The seismicity of the norwegian and Greenland Seas and adjacent continental shelf areas

Tectonophysics, 26 (1975) 55-70 @ Elsevier Scientific Publishing Company,

Amsterdam

- Printed

in The Netherlands

THE SEISMICITY OF THE NORWEGIAN AND GREENLAND SEAS AND ADJACENT CONTINENTAL SHELF AREAS

E.S. HUSEBYEl, 1 NTNF/NO 2 Department

(Submitted

H. GJ@YSTDALl,

RSAR, Kjeller (Norway) of Geology, University

H. BUNGUM’

and 0. ELDHOLM2

of Oslo, Oslo (Norway)

May 6, 1974; revised version

accepted

September

6, 1974)

ABSTRACT Husebye, E.S., Gjeystdal, H., Bungum, H. and Eldholm, O., 1975. The seismicity of the Norwegian and Greenland Seas and adjacent continental shelf areas. Tectonophysics, 26: 55-70. The seismic activity of the Norwegian and Greenland Seas and adjacent areas has been examined in view of the tectonic evolution of the North Atlantic. The 529 earthquakes used covered the period 1955-1972, and for fifteen of these events fault-plane solutions were available. An analysis was made of the location precision which turned out to be better than 20 km in most cases. Expectedly, little new evidence was obtained at the midoceanic ridges and major fracture zones, with possible exceptions of the Knipovich Ridge showing a well-defined seismicity belt supporting the idea of an active spreading ridge, and the Spitsbergen Fracture Zone, which seems to be a system of en-&helon faults. Most interesting is a weak linear event pattern in the Lofoten Basin, possibly giving evidence of unknown structures parallel to the Greenland and Senja Fracture Zones, although sediment loading also may be important. Earthquakes along the shelf edge off Norway are located at or near isostatic gravity belts which may act as hinge lines for the marginal subsidence, thus implying stress release caused by differential subsidence of the continental crust. Part of the seismicity of eastern Greenland and western Norway appears to be related to zones of weakness of :Jre-Cenozoic age. The seismic activity along the edges of the Norwegian Channel is very limited.

INTRODUCTION

It is well known that earthquake occurrence is closely associated with specific geological features, such as faults, island arcs and mid-oceanic ridges. An important objective of seismicity studies is therefore to outline typical tectonic patterns, provided that the epicenter locations are known with satisfactory precision. For example, Isacks et al. (1968) demonstrated how seismicity gave excellent support to the new global tectonics. Hodgson et al. (1965) and Sykes (1965) have investigated the seismic activity in the Arctic. The latter study was based on well-recorded events from the

56

period Jan. 1955-Mar. 1964, and an accuracy of about 10 km in the epicenter solutions was indicated when the number of recording stations was ten or more. He pointed out that the main seismic activity is found along the extension of the Mid-Atlantic Ridge. North of Iceland the earthquakes are confined to a relatively narrow region between the continental margin off Greenland and the Jan Mayen Ridge. Near Jan Mayen the seismicity pattern is shifted to the east which was interpreted as reflecting a major east-west trending fracture zone. North of the fracture zone the seismicity belt mainly runs along the mid-oceanic ridge (Mohn and Knipovich Ridges) and follows the Spitsbergen Fracture Zone into the Arctic Ocean. The more extensive earthquake data now available for the Norwegian-Greenland Sea and adjacent areas clearly indicate that a modest, but significant, seismic activity also takes place outside the dominant mid-oceanic ridge system. The aim of this study is a re-examination of the seismicity in the Norwegian and Greenland Seas, emphasizing the continental margins off Norway, Greenland and Svalbard (Fig. 1). Such an analysis is believed to give new and important information in addition to that of geological and geophysical surveys (e.g., Johnson and Heezen, 1967; Harland, 1969; Talwani and Eldholm, 1974) on past and present tectonic movements in the areas defined above. DATA AND ANALYSIS

Our data base consists of events reported by Sykes (1965) from the time interval Jan. 55-Dec. 60 and NOAA (National Oceanic and Atmospheric Administration, U.S.A.) reported events from Jan. 61-Dec. 72. In addition, a collection of small events localized by I. Noponen (personal communication, 1974) was used to supply our data on the continental shelf off Norway. The epicenters of the latter earthquakes were primarily based on recordings from Scandinavian stations. A list of all events used in the analysis is available upon request from the authors. The frequency-magnitude relationship for these earthquakes is presented in Fig. 2. A limitation in all seismicity studies is the accuracy in the estimated epicenter locations, which depends on the number of stations used in the computation. However, good precision is not obtained unless the stations have a reasonable distribution in azimuth and distance from the source. In order to examine the location accuracy when the number of recording stations decreases, we performed the following experiment. The starting point was two relatively large earthquakes on the mid-oceanic ridge which were recorded by more than hundred stations. For each of these two master events, a number of smaller events in the same area were selected, having a different number of reporting stations. The relevant station, travel time and epicenter data were taken from the International Seismological Centre (ISC) bulletins. The stations were then ranked according to the number of reported events, and such a ranking is assumed to reflect the stations’ relative ability to detect

57

300E \

40°E b

O”N

,

GREENLANa

65%

Fig. 1. Epicenter distribution (1955-1972) in the Norwegian-Greenland Sea and adjacent areas. Epicenters and selected depth contours (in meters) are shown together with main structural features (Talwani and Eldholm, 1974). Earthquakes for which focal mechanisms have been published are numbered from 1 to 15 (Table II).

58

fl= 8.07 io.13 B=-1.26tO.04 50X = 4.13 90% = 4.53

MFlGNITLJDE Fig. 2. Incremental and cumulative frequency-magnitude distributions based on all events used in this study for which magnitude has been reported. The straight line is a least-squares fit through the data between the vertical bars. A and B are coefficients in the cumulative recurrence relationship log N = A + B . mb where mb is magnitude and N is the number of events with magnitude equal to or larger than ml,. The 50% and 90% detection thresholds are indicated.

events from the above region. The next step was to see how the epicenter coordinates of the master events changed when the number of stations used in the location computation was successively decreased. We started with the 23 highest-ranked stations (11 in Scandinavia, 8 in northwest America and 4 in central Europe), and successively masked three or four in each run, ending up with an epicenter location based on merely three arrival times. The actual elimination of stations was performed according to their relative ranking in the way that the stations with the lowest reporting frequency were deleted first. The results are shown in Table I, where the ISC, NOAA and BCIS solutions are also given. For master event I there is little scatter in the epicenter locations when the number of stations decreases from 23 to 4. For master event II there is a shift in the location when the number of stations becomes less than 10. In the latter case the azimuth distribution is poor due to a predominance of Scandinavian stations. Our conclusion is that the epicenter accuracy is not extremely dependent on the number of stations used, provided that the signals are well-recorded so that arrival times are not very uncertain. At least we could say that in the regions considered here more than ten stations will give a precision probably around 20 km, representing the lower limit for the size of the tectonic features which could possibly be investigated.

59

TABLE I Epicenter location estimate using a gradually reduced number of stations, where the stations with the poorest detectability are removed first. The ISC, NOAA and BCIS (Bureau Central International de Seismologic) solutions are also given Master event I Nov. 21,1967

Master event II Nov. 27, 1966 epicenter location

.~

lat. (N)

long. (E)

ESC NOAA BCIS

72.66 72.70 72.80

8.14 8.50 8.50

23 19 15 11 8 4

72.70 72.70 72.70 72.60 72.70 72.70

8.60 8.50 8.60 8.20 8.40 8.50

sta. sta. sta. sta. sta. sta.

epicenter location lat. (N)

.- ~ long. (E)

ISC NOAA BCIS

78.50 78.50 78.50

5.80 5.80 4.50

21 17 14 10 7 3

78.60 78.60 78.60 78.70 79.00 79.30

6.90 7.10 7.00 7.20 9.10 9.20

sta. sta. sta. sta. sta. sta.

Focal-mechanism studies in the areas covered in this paper are difficult because there are few large events and because the coverage of seismic stations is often poor and irregular. However, solutions have been published by Conant (1972), Horsfield and Maton (1970), Lazareva and Misharina (1965), Stauder and Bollinger (1966), Stefansson (1966), Sykes (1967), Sykes and Sbar (1973), Ward (1971) and Zobin (1972). Out of the 529 earthquakes used in this study, these authors have published foci-mechanism solutions for fifteen earthquakes, one of them covered by several authors. These events are presented in Table II, where also the different focal-mechanism solutions are given (see also Fig. 1). All solutions have been derived from information about the first motion of the initial arrival of P-waves. In most cases only P has been used, and then the fault plane can be associated with any of the nodal planes thus obtained, and more information (e.g. seismic, bathymetric, geologic) is needed in order to solve the ambiguity in the fault-plane solution. For the events in Table II, this has been done only in four cases (nos. 1, 2, 13 and 14). Comments on the different focal solutions are incorporated in later sections where the seismicity of the different regions is discussed.

27 Ju167

28 Mar 63

5 May 69 18 Jun 58 28 Ott 60 18 Sep 70 23 Mar 71 9 Sep 60 31 May 71 29 Jan 59 lMar59 26 Ott 70 18 Ott 67

23 Nov 67 26 Nov 71 ~___~

2

3 4 5 6 7 8 9 10 11 12 13

14 15

Date

80.2N 79.4N

66.8N 68.8N 71.3N 71.2N 71.ON 71.8N 72.2N 70.9N 74.8N 79.8N 79.8N

66.1N

64.ON

Lat.

l.OE 17.8W

18.2W 16.5W 9.2w 7.7w 7 .ow 1.2w 1.2E 7.3E 8.1E 2.7E 2.4E

20.1W

20.7W

5.8 5.2

5.2 5.2 5.9 5.1 6.0 4.6 5.5 5.8 5.4 5.6 5.7

5.6

5.0

Long. mu ____

Epicentral solution (NOAA)

1

No.

Horsfield and Maton (1970) Sykes and Sbar (1973)

Stauder and Bollinger (1966) Stefansson (1966) LP(P) Stefansson (1966) SP(P) Stefansson (1966) LP( S) Conant (1972) Lazareva and Misharina (1965) Lazareva and Misharina ( 196 5) Zobin (1972) Conant (1972) Lazareva and Misharina (1965) Conant (1972) Lazareva and Misharina (1965) Lazareva and Misharina (1965) Conant (1972) Horsfield and Maton (1970)

Sykes (1967)

Focal solution determined by

strike-slip/ transform dextral strikeslipltransform strike-slip strike-slip strike-slip strike-slip strike-slip strike-slip strike-slip strike-slip strike-slip strike-slip normal strike-slip strike-slip strike-slip dextral strikeslipltransform strike-slip normal

Type of motion/faulting __-_____

103 107 109 104 112 100 100 155 120 120 52 175 165 138 131

70 79 76 77 72 80 80 75 72 90 54 80 70 86 86

84

17 18 18 12 25 15 10 65 26 30 42 85 70 17 43

40

128

106

78

dip

Plane 2

17

az.

Plane 1

Epicenter and focal-mechanism solutions for fifteen events, with reference to where the solutions are obtained. Where given, the type of motion/faulting and the azimuths and dips of the two planes are also listed

TABLE II

74

77 88 84 81 82 75 80 85 74 90 64 70 70 76 71

86

g

61 EVOLUTION

OF THE NORWEGIAN-GREENLAND

SEA

A new understanding of the tectonic movements in the Norwegian-Greenland Sea was introduced by the theory of sea-floor spreading and the evolution of this area has been discussed by several authors during the last years. Here we want to give a short review of the main features of this complicated process in order to establish a basis for the following discussions. Several authors, among them Johnson and Heezen (1967), Avery et al. (1968), Johnson et al. (1971), Harland (1969), Harlan1 and Gayer (1972) have discussed the development of this area. On the Sasis of magnetic anomalies and major structural features Talwani and Eldholm (1974) recently proposed a model for the tectonic evolution of the Norwegian Sea in which the separation of Greenland and Norway started 60-63 m.y.b.p. and consisted of two major phases. During the first phase the Norwegian Sea opened along the Mohns Ridge and a now extinct axis in the Norwegian Basin. However, the motion in the incipient Greenland Sea was southeast-northwest with Greenland sliding past Svalbard. In the second phase the spreading direction changed to more east-west and the opening started also in the Greenland Sea. They associate this change with Greenland becoming attached to the North American plate. Looking at the present location of the active ridges (Fig. l), it is readily concluded that the ocean floor has not spread symmetrically from the ridge axis, especially between the Jan Mayen Fracture Zone and Iceland where the active zone is situated close to the Greenland margin. An important aspect of ocean-floor spreading is the fracture zones of which the most prominent ones are the Jan Mayen, Greenland-Senja and Spitsbergen Fracture Zones. As may be seen from Fig. 1, only those parts of the fracture zones intermediate between “ridge ends” are now seismically active (Sykes, 1965). McKenzie and Parker (1967) have proposed that the fracture zones represent “flow lines” giving the direction of motion of the ocean floor at the time of generation. THE SEISMICITY

PATTERNS

OF OCEANIC

RIDGES

AND FRACTURE

ZONES

The main seismic activity is confined to a narrow belt along the mid-oceanic ridge and associated fracture zones displacing the ridge (Fig. 1). It has been shown that earthquakes occurring at the ridge are mostly of the normal fault type (Sykes, 1967; Weidner and Aki, 1973), while the transform portions of the fracture zones are associated with strike-slip motions (Wilson, 1965). Fault-plane solutions of focal-mechanism studies on earthquakes in this region are given in Table II (Event nos. l-9). All these solutions, except one, are of the strike-slip type which indicates a predominance of transform faulting. Especially should be mentioned events 5 and 7 with fault planes striking approximately in the direction of the Jan Mayen Fracture Zone which is well-defined bathymetrically. We also note that the seismicity here

62

is restricted to the part of the zone lying between the displaced ridge portions. The only normal fault event (no. 9) is located on the ridge between Jan Mayen and Greenland Fracture Zone (Mohns Ridge). This kind of solution is directly connected to the spreading mechanism at the crest of the ridge. North of the Greenland Fracture Zone the main seismicity belt turns abruptly northwards and continues along the Knipovich Ridge and Spitsbergen Fracture Zone into the Arctic Ocean. The asymmetric position of the Knipovich Ridge has raised the question if there is an offset in the ridge axis where the Greenland-Senja Fracture Zone intersects the ridge. The earthquake data, however, suggest a continuous belt (see also Heezen and Ewing, 1956; Sykes, 1965). This is also consistent with the continuity of free-air gravity minimum over the rift valley (Talwani and Gronlie, in press) and studies of the seismic reflection profiles data (Eldholm and Wind&h, 1974). Obviously, fracture zones of minor offsets may exist and this is partially supported by the only focal-plane solution available at the Knipovich Ridge, which is of the strike-slip type (event no. 11, Table II). The nature of this ridge has been debated in the literature (Johnson, 1972). Our findings show that the seismicity belt follows the ridge, thus supporting the idea that it is, at present, an actively spreading ridge. It may be argued that the seismicity belt is slightly wider here than elsewhere on the mid-oceanic ridge. It is difficult to resolve this problem, but several factors may contribute. We have already mentioned the possibility of minor offsets along the ridge axis. Furthermore, we note that the Knipovich Ridge exhibits a very prominent and relatively wide rift valley when compared with other portions of the mid-oceanic ridge system (Eldholm and Windisch, 1974). This could possibly be interpreted as a wider zone of injection than normal. Finally, Talwani and Eldholm (1974) have indicated a complex continental margin off Svalbard due to the fact that the margin first developed as a sheared margin and later on became a rifted margin. The proximity of the ridge axis am1 the margin makes it difficult, therefore, to distinguish between events from possible zones of weakness along the continental slope (De Geer dextral fault; Harland, 1969) and those associated with the ridge axis. The focal-plane solutions from the area of the Spitsbergen Fracture Zone all show strike-slip mechanisms consistent with the idea that the fracture zone offsets the spreading ridge axis into the Arctic Ocean. The width of the seismicity belt may well indicate a series of en-echelon faults as suggested by Johnson and Eckhoff (1966). It should also be noticed that our seismicity data favor the existence of two fracture zones just north of the Spitsbergen Fracture Zone system (see also Vogt et al., 1970). These fracture zones displace the ridge axis left-laterally into the Nansen Ridge in the Arctic Ocean (Fig. 1). THE SEISMICITY PATTERNS OF EASTERN NORWAY WITH ADJACENT SHELF AREAS

The seismicity

GREENLAND, SVALBARD, AND OCEAN BASINS

of the coastal areas of eastern Greenland

WESTERN

is very modest.

63 The

few earthquakes occurring are located in two zones, namely, west of the Iceland-Jan Mayen Ridge and at the northeastern comer of Greenland. Noteworthy is that the bulletins from the seismological stations Scoresbysund (70.48N, 21.95W) and Nord (81.60N, 16.41W), published by Geodetic Institute, Copenhagen, indicate a significantly higher seismic activity in the two areas than is apparent from the NOAA earthquake files. For example, during 1962 these stations reported respectively 61 and 167 small, local events, but no epicenter information was available due to lack of a sufficiently dense local network. The tectonic forces instrumental in the seismic activity may be related to vertical motions due to changes in the Greenland icecap. This statement is based on the normal fault solution for event no. 15 in Table II (Sykes and Sbar, 1973). The actual strain release causing the earthquakes would then take place along zones of weakness in the crust which in this case possibly may have originated from the extensive folding in the coastal areas of eastern Greenland during the Caledonian orogeny (Harland and Gayer, 1972). The earthquakes located in the Svalbard area together with major structural features are shown in Fig. 3. The fault lines originate from the Late Caledonian

81 -

00 -

79 -

76 -

. -

MAX)R FAULT

_

FAULT

LINES

& FOLDING

.

EARIHQUAKES

t

VOLCANIC

. . BELT

\ I

ACTIVITY

I

Fig. 3. Epicenter distribution (1955-1972) in the Svalbard area. Epicenters and selected depth contours (in meters) are shown together with local structural features.

64

and Devonian times and in a small area there are volcanoes and hot springs of Quaternary age. The fault and folding belt in the western part of Svalbard (hatched area in Fig. 3) reflects tectonic motions through Late Devonian to Tertiary times. In particular, the faults here are approximately parallel to the Spitsbergen Fracture Zone, and thus could be related to the Tertiary separation between Svalbard and Greenland. The tectonic history of this area is complex, and has been discussed in numerous papers, notably by Birkenmayer (1972a, b), Harland (1969), Harland and Gayer (1972), McWhae (1953) and Orvin (1940). It may be argued that a slight correlation exists between the outlined tectonic fault (weakness zones) pattern and some of the earthquakes, thus indicating an almost NNW-SSE alignment. The data may also be interpreted as exhibiting an apparent NE-SW trending zone of earthquakes from the southern part of Svalbard across the continental margin and Knipovich Ridge towards the Greenland Fracture Zone (Fig. 1). We do not know the significance of this trend which, if it is real, may point out an incipient change in structural pattern. Furthermore, we note that the above trend coincides with the directions of the principal stress components on Svalbard as measured by Hast (1972). A most remarkable feature is the seismic activity in the Lofoten Basin, that is, the part of the Norwegian Sea lying north of the Jan Mayen Fracture Zone and east of Mohns Ridge. Talwani and Eldholm (1971) discussed the possibility of a counterpart to the Greenland Fracture Zone and they associated a prominent gravity high along the lower slope off the Barents Sea with such a feature (Senja Fracture Zone). On the other hand, Eldholm and Windisch (1974) could not observe any expression of the fracture zone in the profiles data but pointed out that the Barents Sea has been a major source of sediment with exceptionally thick deposits on the lower slope and in the northeastern part of the Lofoten Basin. The easternmost events appear to have a trend similar to that of the Senja Fracture Zone, whereas the seismic activity in the Lofoten Basin proper appears to trend in a NW-SE direction, which is approximately parallel to the flow lines as defined by the Greenland and Senja Fracture Zones. Also these earthquakes cannot have connection with old zones of weakness as suggested for some of the events on the continental crust because they are all located within the recently generated oceanic crust of the Norwegian Sea. One explanation could be that the oceanic crust in the Lofoten Basin is subject to a much greater amount of loading than elsewhere due to the thick sediments and that the crust is too weak to withstand it without cracking. The reason could also be a compressional stress field normally existing inside tectonic plates, as shown by Sykes and Sbar (1973). They also show that such intraplate earthquakes are usually of the thrust-fault type, due to the maximum horizontal stress. Interesting here is the focal solution of event 10 in Table II, showing a strike-slip movement in a nearly vertical plane with one nodal plane striking approximately in the same direction as the Senja Fracture Zone. This could be an indication that a hidden structure may in fact exist and would in that

65

case be located along the weak lineament of seismicity recognized above (Fig. 1). We would like to emphasize the great importance of additional focal-plane solutions in this interesting region. The dominant structural features on the continental margin and coastal areas off Norway and the NOAA reported earthquakes are shown in detail in Fig. 4. The known seismic activity in the coastal area is also indicated (Bath, 1953, 1956; Kvale, 1960a). This area has been mapped in considerable detail geophysically, notably by Gr#nlie and Ramberg (1970), Am (1970), Talwani and Eldholm (1972). Admittedly, speculation about the detailed causes of the seismicity in Fig. 4 is perhaps premature; however, some interesting findings emerge in view of recent geophysical data. Talwani and Eldholm (1972) mapped two major structural discontinuities, the V&ing Plateau and Faeroe--Shetland escarpments (Fig. 4), which they believed marked the boundary between oceanic and continental crust. At the Y@ing Plateau a similar structure was reported by Hinz (1972), although he interpreted the area to be continental with intrusion of oceanic material on the inner portion of the plateau. Regardless, Fig. 4 shows quite clearly that no seismicity is associated with the escarpments. We further note that the existing seismicity appears to be associated with the area near the shelf edge between 62” and 68.5”N. In addition, there is a small cluster of epicenters at about 62”N, 4”E and 66.5”N, 13”E. It is possible, though, that some of the events in the latter area are explosions. Furthermore, the shelf-edge earthquakes are located at or near isostatic gravity belts (Talwani and Eldholm, 1972) and limited to the portion of the belts where there is typical oceanic sea floor on the seaward side (e.g., none where the Faeroe-Shetland Ridge approaches the continental margin). Talwani and Eldholm (1972) interpreted these gravity anomalies as reflecting intrabasement high-density belts and suggested that these belts always had subsided relatively less than the surrounding area. When the Norwegian Sea opened, the belts also acted as a hinge line for the post-opening marginal subsidence. If their explanation is valid and a differential subsidence still exists, these earthquakes could result due to stress release within the continental crust. The confinement of the seismicity in the area where we have normal development of the oceanic crust and consequently maximum post-opening subsidence, indicates that at present the differential vertical motion is greatest here and that the intrabasement belts act as a zone of weakness in the crust. The earthquakes near the coastline are clustered in two separate areas as are those on the Greenland Shelf. Some of these earthquakes have been explained as occurring on fault lines associated with the Tertiary uplift of Fennoscandia (Kvale, 1960b). At that time, the coastal areas of western and northern Norway were elevated more than 1000 m supposedly along faults which were mostly parallel to the present coastlines. However, the mere fact that these events are local in character whereas the faults were to be found all along the coastline argues against the idea as do recent geophysical investigations. We will point out that a reconstruction of the Norwegian Sea (Talwani

66

. l

\

**

\

.

\

I\ ,660

,6Z"

\ I*150

,60°

-59

0*

_

Ji loo

Fig. 4. Epicenter distribution (1955-1972) on the continental margin off Norway. Epicenters and selected depth contours (in meters) are shown together with the main structural features (Taiwani and Efdholm, 1972). Zones of known seismic activity in the coastal areas (Kvale, 1960a; BHth, 1953, 1956) are indicated by shading.

67

and Eldholm, 1974) shows that the seismicity areas are approximately juxtaposed, thus possibly indicating zones of weakness in the crust of preopening age. The zones may have been regenerated either by stresses in response to cooling of the plates moving away from the ridge axis or to glacial rebound. There is no apparent activity at the landward end of the Jan Mayen Fracture Zone. Where the Senja Fracture Zone approaches the continental margin some e~hquakes occur. Sundvor (1971) reports an abnormal basement configuration, perhaps faulting, here and Talwani and Eldholm (1972) suggested that the faulting at And@ya and on the shelf may be related to the landward extension of the fracture zone. The present study shows that there is also noticeable seismicity. Finally, we would like to stress that the two epicsntinental seas bounding the Norwegian margin show limited seismicity. In the Bare&s Sea there are no recorded events in the period of this study except for the area in the vicinity of Svalbard and a cluster of events located at the test sites in Novaya Zemlya. Some randomly distributed earthquakes are reported in the North Sea. It has been tacitly assumed by many authors that there is seismicity along the edges of the Norwegian Channel, which is a prominent depression in the North Sea along the southern coast of Norway. Although our data do not show such a relationship, a number of smaller earthquakes has been located in this area using local stations (Bath, 1972). CONCLUSIONS

By collecting seismicity data from various sources for the time period 1955-1972 (529 events) and focal solutions for fifteen earthquakes, we have arrived at the following conclusions: (1) The precision in epicenter estimates is more dependent on station distribution than on the number of recordings. It is usually better than 20 km, which is a limit for the size of the features to be studied. (2) Only one of the focal solutions for events belonging to the active ridge-transform fault-zone system is of the normal type, i.e., associated with the spreading mechanism of the active rift. The strike-slip type events can mostly be related to the transform portion of known fracture zones. (3) The seismicity data appear to support the idea that the Knipovich Ridge is at present an active generator of new sea floor, although a strike-slip focal solution shows that minor offsets may exist. It should be stressed, however, that the Knipovich Ridge area seems to be characterized by a relatively complex stress pattern, possibly due to old zones of weakness along the margin west of Svalbard. The seismicity supports the idea that the Spitsbergen Fracture Zone is of en-e’chelon nature, and the seismicity also defines two fracture zones between the Spitsbergen Fracture Zone and Nansen Ridge. (4) In the Svalbard region there may be a weak correlation between seismicity lineations and north-south trending fault lines. However, the

68

seismicity pattern also shows a NE-SW linear trend that continues into the Greenland Sea and intersects the Knipovich Ridge. We cannot associate these epicenters with any known structure. (5) Outside the active ridge axis and transform part of the fracture zones, the oceanic crust is seismically inactive. An exception is the northern part of the Lofoten Basin where there is notable activity. This may be attributed to an extreme amount of loading due to the thick sedimentary sequence in this area. On the other hand, we point out that a strike-slip event with a nodal plane sub-parallel to the Senja Fracture Zone together with the apparent SE-NW trend of the epicenters may be an indication of hidden structures along the flow lines. (6) No seismicity is associated with the marginal escarpments suggested to mark the boundary between oceanic and continental crust. (7) Earthquakes confined to the shelf edge off Norway are located at or near isostatic gravity belts which probably act as hinge lines for marginal subsidence (Talwani and Eldholm, 1972). The seismic activity is believed to be caused by stress release associated with differential subsidence taking place within the continental crust. (8) The seismic activity in eastern Greenland is confined to two regions as is the seismicity near the Norwegian coastline. These regions were approximately juxtaposed prior to the opening of the Norwegian Sea. This supports the idea that the earthquakes are related to zones of weakness of pre-opening age, possibly activated by stresses due to glacial rebound or plate cooling. (9) There is very limited seismic activity in the North Sea and the Barents Sea. ACKNOWLEDGEMENTS

We would like to thank M. Talwani for commenting on the paper. The NORSAR project was sponsored by the United States Air Force and monitored by the European Office of Aerospace Research and the Air Force Office of Scientific Research, Air Force Systems Command, under contract number F44620-74-C-0001.

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