessity of radically specifying their location. This is especially true for the Laptev Sea where a previous 8 MSK intensity zone was marked along its central axial part.

It is obvious that the creation of the electronic Arctic Seismological Data Bank which provided the basis for this work, taking advantage of the opportunity of computer map making for epicenters realized in this work, substantiation of tectonic nature of arctic earthquakes carried out in this work in association with other advanced geoscientific information provide premises for transition to qualitatively different level of seismic hazard assessment in the Arctic Region. The entire work demonstrates that by the seismic activity level the seismic zones of the Arctic may be ranked as follows:

- The Mid-Arctic Earthquake Belt including the entire shelf of the Laptev Sea, Western and Northern Alaska (primarily, Brooks Range), Baffin Bay where earthquakes with magnitude 6 and more are known to be frequent;

- The Lofoten Basin, Svalbard, Northeastern Greenland, transverse troughs of the continental slope of Eurasia, Beaufort Sea, Mackenzie Mountains where earthquakes with magnitude 5 and more are not rear;

- Northern Sweden and Norway, western Kola Peninsula, local swarms of epicenters within the Canadian Arctic Archipelago (e.g. northeast of Melvill Island or on the northeast cost of the Baffin Island) where earthquakes with magnitude up to 5 are established.

Scrutinized shall be Chukchi Peninsula which is clearly active but poorly known.

Acknowledging ranking in the seismic activity level it should be yet noted that this is not the top priority factor for selection of the region in terms of seismic hazard assessment. On the foreground is the industrial exploration potential of the region which defines a different layout of the above regions. The most demanding are seismic hazard assessments of the Norwegian and Beaufort seas where oil and gas fields are being explored, Barents Sea rich in oil and gas fields and oil- and gas -prone structures, Brooks Range where a large polymetallic deposit has been discovered. For many reasons including great fuel resources potential, and intensive industrial development of the offshore areas, and high level of seismic activity, the seismic zonation of the Laptev Sea and adjacent offshore areas seems to be the most demanding. At the same time, despite the high level of seismic activity, in our view, in the foreseeable future the seismic hazard assessment of the Arctic deep-sea parts will be mainly cognitive.

It should be kept in mind that assessing seismic hazard for any local industrial or residential site it is necessary to conduct seismic microzonation estimating the impact of topographic features, soil conditions and ground water level, etc. on the earthquake intensity. In unfavorable cases the above features may result in enhanced intensity by 2-3 MSK and form seismically hazardous areas within generally safe region.

Closing up the study it should be noted that the existing network of national and foreign telemetric stations in to a certain extent sufficient for overall monitoring of seismic regime in the Arctic Region in general, and collection of statistic data on known features is absolutely insufficient for demanding transition to a new level of investigations, namely more detailed studies of the most scientifically and industrially interesting and significant key units of seismic zones. In this connection, in our view, strong efforts should be committed to the task of creating a program for fairly short-term but large-scaled specialized field observations in certain areas of the Arctic using portable land and bottom stations. Evidently necessary is concentration of efforts of all nations and organizations interested in exploration and management of the Arctic Region.