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Historical background of seismological researches into the Arctic
1.1. Development of the permanent seismograph
observation network in the Arctic
It is evident that the notion "Arctic observation network" is fairly indefinite and requires a special comment. In this paper, in accordance with the adopted position of the Arctic border as mentioned above, by Arctic observation network we imply a group of stations located within the area confined by the North Polar Circle and the area located in its close vicinity (fraction of degree) southward. Referring to the specifications of the network one should bear in mind the contribution to the knowledge of the Arctic seismicity made by the global seismic observation network, and particularly, those fragments which may be called subarctic (north of 60° N). The latter should definitely include NORSAR and NORESS networks in Fennoscandia, Western in Alaska, Yellowknife in Canada, stations in Iceland, etc.
Seismological observation in the Arctic began with the installation of Abisko Station of Sweden (1906). This station as well as Scoresbysund in Greenland (1928) had been the only polar stations until 1950 (Fig.1) (Table 1) except for observations of 1911-1912 in Advent Bay, Svalbard (7 months) (Austegard,1974). Even with allowance of the stations Ivigtut (1929), Reykjavik (1909), Bergen (1904), Helsinki (1924), College (1935) located north of 60° N each station fell within a vast area of 5 million square kilometers. Such a sparse network provided completeness threshold only for very strong events of magnitudes 5.5-6, which could only give an initial understanding of the epicentral distribution.
Development of the network of Arctic stations
The first remarkable expansion of the Arctic observation network occurred between 1950 and 1960, which appear to be largely due to the preparation and holding of the International Geophysical Year. The big contribution made to this project by the Eastern Hemisphere nations is evident. With only three new stations in Arctic Canada (Resolute, 1950), and Greenland (Thule, Nord, 1957), seven stations were installed in Eurasian Arctic: on Svalbard (Isfjord, 1958), Norway (Tromso, 1960), Sweden (Kiruna, 1951), Finland (Sodankyla, 1956), and three Soviet stations (Apatity, Tiksi, 1956 and Kheis, 1957) (Fig.1). Frankly speaking, the Swedish Kiruna has replaced the Arctic veteran Abisko closed in 1951. Despite the fact that the number of Arctic stations increased over the decade more than five times, the
network is still rather sparse and irregular being more densely spaced off Scandinavia. The completeness threshold has diminished on average down to M=4.5-5, whereas in Fennoscandia even much weaker events with M=3-3.5 (The Arctic Ocean Region..,1990) were confidently recorded.
During 1961-1970, the installation of new Arctic stations was continued, however, they were still very few, and their recording capacity did not exceed the first years or even months. Thus in Greenland, among three new stations, two Ice Cap and Camp Century were in operations for 3 and 13 months, respectively (Fig.1). Bearing in mind that after the installation in 1962 of the Godhavn Station on the central western coast of the island the Thule Station was closed in 1966, the number of Greenland stations did not exceed 3 in 1970. The network of three stations spaced at 4 km deployed in 1961 on Jan Mayen Island.
A fairly essential expansion of the network took place during the decade in Arctic Canada. There appeared 4 stations, such as Alert and Mould Bay (1961), Coppermine (1963, closed in December 1969), and Inuvik (1969). The Barrow Station was installed in Alaska in 1969.
Similar quantitative changes of the network occurred in the Eastern Hemisphere. The stations Kevo (1969) and Kirkenes (1964-1969) were set up in Fennoscandia. The Kingsbay Station was installed on Svalbard in 1967. In the huge area of Northern Russia two stations, Norilsk and Iultin brought about in 1964. The Amderma Station installed in 1962 was in operation until 1964.
Therefore, by early 1970s there were 20 stations in the Arctic, that is one station per 1 million square kilometers on average. Essential deviations from this average value are apparent primarily due to the fact that the vast Central Arctic offshore area (6-7 million square kilometers) cannot be covered by a network of permanent stations whatsoever. The completeness threshold of earthquakes was generally M=4-4.5, and 3-3.5 in the Arctic and Fennoscandia, respectively (The Arctic Ocean Region..,1990).
During the next decade (1971-1980) the project on the expansion of the Arctic observation network was only carried out in North America, primarily, in Alaska (Table 1, Fig.1). Eight new stations were installed there, and are still in operation with Fort Yukon being a system of 4 recording points spaced at 100-150 km. In 1975-1978 the Barter islands System was also operated (4 recording points spaced at about 120 km). Observations from the Barrow Stations were terminated in 1975.
Three stations were installed in Northern Canada with two of them, Igloolik (1975) and Sachs Harbour (1980) located on Canadian Arctic Archipelago suffering lack of observations.
The total number of Greenland stations has been unchanged. Instead of the Nord Station operated until 1972 on the northwestern coast, the more southern Danmark Havn Station was started up in the same year, while Kap Tobin went on recording from nearly the same point as the Scoresbysund Station terminated in 1980.
The only change in the network of the Arctic stations in the Eastern Hemisphere occurred on Svalbard where the Isfjord Station was closed in 1976, and the Edge Oya Station was opened in 1977 near the felt earthquake dated January 18, 1976. The Arctic observation networks in Fennoscandia and Russia had remained intact.
It is clear that fairly negligible expansion of the network could not have heavily affected the general completeness threshold of Arctic earthquakes (M=4-4.5). Concurrently, in marginal zones of Canada and Alaska it dropped down to 3.5-3.7 (The Arctic Ocean Region..,1990).
During the subsequent period the number of Arctic stations substantially increased showing irregular distribution which proved the tendency of the previous years. Condensed sites were observed in Northern Alaska and Northern Canada, and off European Arctic. (Fig.1).
Over the period between 1981 and 1990, two stations were installed in Alaska: Bench (1985) and Barrow (after 8 year break), and three stations in Canada: Nicholson and Komakuk (1981), and Shingle Pt (1982). Among these station only Barrow was located north of the seventies parallel. In 1984 the Tuktoyaktuk Station of Canada was terminated. The observation network of Greenland remained intact.
As to he Eastern Hemisphere, in late 1980s, three stations were installed in the territory of Norway: Moi Rana (1986; in 1989 transformed into the 4-point system), Lofoten (1987), and Kautokeino (1989), as well as Norwegian station Hornsund (1984) on Svalbard.
In Russia, the stations Barentsburg (1982) and Pyramida (1984) on Svalbard, Polar Dawns and Polar Circle on the Kola Peninsula and renewal after a twenty-year break of the Amderma Station (1983) were organized through the efforts of the Kola RAS Affiliate.
Great and efficient efforts of the Yakutia RAS Affiliate resulted in deployment in 1985-1988 of a regional seismic network in northern Yakutia comprising 8 recording points. Though not in a position to meet all requirements of the standard station it substantially increased the recording efficiency of local earthquakes near the continental exposure of the Mid-Arctic Belt and brought the completeness threshold to the level of Energy Class K=7-8.
Therefore, in late 1980s, 43 stations were operated in the Arctic Region of which 22 and 21 were located in the Western and Eastern hemispheres, respectively. The breakdown by region was as follows: Greenland - 3, Jan Mayen -1, Northern Canada - 9, Northern Alaska - 9, Fennoscandia (outside Russia) - 7, Sweden -1, Norway -4, Finland - 2, Svalbard -5 (2 Russian and 3 Norwegian), Russia - 7 (other than on Svalbard). On average, a station fell within an area of 0.5 square kilometers, which ensured completeness threshold of earthquakes of M not lower than 3.9-4. This fairly high value seems to be due to the situation in the Central Arctic Basin with the poorest coverage, and sorry to say, on northern Russian margin where the density of coverage is twice as low as in the Arctic on average. At the same time, in Northern Canada and Northern Alaska, the total area of which is about 4 million square kilometers including immediately adjacent offshore areas. The above parameters are estimated at 0.2 million square kilometers and min. M =3-3.5, respectively (Drysdale and Horner,1986). The situation in Fennoscandia is much better with one station falling within an area of 700 square kilometers and completeness threshold at M=2-2.5 (Panasenko,1991).
Unfortunately, in 1990s the conditions of the Russian Arctic observation network dramatically deteriorated. Due to lack of financing the northern Russian station Kheis on Franz Joseph Land, Piramida on Svalbard and some stations in the Tiksi¸s area were terminated.
In general, it should be admitted that the existing network of permanent national and foreign Arctic and Subarctic stations is basically sufficient for overall monitoring of the seismic regime in the Arctic Region and collecting statistic data on the features which have been already known. However, it is absolutely insufficient for the promotion to the qualitatively new level of detailed investigations of key fragments of seismically active zones which are interesting in terms of research and application. Taking into account the fact that installation of even a single station, especially in Far North would require considerable financial support, one can hardly expect essential upgrade of the Arctic seismic network in the forthcoming years, and for Russia, decades. That is why the arrangement in pre-determined areas of large-scale but shirt-term seismological observations using portable onland and probably, bottom stations, are particularly demanding. It the past years similar studies were not infrequent including those using bottom seismographs (Chan and Mitchell,1985;Kovachev and others,1994; Panasenko and others,1987; Reid and Falconer,1982 etc.). In this country, seismological observations were carried out most regularly and for a long while solely by Sevmorgeologia Research Association of Saint Petersburg.
1.2. Sevmorgeologia¸s field observations
Seismological research of Sevmorgeologia Association for Research and Production (referred to as Research Institute for Arctic Geology until 1972) began in 1968 seeking to obtain auxiliary information necessary for compilation of Franz Joseph Land seismic zoning chart and seismic microzoning of its westernmost island Alexandra Land. On July 28, 1968, at Nagurskaya point (80.8° N and 46.8° E) a seismic station Arkticheskaya was installed. It was equipped with oscillographs, three-component seismograph array VEGIK (from July 1969, USF-3M) and galvanic sensors GB-IV to keep recording for 2 years. The station had a table-shaped frequency within the range between 0.1 and 1.2 s. A rather high level of microseisms was recorded mainly deriving from oceanic rolling and ice drifting (especially in summer ), industrial interference and permanently operating engines at the Polar Station (increasing in winter after the upper ground layer has frozen) and glacial motions recurring almost every day at maximum distance of 15-20 km from the station. As a result, the maximum increase of the record did not exceed 3-4x104.
Over the entire observation period only few weak earthquakes were recorded northwest of the station near the subsea Franz Victoria Trough where two strong events had been reported from its northern part. The above weak earthquakes were recorded by the Kheis Station located 200 km eastward. On September 2, 1969 both stations provided first sufficiently confident record of an earthquake east of the Archipelago near the subsea St.Anna Trough. In the subsequent years 1 to 2 events per annum were recorded, and a series of 6 pulses with K intensity of 9 to 10 was recorded in September -November, 1983 (Kochetov and Lazareva,1986).
Apart from the data on the local seismicity the deep upper mantle crustal structure was estimated near the stations Kheis and Arkticheskaya through the method of converted wave data based on the reliable record of distant events. It is noteworthy that due to its geological orientation the NIIGA-Sevmorgeologia institution has been setting forth this goal as primary both for substantiation of seismic observation and for selection of study areas and recording array deployment.
After a one-year break seismological records were resumed in the summer field seasons of 1972-1976 on New Siberian Islands as part of comprehensive study aimed at creating geoscientific marks required further for further research into the Arctic.
During the season of 1972, observations were carried out at one point near the Temp Polar Station , and during subsequent years recording was concurrently effected at two points one of the station being shifted in the season of 1976. Various premises were used for observation purposes, such as a specially equipped tent, empty houses of the effective or abandoned polar stations, hunter log-huts or hunter household. In total, 8 observation points had been operated over the above period (Fig.2, Table 2).
The equipment was the same as at the Arkticheskaya Station. During April to June before marine ice opening and permafrost melting observations were carried out at maximum increase (9-10x104) and practically absent interference background while during the summer months the increase was two-to three times lower.
Locations and data of seismological observations on New Siberian Islands
During the observation period totaling 10 months some 120 earthquakes were recorded near New Siberian Islands previously regarded as non-seismic. Epicenters of 32 earthquakes were determined. It has been established that the epicenters fairly confidently trace the contacts of tectonic block consolidated in different times. The induced or passive nature of seismic activity in this region appears to be attributed a discharge in the weakened lithoshperic sites of tectonic stresses generated in the zone of the Central Laptev Sea rifting. The deep structure of the archipelago and adjacent areas has been estimated from the evidence on distant earthquakes (Avetisov,1983a).
During the period of 1974-1976 seismic observations were conducted on the meridional profile in Western Yakutia ranging from the town of Mirnyi to the Laptev Sea through the efforts of almost the same leading specialists using the same instruments. The main objective of the investigations was to collect data on the deep lithospheric structure further used for elaboration of forecasting criteria for hostrock diamond deposits. Three northernmost stations Udzha, Khalgannakh, and Tchochurdakh (Table 3) recorded earthquakes off the south coast of the Laptev Sea and the mouth of Khatanga Bay.
Locations and data of observations in the north Yakutia
In 1976 and 1977 Soviet mines Barentsburg and Pyramida were investigated, respectively. A very heavy ambient seismic noise coming from the open sea prevented from reliable recording of distant earthquakes, at the time local events were recorded nearly every day.
In 1978 short-term observations were carried out on the southern island of the Novaya Zemlya Archipelago. Unfortunately, for various organizational reasons the observational materials of Svalbard and Novaya Zemlya have not been finally processed.
In 1979 the team was supplied with the most advanced domestic instruments provided with magnetic recording "Tcherepakha" intended for field seismological operations. During field seasons of 1979-1983 seismological and seismic observations (Deep seismic sounding) were conducted in order to gain a better knowledge of the deep earth crustal structure and upper mantle of the Norilsk mining area (Fig.3). Over the entire period of observations in the said region, no local or proximal earthquakes had been recorded.
In April to May, 1984, observations were carried out on the Arctic Cape - the northernmost point of Severnaya Zemlya Archipelago. A few earthquakes were recorded on the Mid-Arctic Ridge and a considerable abundance of seismic events was recorded at a distance of a few kilometers and dozens of kilometers , most of which are rather due to the processes occurring in the nearest glacial cupola..
During the field seasons of 1985-1988 seismological team supplied with 12 recorders "Tcherepakha" conducted landing profile and areal observations in the Lena delta and on the coast of Buor-Khaya Bay (Fig.4) using helicopters MI-8 having their base at the airport of Tiksi. Over the entire period of observations totaling more than 9 months, some 140 local and proximal earthquakes were recorded being located 121 of which. Owing to the abundance of recorders, unbiased Vadati and Treskov (Treskov,1964) methods were successfully applied for hypocenter location solution, which is especially important for the areas of unknown travel-time curve. Based on the acquired information some new assumptions have been made regarding geodynamics of this region, which to a certain extent have changed the previous views. In addition, detailed data on the epicentral distribution are of unconditional importance and should be used for the assessment of seismic hazard of this fast growing and promising region.
Closing up the brief historical description of the field seismological investigations conducted by NIIGA-PGO Sevmorgeologia in the Arctic we are sorry to confess that despite a wide scope of unsolved problems which are fairly interesting and important in terms of science and application, the above investigations have been completely terminated after 1988 and do not seem to be resumed in the foreseeable future.
1.3. A review of basic summary publications
In general, the first publications regarding the seismicity of the Arctic and Subarctic covered single strongest events where instrumental evidence were available. In the summary by the classic seismologist John Miln reported at the British Seismological Association in August, 1907, among 474 epicenters only three were located within the Arctic. They all were close to Jan Mayen Island. Papers by B.B.Golitsin (1911) and I.I.Vilip (1913) were devoted to two earthquakes from Iceland of January 22, 1910 and May 6, 1912, respectively.
In 1914 the Beitrage Geophysik Journal (Hodgson,1930) published the results of operation during 1911-1912 of seismological station on Svalbard which over 7 months of observations had detected 6 essential earthquakes first proving a higher seismicity of the archipelago.
In 1922 E.Tams reported on 26 earthquakes of which 21 occurred in the Arctic during 1904-1921. The map included in the paper shows a linear distribution of the epicenters to the north of the North Atlantic across Iceland, Jan Mayen, west of Svalbard, and farther north of Franz Joseph Land and Severnaya Zemlya.
The paper by E.Tams of 1927 also published in the German language presented the locations of 5 earthquakes off the Laptev Sea over the period of 1909-1925. The epicenters were shown on the tectonic map of Siberia by V.A. Obruchev, however, the author did not raise the question of their tectonic nature.
In his paper E.Hodgson (1930), apart from the information on the aforesaid John Miln¸s publication, summarized 36 Arctic earthquakes over the period between 1911 and 1927.The summary was made on the basis of catalogues produced by the Seismological observatories in Ottawa and Oxford. These data show that along the seismically active region of Iceland-Jan Mayen another two existed in the mouth of Lena river where two pulses were recorded on May 30, 1923, and Mackenzie River delta where one pulse was detected on November 16, 1920.
A sound contribution to the understanding of the seismicity in the Arctic was the paper by N.V.Raiko and N.A.Linden (1935) which apart from detailed information on the strongest earthquake in the Arctic of November 20, 1933 in the Baffin Bay, presented a catalogue and map containing 88 Arctic events over the period of 1910-1934, both compiled from national and foreign bulletins. These provide a reliable evidence on the existence of seismic belt extending from Iceland through Jan Mayen Island, western rim of Svalbard and Franz Joseph Land to the north coast of the Asian continent. The picture of the earthquake focal distribution in the Arctic and their number turned out to be unexpected for the authors and caused an attempt made by D.I.Mushketov (1935) to explain it for the first time accounting them to intensive contemporary uplifting and subsiding.
Another attempt to specify and summarize the data on the Arctic seismicity was made by D.I.Mushketov in his report presented to the session of the Arctic Commission for the preparation to the International Geophysical Year on March 19, 1937. The catalogue made by D.I.Mushketov and associates contained 178 earthquakes (with the vast majority beyond the Polar Circle) and was as of the date the most complete summary of the Arctic seismicity. Despite of the above, the author admitted premature to hypothetically and theoretically expatiate on the tectonic nature of the higher seismicity bearing the responsibility of collecting factual material as complete as possible and just indicating the correlation between the epicenters and the zone of young alpine motions. The author emphasized the importance of looking at the Arctic seismicity and necessary arrangement of seismic stations in the Arctic, particularly on Franz Joseph Land, the Tcheluskin Cape and Matochkin Shar, as well as in northern Norway, Finland and Greenland.
Among publications on the Arctic seismicity the paper by K.Emery (1949) should be definitely referred to. This paper covers a large scope of problems with regard to bathymetry of the Arctic Ocean Basin, the distribution and composition of sediments and the source of discharge, topography of the adjacent land and peculiarities of glaciers, earthquake epicentral locations and many other problems. Noteworthy is that the author based on the collected material and the fact that the epicentral belt extends from the subsea Mid-Atlantic Ridge through poorly known Arctic Basin to the mount of the Lena river where also go through a mountainous area, suggests the topographic complexity of the basinal part crossed by the belt, thus predicting the existence of the Mid-Arctic Ridge.
Summaries of the Arctic earthquakes are given in publications by B.Gutenberg and K.Richter with regard to the Earth¸s seismicity (1948; 1954), the latter presenting a catalogue and a map indicating magnitudes and precision of the location.
As it has been mentioned above, the crucial point in the history of seismological studies of the Arctic was International Geophysical Year (1957-1959), for the purpose of which several seismic stations were installed in the Arctic. The IGY program is associated with the advent of fairly remarkable and still topical publications by N.A. Linden (1959; 1963) and A.P. Lazareva (1963; 1965).
The paper (Linden,1959) based on the deliberate selection and revision of the factual material presents the most detailed and reliable as of the above period a summary and map of the Arctic earthquakes over the period of 1908-1958 which have been for a few decades a factual basis for many other papers dedicated to the Arctic seismicity. In the next paper N.A. Linden (1965) presents a detailed historical review of national and foreign scientific literature on the point and catalogues of string and weak Arctic earthquakes recorded over the period of the IGY (1957-1959).
The paper by A.P. Lazareva (1963) presents a first attempt to compute Arctic earthquake hypocentral depths. Prior to this noteworthy are only the assessments made by D.I.Mushketov from the travel-time curve for 13 earthquakes from his catalogue providing approximate values of 70-100 km. From seismograms of the Pulkovo Station based on the sP phase picking the focal depths of 102 from 246 earthquakes presented in N.A.Linden¸s catalogues are determined, however only 29 determinations supported by data obtained from other stations (Moscow, Tiksi, Kheis) have been published, which is an evidence of the highest commitment of the research worker. This information on the Arctic earthquake depths has been only available for a long time.
Among publications summing up the IGY the paper (Hodgson,1965) by an international seismological team was published as late as in 1965 and contained the most valuable information on the location and recording parameters of all Arctic and Subarctic stations. The catalogues presented in the paper on the Arctic earthquakes although citing some events missing from other publications are in general less comprehensive than those by N.A.Linden.
A catalogue and a map of 281 Arctic earthquakes over the period of January 1955 to March 1964 was presented in the paper by L.Sykes (1965) published in the same year basing on the computer determinations and re-determinations of epicentral locations. Based on a fairly large amount of seismological information, the author gave a detailed description of the conduct of the major Arctic Seismic Belt, while the discovery by that time of the world rifting system , particularly the Mid-Arctic Gakkel Ridge enabled him confidently to correlate the higher seismicity with the oceanic rifting processes.
A qualitatively new stage of seismic investigations of the Arctic originated from the papers by A.P.Lazareva and L.A.Misharina (1965) and L.A.Misharina (1967) as the first studies of the focal mechanism solutions in the Arctic earthquakes.
In subsequent years the Arctic seismicity maps were published by a number of scientists (Tarr,1978; Wetmiller and Forsyth,1978, etc.). In our view, the paper by Avetisov and Golubkov (1971) is worth of special notice presenting tectono-seismic zoning of the Eurasian subbasin off the Arctic ocean and adjacent areas with assessment of seismic hazards for eventual residential and industrial buildings. Similar to the above were the following papers by G.D.Panasenko et al., (1983), B.A.Assinovskaya and S.L.Soloviev (1993), B.A. Assinovskaya (1990;1994) about the Barents Sea and by I.P.Kuzin (1989) about the Laptev Sea.
Beginning in 1960-1970s the number of publications addressing seismicity of different Arctic areas grew abruptly, especially with regard to the Norwegian-Greenland Basin provided with the best observation coverage. Among investigations of that time definitely noteworthy are those by P.Wetmiller and D.Forsyth (1978) and the doctoral theses by A.P.Lazareva (1977).
The former summarizes data on more than 3400 Arctic earthquakes over the period of 1908-1975. The authors present maps of epicentral locations for the entire Arctic and its separate areas in conjunction with bathymetry data accompanied with the analysis of seismicity and its tectonic nature.
A.P.Lazareva presents a catalogue of Arctic earthquakes over the period of 1964-1974 preceded by the catalogue by L.R.Sykes, and a fairly detailed catalogue of the Chukchi Peninsula earthquakes which constituted the basis for seismic zoning of this intensely explored and poorly known territory.
A great event providing better opportunities for scientists has become the arrangement of regular publication of Arctic earthquake catalogues where the most notable are annuals "Earthquakes in the USSR", "Canadian Earthquakes", publications by G.D.Panasenko "Earthquakes in Fennoscandia" and of course, catalogues and bulletins produced by the International Seismological Center.
The latest basic work regarding geoscientific research into the Arctic region, including its seismicity was a magnificent edition of the team paper by American, Canadian and Scandinavian scientists "The Arctic Ocean Region..." (1990).