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Intraplate earthquakes in Northern Svalbard
181
~ec~~~o~~~~ic~, 114 ((1985)
181-191
Elsevier Science Publishers
B.V., Amsterdam
INTRAPLATE
W. WINSTON
- Printed
EARTHQUAKES
CHAN
in The Netherlands
IN NORTHERN
SVALBARD
and B.J. MITCHELL
Depurtment of E~t-th and Atmospheric Sciences, Saint Louis ~ffi~~ersj~~, St. Louis, MO
(Received
August
26, 1983; accepted
January
63156 (U.S.A.)
10, 1984)
ABSTRACT
Chart W.W. and Mitchell, Johnson
and
B.J., 1985. Intraplate
Y. Kristoffersen
(Editors),
earthquakes
on northern
Geophysics
of the Polar
Svalbard.
In: E.S. Husebye,
Regions.
G.L.
Tectonoph.vsrcs. 114:
181-191. A temporary recorded concentrated activity
network
numerous
during
zones in restricted
portions
also occurred
locations,
of mapped portions
these are intrapfate Composite greatest
of mapped solutions
at least roughly,
principle
south. The consistency
fault segments
along
faults
occurred
Nordaustlandet,
the uncertainty
of the epicentral
and even entire
spatial
locations
in response
faults,
lines of activity, to an applied
with maximum suggests
and trend of a mapped vary between
stress directions
of maximum
determined
principle
that the earthquakes
N90”W
that
stress
field.
which can of
and N55OW. directions
for earthquakes
there occur in response
suggest
fault. The directions
for a region about
stress obtained
however.
of these earthquakes
for the three most active regions each include one nodal-plane with a trend of seismicity
in
but minor
by glaciers could be associated
of throughgoing
existing
part of Svalbard
of the earthquakes
in central
Within
The concentrated
faults, and the absence
of the directions
region of Svalbard
of plate tectonic
are numerous
occurred.
occurring
in the northern
Most
on land in regions not covered
stress for the three fault plane solutions
which agree approximately a broad
There
earthquake
earthquakes
fault-plane
be associated,
deployed
of 1982.
of three fault complexes
which occurred faults.
which no recorded
along preferred
seismographs
the summer
to the east and west of these complexes.
most earthquakes
with segments along
of microearthquake
earthquakes
200 km to the throughout
to a stress field which is
origin.
INTRODUCTION
Svalbard is an archipelago situated on the northwestern corner of the Barents shelf. It consists of the main island of West Spitsbergen, along with the islands of Nordaustlandet, Kong Karls Land, Edgeoya, Barentsoya, and several smaller islands. Although it is a shelf region with a continental-type crustal structure (e.g. Chan and Mitchell, 1982), it lies close to the mid-Atlantic ridge system. Studies of seismicity patterns and fault-plane solutions there should therefore provide important information on the nature of stress patterns near an oceanic spreading center. ~-1951/85/$03.30
Q 1985 Elsevier Science Publishers
B.V.
182
Several
major
1969; Harland addition,
faults
traverse
Svalbard
in a NNW-SSE
et al., 1974). The more important
the Hornsund
direction
et al.,
in Fig. 1. In
fault zone lies less than 20 km to the west of the western-
most fault on the map. That fault zone lies near the transition and oceanic crust (Myhre et al.. 1982). The faults are thought the Paleozoic,
(Flood
of these are shown
but were reactivated
in Mesozoic
between
continental
to have formed during
and Tertiary
times. There
is little
contemporary earthquake activity associated with most of the mapped faulting; however, a region of high activity occurs in Heer Land, near the eastern coast of West Spitsbergen (Mitchell et al., 1979), and another in Nordaustlandet (Bungum et al.. 1982). A few earthquakes also appear to he associated with mapped faults in central and western Spitsbergen. but their nutnber is very smah. Mitchell et al. (1979) and Bungum et al. (1982) suggest that the earthquakes in Svalbard can be explained as a reaction along local zones of weakness to a regional stress field of
Fig. 1. Map of the main islands of Svalbard Spitsbergen and Nordaustlandet
showing
major mapped
faults (dashed
lines) in West
183
plate tectonic
origin.
who associate
Spitsbergen
separates
a portion
seismicity
patterns
tinguish
between
quakes
in Svalbard
not possible of instruments
This view contrasts earthquake
activity
of Spitsbergen and
with that of Savostin
from
with a proposed
the Eurasian
plate.
and Karasik
(1981)
plate margin
which
Detailed
fault-plane
solutions
in Svalbard
shoufd
these alternatives.
Although
some single-station
studies US to
allow studies
of dis-
of earth-
were conducted
more than 20 years ago (Sellevoll,
1960), it was
to study the seismicity
there in detail until the instahation
of networks
in 1976 and later (Mitchell
et al., 1979: Bungum
and Kristoffersen,
1980: Bungum et al., 1982). The first networks were operated during short periods of the summers of 1976 and 1977 (Mitchell et al., 1979) and led to the first detailed delineation of a concentrated
earthquake
gen. Although
zone in Heer Land on the eastern
those instruments
surrounded
coast of West Spitsber-
the Heer Land
region
and permitted
reliable locations, they could not be operated for very long durations because of the remoteness of the field sites and shortness of the summer field season. Additional instruments
were operated
beginning
in late 1979 by the Norwegian
Polar Institute
and NORSAR in mining communities several ten’s of kilometers from Heer Land. Although these instruments did not permit precise earthquake locations, they did operate continuously for nearly a year; thus they provided valuable data on patterns of earthquakes in Svalbard over an extended period of time. In an attempt to record over a long time period and yet have good azimuthal coverage installed
with stations near the Heer Land zone. a telemetered seismic network was by Saint Louis University in cooperation with the Norwegian Polar
Institute during the summer of 1979. It consisted initially of seven stations, all of which transmitted data to a central recording station at Svea, a Norwegian mining community. Those data confirmed the previous conclusions of Mitchell et al. (1979) concerning seismic Svalbard
the extent
zone.
and orientation
In addition,
of the earthquake
the network
as well as along the northern
series of events was observed
recorded Atlantic
in Nordaustlandet,
patterns
earthquakes
in the Heer Land in other
ridge system. A particularly
regions
of
striking
about 200 km north of the network
(Bungum et al., 1982) in a region where a small number of earthquakes had been reported previously (Bungum and Kristoffersen, 1980). Because of the distances of the stations
from Nordaustlandet
located precisely enough with mapped faulting.
in the above studies,
to delineate
definite
the earthquakes
trends or to associate
could not be
the earthquakes
In the present study we have attempted to precisely locate earthquakes in Nordaustlandet, to associate those locations with mapped faults when possible, to determine the nature of faulting there, and to determine the orientation of the regional stress field in northern Svalbard. Although our field season was short and the number of recorded events was limited, it was possible to achieve our goals because of the high level of seismicity and the low level of seismic noise in Nordaustlandet.
1x4
PREVIOUS
LOCATIONS
The Saint Louis University and surrounding
Regional
regions recorded
Telemetered
numerous
Seismic Network
earthyuakes
in Heer Land
in Nordaustlandet
in 1979.
Those events were about 200 km from the network, thus they could not be located very precisely. The available data suggested that the earthquakes occurred throughout a zone which was 40-50 km in E-W extent and 15-20 km in N-S extent (Bungum et al.. 1982). Those earthquakes had been located using a method developed by Mitchell (1980) which solves for apparent velocity and azimuth of curved wave fronts across a network
at some distance
that a refation
S-P times and distance
between
from an earthquake.
The Nordaustlandet earthquakes recorded studied by Bungum et al. (1982) are shown differ
somewhat
different
S-P
from those originally
versus distance
relation
The method
by the Heer Land network in Fig. 2. Those locations
plotted
assumes
is known.
by Bungum
is used. We found
in 1979 and on the map
et al. (1982) because that the relation
a
used by
790 0 j.
Fig. 2. Earthquakes network
in 1979
located
in Nnrdaustlandet
using
phases
recorded
by the Heet
Land
telemetered
185
Mitchell
et al. (1979) produces
the original
locations
with locations
locations
of Bungum
determined
which are slightly
further
to the north than
et al. (1982) and which correspond
in the present
report by the Nordaustlandet
more closely network.
NEW DATA
In order to delineate seismicity patterns in Nordaustlandet in greater detail, Saint Louis University personnel conducted a microearthquake survey there in the summer of 1982. A temporary four-station network of portable seismographs monitored earthquake activity in western Nordaustlandet for a period of three weeks from July 4 through July 23, 1982. The station locations are listed in Table 1 and plotted in Fig. 3. The instruments used are MEQ-800 portable seismographs and Mark Products L4C seismometers. The systems were powered by air cell batteries and recording was done by pen and ink on paper. The high cutoff filter was set at 10 Hz and the low cutoff was set at 5 Hz at all stations. Because the sites were so quiet through
this frequency
impulsive disturbance
range, it was possible
phases were recorded
to set the gains at very high values and
even for very small earthquakes.
were those due to intermittent
wind and the nearby
The only sources of passage of animals.
P- and S-wave arrivals were read and their times were used as input to the FASTHYPO program of Herrmann (1979). In all cases a minimum of five phases from at least three stations was used for the locations. The depths of the earthquakes were fixed at 4 km as suggested by earlier results of Mitchell and Ghan (1978) for the Heer Land earthquakes. The crustal model used for the locations is that of Chan and Mitchell (1982) obtained from refraction studies in central Spitsbergen. That model is continental in structure having a total crustal thickness of 30 km.
TABLE Station
1 information
for the Svalbard
network
Station
Lat.
Long.
Year of
code
C’N)
(“Et
operations
SVE
77.893
16.693
1980,1981
UTB
77.555
17.472
1980
RST
71.555
17.472
1981
DAL
77.982
17.454
1980
SKG
77.669
17.467
1980, 1981
BER
77.722
15.437
1980,1981
DEP
80.386
19.484
1982
KIN
80.049
18.264
1982
FAK
79.567
17.749
1982
RIP
80.216
22.467
1982
.,...,._ I_-_.-___. . llli,. i
187
TABLE 2 (continued) r.m.s.
Time
Lat.
Long.
(h.m.s.)
(“N)
(“E)
82.07.18
03.27.51
80.14
20.36
0.43
1.51
82.07.18
03.45.27
80.09
21.49
0.99
0.64
82.07.18
04.06.00
80.15
19.91
0.82
1.16
82.07.18
04.09.53
80.15
19.96
0.76
1.15
82.07.18
04.43.16
80.16
21.31
0.72
1.26
82.07.18
08.13.54
80.37
17.24
0.01
0.97
82.07.18
20.06.43
80.11
20.18
0.60
1.27
82.07.18
22.48.16
80.15
20.24
0.63
1.61
82.07.18
22.58.45
80.15
20.19
0.74
1.82
82.07.18
23.03.45
80.05
20.23
0.76
1.61
82.07.19
01.55.02
80.27
21.48
0.99
1.26
82.07.19
03.06.13
80.15
20.28
0.92
1.36
82.07.19
15.35.44
79.95
18.79
0.74
2.77
82.07.19
17.37.41
80.22
20.34
0.33
1.42
82.07.19
20.35.30
79.89
18.88
0.71
2.26
82.07.19
21.06.25
80.09
25.02
0.99
2.08
82.07.19
22.35.49
19.92
18.78
0.57
2.08
82.07.19
23.52.51
80.28
20.70
0.86
0.61
82.07.20
07.37.32
79.91
18.99
0.84
1.63
82.07.20
08.22.49
80.16
20.32
0.70
2.10
Date
Ml_
82.07.20
14.37.58
80.18
20.26
0.88
1.63
82.07.20
17.37.32
80.15
20.38
0.46
1.84
82.07.20
19.24.52
80.14
20.26
0.66
1.79
82.07.20
19.47.27
80.13
20.26
0.64
1.67
SEISMICITY
PATTERNS
A total of over 100 microearthquakes of which were locatable.
were detected
All of the located
events
over a threeweek
are listed
in Table
period,
68
2 and are
plotted in Figure 3, along with mapped faults of the region obtained from Flood et al. (1969). The error in epicentral locations of these earthquakes is less than 7 km in all cases, with a 95% confidence limit, and the depth uncertainties for them is about 3 km. The earthquake locations in Fig. 3 are not associated with a single zone, as suggested by the locations obtained earlier using more distant stations. With better locations
made possible
by means of a nearby
network,
the earthquakes
are found to
occur in at least four distinct zones throughout the broader region found by Bungum et al. (1982). A zone of high activity occurs near the center of the temporary network, where over 30 events were located (Regions 1 and 2). The northern portion of the zone (Region 1) parallels and lies near a N-S trending fault mapped by Flood et al. (1969). The southern portion of the zone shows some scatter but indicates an approximate E-W trend in agreement with a small fault mapped by
Flood
et al. (1969).
trends
in a N-S
A less active region
direction;
faults in the area. Another island
coincide
the sea. Two other minor
to the southeast
seismic zone (Region
where the fault system extends
sea. The earthquakes
occurs
these events may be associated
3) lies closer to the center
from the northern
part of the island
of the into the
closely with this system of faults both on land and in
zones are located
island. No faults are mapped
of this zone and
with a system of parallel
in the eastern
near the eastern
ity of the area for geologic mapping.
zone. probably
and western
parts of the
due to the inaccessibil-
The minor zone to the west may be associated
with one of the mapped faults in the region, but because of the complexity of the fault system in this region it is not possible to definitely relate earthquakes with any particular fault. Other scattered events are found in the offshore few in number to be associated with any concentrated zone.
area. but are too
All earthquakes tend to be located further north than those indicated by Bungum et al. (1982). As discussed earlier, this suggests that the S-P relation of Bungum and Kristoffersen (1980) is not applicable for paths between Nordaustlandet and the telemetered network near Heer Land. The magnitudes for the events located by the microearthquake network are also listed in Table 2 and a cumulative magnitude-frequency relationship is plotted in Fig. 4. A least-squares fit to the data gives a h-value of 0.82. a value which is
Fig. 3. Detailed obtained
during
and dashed
map
of epicenters
the summer
lines indicate
and composite
of 1982. Triangles
major mapped
28.0
Z’t*O
20.0
faults.
fault-plane
denote the stations
solutions
for three
of the temporary
regions seismograph
using
data
network
189
00
0.75 Fig. 4. Cumulative
frequency-magnitude
line is a least-squares
plot for Nordaustlandet
earthquakes
recorded
in 1982. The solid
fit to the data and yields a b-value of 0.82.
significantly higher than the value of 0.5 reported earlier by Bungum et al. (1982). The smaller value found in the earlier study is probably due to the smaller number of earthquakes used in that study and to the occurrence of an unusually large number obtained
of larger earthquakes during the recording period in 1979. The b-value in this study is similar to that obtained earlier for the Heer Land seismic
zone. FAULT-PLANE
Earthquake
SOLUTIONS
fault-plane
solutions
give valuable
information
concerning
the physi-
cal mechanism associated with a fault. Because of the low magnitude of the events in Nordaustlandet, only composite fault-plane solutions using locally recorded waves, could be obtained. Take-off angles for ray paths leaving the source were calculated using the crustal model of Chan and Mitchell (1982). Among the seismic zones delineated in the previous section, three of them produced which were unambiguous enough to permit the determination In the highly active zone to the west the earthquakes
first-motion readings of focal mechanisms.
were divided
into northern
and southern zones, as suggested by the location and orientation of mapped faults. The northern zone trends N-S and the southern zone approximately E-W. The first-motion data for these earthquakes are plotted for the northern and southern trends on the left side and center, respectively, of Fig. 5. These plots show that the northern zone can be explained by a strike-slip fault striking either N45”W or N45”E. The former solution comes closest to agreeing with the fault mapped above the earthquake epicenters, but still differs by about 40” from the trend of those
’:
Fig. 5. Composite
ii
focal mechanisms
from left to right, to regions plotted
for earthquakes
m three regions
I, 2, and Z in Fig. 3. Compressions
on the lower hemisphere
of Nordaustlandet.
(circles) and dilatations
of the focal sphere. Some of the points
are offset slightly
The plots refer. (triangles)
are
for clarity.
epicenters. The latter solution gives a faultpiane which closely parallels a fault mapped about 5 km north of the present epicenters. Since the uncertainty in earthquake locations is about 7 km, they could be associated with that fault. The maximum compressive stress acts in an E-W direction and the minimum stress in a N-S direction. The southern zone can be characterized by a fault with a significant dip-slip component and N5O”E strike direction, again in rough agreement with the seismicity pattern and a mapped fault. The maximum compressive stress acts at a direction of N55”W and the minimum stress acts in a NE direction. A comparison of the seismicity patterns and the fault patterns in region 3 suggests that the earthquakes occurring there may be associated with a complex pattern of faults. If all of those earthquakes which occurred on the landward side of the fjord are considered together, the fault-plane solution on the right in Fig. 5 is obtained. The plane which corresponds most closely to the seismicity pattern and mapped direction
faults strikes N60’E. The maximum compressive stress is oriented in a of N75”W and the minimum compressive stress lies in a direction of
N15”E. CONCLUSIONS
AND
DISCUSSION
The seismicity of Nordaustlandet can be associated with mapped faulting wherever sufficiently detailed maps are available. The activity is mainly associated with segments of three fat& complexes in central Nordaustlandet while minor activity occurs in the eastern and western portions of the island. There are also numerous faults and fault segments along which no earthquakes were located during the recording period. The concentrated nature of the seismically active zones and the absence of any major through-going hnes of activity suggest that the earthquakes there are intra-plate in nature and do not occur along a plate boundary. Fault-plane solutions in the three most active regions of Nordaustlandet each yield one nodal plane which can, at least approximately, be associated with a trend
I91
of seismicity and strike of a mapped fault. The directions of the greatest principal stress axes for the three areas range between N90”W are consistent with the direction from an in situ measurement greatly from the direction Heer Land earthquake
of maximum
in central
and N55”W.
These directions
compressive stress (N78”W)
Svalbard
(Hast,
compressive stress (N 11X”W)
of maximum
(Bungum and Kristoffersen,
found
1969) and do not differ found for a
1980). The rough consistency of
these observations over widely separated regions of Svalhard suggests that the stress field in that region of the Barents shelf is of plate
tectonic origin.
ACKNOWLEDGMENTS
We are grateful to the Norwegian
Polar Institute for providing information
location of huts in northern Svalbard and for providing Zollweg of the University
of Washington
and Karl Chauff of Saint Louis University
operated two of the field instruments and read many This research was sponsored by the National Programs. under Grant
on the
logistical assistance. James
of the recorded
Science Foundation,
arrival
times.
Division of Polar
DPP-8121478.
REFERENCES H. and Kristoffersen, Y., 1980. A microearthquake survey of the Svalhard
Bungum.
Phase I. NORSAR Bungum,
H., GJoystahl.
preliminary
H., Hokland.
report
NTN F/NORSAR, Bungum.
on
B. and Kristoffersen.
microearthquake
W.W. and
B.J. and
Kristoffersen.
B.J. Mitchell,
Harland.
and central
parts.
W.B., Cutbill,
Wallis,
R.H..
lineament.
NORSAR
Y., 19X2. Concentrated
1982. Synthetic
Tectonophysics,
of the Svalbard
Tech.
Rep.,
earthquake
Mitchell.
T.L., Friend,
T.P.F.,
1974. The Billefjorden
Nor. Polarinst.
seismogram
and
and T.S. Wmsnes,
Nor. Poiarinst.
in Svalhard.
surface
wave constraints
on crustaf
1969. The Geology
of Nordaustlandet.
Skr., 146: 139 pp. Gobelt,
D.J.. Holliday.
fault zone, Spitshergen,
D.W.. Maton, the long history
P.I., Parker, of a major
R.B.. 1979. FASTHYPO-A
Missouri. Mitchell,
Spitsbergen. B.J.. Zollweg,
the Svalbard Myhre. Savostin.
A.M.. Eldholm,
program.
tectonic
1978. Characteristics
Earthquake
8: 169-21 I.
Notes. 50: 25-37.
velocity using curved wave fronts across a regional Earthquake
Hazard
Studies
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No. 14-08-~1-16708,
of earthquakes
in the Heerland
seismic
zone of
48: 31-40.
J.E., Kohsman, J.J., Cheng, C.C. and Haug. J. Geophys.
0. and Sundvor, from plate tectonics.
L.A. and Karasik.
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W.W.,
Polarforschung.
Archipelago,
zones: Implications
and apparent
location
Tech. Rep., U.S. Geol. Surv., Contract
B.J. and Ghan,
eastern Mitchell.
Semi-annu.
location
hypocenter
R.B. Herrmann and B.J. Mitchell.
In: W. Stauder,
J.R. and
Skr.. 161: 72 pp.
B.J.. 1980. Earthquake
array,
a
Z/7%
zones
Hast. N., 1969. The state of stress in the upper part of the earth’s crust. Tectonophysics, Herrmann.
region:
No.
89: 51-X.
B., D.G. Gee, A. Hjelle. T. Siggerud
northern
Y., 1978. Seismicity
activity.
82: 175-18X.
models of Spitsbergen. Flood.
the
region. Final Report,
KJeller. Norway.
Kjeller, Norway.
H., Mitchell.
Tectonophysics. Chan,
Tech. Rep., No. I/X0--NTNF/NORSAR.
E.J.. 1979. Intraplate
earthquakes
in
Res., 84: 5620-5626. E.. 1982. The margin between Tectonophysics.
A.M., 1981. Recent plate tectonics
Senja and Spitsbergen
fracture
89: 33-50. of the Arctic
Basin and northeastern
Asia.
74: 111-145.
M.A., 1960. Seismiseten
i Svalbardomradet,
Jordskelvstasjonen.
University
of Bergen,
Bergen.
~ec~~~o~~~~ic~, 114 ((1985)
181-191
Elsevier Science Publishers
B.V., Amsterdam
INTRAPLATE
W. WINSTON
- Printed
EARTHQUAKES
CHAN
in The Netherlands
IN NORTHERN
SVALBARD
and B.J. MITCHELL
Depurtment of E~t-th and Atmospheric Sciences, Saint Louis ~ffi~~ersj~~, St. Louis, MO
(Received
August
26, 1983; accepted
January
63156 (U.S.A.)
10, 1984)
ABSTRACT
Chart W.W. and Mitchell, Johnson
and
B.J., 1985. Intraplate
Y. Kristoffersen
(Editors),
earthquakes
on northern
Geophysics
of the Polar
Svalbard.
In: E.S. Husebye,
Regions.
G.L.
Tectonoph.vsrcs. 114:
181-191. A temporary recorded concentrated activity
network
numerous
during
zones in restricted
portions
also occurred
locations,
of mapped portions
these are intrapfate Composite greatest
of mapped solutions
at least roughly,
principle
south. The consistency
fault segments
along
faults
occurred
Nordaustlandet,
the uncertainty
of the epicentral
and even entire
spatial
locations
in response
faults,
lines of activity, to an applied
with maximum suggests
and trend of a mapped vary between
stress directions
of maximum
determined
principle
that the earthquakes
N90”W
that
stress
field.
which can of
and N55OW. directions
for earthquakes
there occur in response
suggest
fault. The directions
for a region about
stress obtained
however.
of these earthquakes
for the three most active regions each include one nodal-plane with a trend of seismicity
in
but minor
by glaciers could be associated
of throughgoing
existing
part of Svalbard
of the earthquakes
in central
Within
The concentrated
faults, and the absence
of the directions
region of Svalbard
of plate tectonic
are numerous
occurred.
occurring
in the northern
Most
on land in regions not covered
stress for the three fault plane solutions
which agree approximately a broad
There
earthquake
earthquakes
fault-plane
be associated,
deployed
of 1982.
of three fault complexes
which occurred faults.
which no recorded
along preferred
seismographs
the summer
to the east and west of these complexes.
most earthquakes
with segments along
of microearthquake
earthquakes
200 km to the throughout
to a stress field which is
origin.
INTRODUCTION
Svalbard is an archipelago situated on the northwestern corner of the Barents shelf. It consists of the main island of West Spitsbergen, along with the islands of Nordaustlandet, Kong Karls Land, Edgeoya, Barentsoya, and several smaller islands. Although it is a shelf region with a continental-type crustal structure (e.g. Chan and Mitchell, 1982), it lies close to the mid-Atlantic ridge system. Studies of seismicity patterns and fault-plane solutions there should therefore provide important information on the nature of stress patterns near an oceanic spreading center. ~-1951/85/$03.30
Q 1985 Elsevier Science Publishers
B.V.
182
Several
major
1969; Harland addition,
faults
traverse
Svalbard
in a NNW-SSE
et al., 1974). The more important
the Hornsund
direction
et al.,
in Fig. 1. In
fault zone lies less than 20 km to the west of the western-
most fault on the map. That fault zone lies near the transition and oceanic crust (Myhre et al.. 1982). The faults are thought the Paleozoic,
(Flood
of these are shown
but were reactivated
in Mesozoic
between
continental
to have formed during
and Tertiary
times. There
is little
contemporary earthquake activity associated with most of the mapped faulting; however, a region of high activity occurs in Heer Land, near the eastern coast of West Spitsbergen (Mitchell et al., 1979), and another in Nordaustlandet (Bungum et al.. 1982). A few earthquakes also appear to he associated with mapped faults in central and western Spitsbergen. but their nutnber is very smah. Mitchell et al. (1979) and Bungum et al. (1982) suggest that the earthquakes in Svalbard can be explained as a reaction along local zones of weakness to a regional stress field of
Fig. 1. Map of the main islands of Svalbard Spitsbergen and Nordaustlandet
showing
major mapped
faults (dashed
lines) in West
183
plate tectonic
origin.
who associate
Spitsbergen
separates
a portion
seismicity
patterns
tinguish
between
quakes
in Svalbard
not possible of instruments
This view contrasts earthquake
activity
of Spitsbergen and
with that of Savostin
from
with a proposed
the Eurasian
plate.
and Karasik
(1981)
plate margin
which
Detailed
fault-plane
solutions
in Svalbard
shoufd
these alternatives.
Although
some single-station
studies US to
allow studies
of dis-
of earth-
were conducted
more than 20 years ago (Sellevoll,
1960), it was
to study the seismicity
there in detail until the instahation
of networks
in 1976 and later (Mitchell
et al., 1979: Bungum
and Kristoffersen,
1980: Bungum et al., 1982). The first networks were operated during short periods of the summers of 1976 and 1977 (Mitchell et al., 1979) and led to the first detailed delineation of a concentrated
earthquake
gen. Although
zone in Heer Land on the eastern
those instruments
surrounded
coast of West Spitsber-
the Heer Land
region
and permitted
reliable locations, they could not be operated for very long durations because of the remoteness of the field sites and shortness of the summer field season. Additional instruments
were operated
beginning
in late 1979 by the Norwegian
Polar Institute
and NORSAR in mining communities several ten’s of kilometers from Heer Land. Although these instruments did not permit precise earthquake locations, they did operate continuously for nearly a year; thus they provided valuable data on patterns of earthquakes in Svalbard over an extended period of time. In an attempt to record over a long time period and yet have good azimuthal coverage installed
with stations near the Heer Land zone. a telemetered seismic network was by Saint Louis University in cooperation with the Norwegian Polar
Institute during the summer of 1979. It consisted initially of seven stations, all of which transmitted data to a central recording station at Svea, a Norwegian mining community. Those data confirmed the previous conclusions of Mitchell et al. (1979) concerning seismic Svalbard
the extent
zone.
and orientation
In addition,
of the earthquake
the network
as well as along the northern
series of events was observed
recorded Atlantic
in Nordaustlandet,
patterns
earthquakes
in the Heer Land in other
ridge system. A particularly
regions
of
striking
about 200 km north of the network
(Bungum et al., 1982) in a region where a small number of earthquakes had been reported previously (Bungum and Kristoffersen, 1980). Because of the distances of the stations
from Nordaustlandet
located precisely enough with mapped faulting.
in the above studies,
to delineate
definite
the earthquakes
trends or to associate
could not be
the earthquakes
In the present study we have attempted to precisely locate earthquakes in Nordaustlandet, to associate those locations with mapped faults when possible, to determine the nature of faulting there, and to determine the orientation of the regional stress field in northern Svalbard. Although our field season was short and the number of recorded events was limited, it was possible to achieve our goals because of the high level of seismicity and the low level of seismic noise in Nordaustlandet.
1x4
PREVIOUS
LOCATIONS
The Saint Louis University and surrounding
Regional
regions recorded
Telemetered
numerous
Seismic Network
earthyuakes
in Heer Land
in Nordaustlandet
in 1979.
Those events were about 200 km from the network, thus they could not be located very precisely. The available data suggested that the earthquakes occurred throughout a zone which was 40-50 km in E-W extent and 15-20 km in N-S extent (Bungum et al.. 1982). Those earthquakes had been located using a method developed by Mitchell (1980) which solves for apparent velocity and azimuth of curved wave fronts across a network
at some distance
that a refation
S-P times and distance
between
from an earthquake.
The Nordaustlandet earthquakes recorded studied by Bungum et al. (1982) are shown differ
somewhat
different
S-P
from those originally
versus distance
relation
The method
by the Heer Land network in Fig. 2. Those locations
plotted
assumes
is known.
by Bungum
is used. We found
in 1979 and on the map
et al. (1982) because that the relation
a
used by
790 0 j.
Fig. 2. Earthquakes network
in 1979
located
in Nnrdaustlandet
using
phases
recorded
by the Heet
Land
telemetered
185
Mitchell
et al. (1979) produces
the original
locations
with locations
locations
of Bungum
determined
which are slightly
further
to the north than
et al. (1982) and which correspond
in the present
report by the Nordaustlandet
more closely network.
NEW DATA
In order to delineate seismicity patterns in Nordaustlandet in greater detail, Saint Louis University personnel conducted a microearthquake survey there in the summer of 1982. A temporary four-station network of portable seismographs monitored earthquake activity in western Nordaustlandet for a period of three weeks from July 4 through July 23, 1982. The station locations are listed in Table 1 and plotted in Fig. 3. The instruments used are MEQ-800 portable seismographs and Mark Products L4C seismometers. The systems were powered by air cell batteries and recording was done by pen and ink on paper. The high cutoff filter was set at 10 Hz and the low cutoff was set at 5 Hz at all stations. Because the sites were so quiet through
this frequency
impulsive disturbance
range, it was possible
phases were recorded
to set the gains at very high values and
even for very small earthquakes.
were those due to intermittent
wind and the nearby
The only sources of passage of animals.
P- and S-wave arrivals were read and their times were used as input to the FASTHYPO program of Herrmann (1979). In all cases a minimum of five phases from at least three stations was used for the locations. The depths of the earthquakes were fixed at 4 km as suggested by earlier results of Mitchell and Ghan (1978) for the Heer Land earthquakes. The crustal model used for the locations is that of Chan and Mitchell (1982) obtained from refraction studies in central Spitsbergen. That model is continental in structure having a total crustal thickness of 30 km.
TABLE Station
1 information
for the Svalbard
network
Station
Lat.
Long.
Year of
code
C’N)
(“Et
operations
SVE
77.893
16.693
1980,1981
UTB
77.555
17.472
1980
RST
71.555
17.472
1981
DAL
77.982
17.454
1980
SKG
77.669
17.467
1980, 1981
BER
77.722
15.437
1980,1981
DEP
80.386
19.484
1982
KIN
80.049
18.264
1982
FAK
79.567
17.749
1982
RIP
80.216
22.467
1982
.,...,._ I_-_.-___. . llli,. i
187
TABLE 2 (continued) r.m.s.
Time
Lat.
Long.
(h.m.s.)
(“N)
(“E)
82.07.18
03.27.51
80.14
20.36
0.43
1.51
82.07.18
03.45.27
80.09
21.49
0.99
0.64
82.07.18
04.06.00
80.15
19.91
0.82
1.16
82.07.18
04.09.53
80.15
19.96
0.76
1.15
82.07.18
04.43.16
80.16
21.31
0.72
1.26
82.07.18
08.13.54
80.37
17.24
0.01
0.97
82.07.18
20.06.43
80.11
20.18
0.60
1.27
82.07.18
22.48.16
80.15
20.24
0.63
1.61
82.07.18
22.58.45
80.15
20.19
0.74
1.82
82.07.18
23.03.45
80.05
20.23
0.76
1.61
82.07.19
01.55.02
80.27
21.48
0.99
1.26
82.07.19
03.06.13
80.15
20.28
0.92
1.36
82.07.19
15.35.44
79.95
18.79
0.74
2.77
82.07.19
17.37.41
80.22
20.34
0.33
1.42
82.07.19
20.35.30
79.89
18.88
0.71
2.26
82.07.19
21.06.25
80.09
25.02
0.99
2.08
82.07.19
22.35.49
19.92
18.78
0.57
2.08
82.07.19
23.52.51
80.28
20.70
0.86
0.61
82.07.20
07.37.32
79.91
18.99
0.84
1.63
82.07.20
08.22.49
80.16
20.32
0.70
2.10
Date
Ml_
82.07.20
14.37.58
80.18
20.26
0.88
1.63
82.07.20
17.37.32
80.15
20.38
0.46
1.84
82.07.20
19.24.52
80.14
20.26
0.66
1.79
82.07.20
19.47.27
80.13
20.26
0.64
1.67
SEISMICITY
PATTERNS
A total of over 100 microearthquakes of which were locatable.
were detected
All of the located
events
over a threeweek
are listed
in Table
period,
68
2 and are
plotted in Figure 3, along with mapped faults of the region obtained from Flood et al. (1969). The error in epicentral locations of these earthquakes is less than 7 km in all cases, with a 95% confidence limit, and the depth uncertainties for them is about 3 km. The earthquake locations in Fig. 3 are not associated with a single zone, as suggested by the locations obtained earlier using more distant stations. With better locations
made possible
by means of a nearby
network,
the earthquakes
are found to
occur in at least four distinct zones throughout the broader region found by Bungum et al. (1982). A zone of high activity occurs near the center of the temporary network, where over 30 events were located (Regions 1 and 2). The northern portion of the zone (Region 1) parallels and lies near a N-S trending fault mapped by Flood et al. (1969). The southern portion of the zone shows some scatter but indicates an approximate E-W trend in agreement with a small fault mapped by
Flood
et al. (1969).
trends
in a N-S
A less active region
direction;
faults in the area. Another island
coincide
the sea. Two other minor
to the southeast
seismic zone (Region
where the fault system extends
sea. The earthquakes
occurs
these events may be associated
3) lies closer to the center
from the northern
part of the island
of the into the
closely with this system of faults both on land and in
zones are located
island. No faults are mapped
of this zone and
with a system of parallel
in the eastern
near the eastern
ity of the area for geologic mapping.
zone. probably
and western
parts of the
due to the inaccessibil-
The minor zone to the west may be associated
with one of the mapped faults in the region, but because of the complexity of the fault system in this region it is not possible to definitely relate earthquakes with any particular fault. Other scattered events are found in the offshore few in number to be associated with any concentrated zone.
area. but are too
All earthquakes tend to be located further north than those indicated by Bungum et al. (1982). As discussed earlier, this suggests that the S-P relation of Bungum and Kristoffersen (1980) is not applicable for paths between Nordaustlandet and the telemetered network near Heer Land. The magnitudes for the events located by the microearthquake network are also listed in Table 2 and a cumulative magnitude-frequency relationship is plotted in Fig. 4. A least-squares fit to the data gives a h-value of 0.82. a value which is
Fig. 3. Detailed obtained
during
and dashed
map
of epicenters
the summer
lines indicate
and composite
of 1982. Triangles
major mapped
28.0
Z’t*O
20.0
faults.
fault-plane
denote the stations
solutions
for three
of the temporary
regions seismograph
using
data
network
189
00
0.75 Fig. 4. Cumulative
frequency-magnitude
line is a least-squares
plot for Nordaustlandet
earthquakes
recorded
in 1982. The solid
fit to the data and yields a b-value of 0.82.
significantly higher than the value of 0.5 reported earlier by Bungum et al. (1982). The smaller value found in the earlier study is probably due to the smaller number of earthquakes used in that study and to the occurrence of an unusually large number obtained
of larger earthquakes during the recording period in 1979. The b-value in this study is similar to that obtained earlier for the Heer Land seismic
zone. FAULT-PLANE
Earthquake
SOLUTIONS
fault-plane
solutions
give valuable
information
concerning
the physi-
cal mechanism associated with a fault. Because of the low magnitude of the events in Nordaustlandet, only composite fault-plane solutions using locally recorded waves, could be obtained. Take-off angles for ray paths leaving the source were calculated using the crustal model of Chan and Mitchell (1982). Among the seismic zones delineated in the previous section, three of them produced which were unambiguous enough to permit the determination In the highly active zone to the west the earthquakes
first-motion readings of focal mechanisms.
were divided
into northern
and southern zones, as suggested by the location and orientation of mapped faults. The northern zone trends N-S and the southern zone approximately E-W. The first-motion data for these earthquakes are plotted for the northern and southern trends on the left side and center, respectively, of Fig. 5. These plots show that the northern zone can be explained by a strike-slip fault striking either N45”W or N45”E. The former solution comes closest to agreeing with the fault mapped above the earthquake epicenters, but still differs by about 40” from the trend of those
’:
Fig. 5. Composite
ii
focal mechanisms
from left to right, to regions plotted
for earthquakes
m three regions
I, 2, and Z in Fig. 3. Compressions
on the lower hemisphere
of Nordaustlandet.
(circles) and dilatations
of the focal sphere. Some of the points
are offset slightly
The plots refer. (triangles)
are
for clarity.
epicenters. The latter solution gives a faultpiane which closely parallels a fault mapped about 5 km north of the present epicenters. Since the uncertainty in earthquake locations is about 7 km, they could be associated with that fault. The maximum compressive stress acts in an E-W direction and the minimum stress in a N-S direction. The southern zone can be characterized by a fault with a significant dip-slip component and N5O”E strike direction, again in rough agreement with the seismicity pattern and a mapped fault. The maximum compressive stress acts at a direction of N55”W and the minimum stress acts in a NE direction. A comparison of the seismicity patterns and the fault patterns in region 3 suggests that the earthquakes occurring there may be associated with a complex pattern of faults. If all of those earthquakes which occurred on the landward side of the fjord are considered together, the fault-plane solution on the right in Fig. 5 is obtained. The plane which corresponds most closely to the seismicity pattern and mapped direction
faults strikes N60’E. The maximum compressive stress is oriented in a of N75”W and the minimum compressive stress lies in a direction of
N15”E. CONCLUSIONS
AND
DISCUSSION
The seismicity of Nordaustlandet can be associated with mapped faulting wherever sufficiently detailed maps are available. The activity is mainly associated with segments of three fat& complexes in central Nordaustlandet while minor activity occurs in the eastern and western portions of the island. There are also numerous faults and fault segments along which no earthquakes were located during the recording period. The concentrated nature of the seismically active zones and the absence of any major through-going hnes of activity suggest that the earthquakes there are intra-plate in nature and do not occur along a plate boundary. Fault-plane solutions in the three most active regions of Nordaustlandet each yield one nodal plane which can, at least approximately, be associated with a trend
I91
of seismicity and strike of a mapped fault. The directions of the greatest principal stress axes for the three areas range between N90”W are consistent with the direction from an in situ measurement greatly from the direction Heer Land earthquake
of maximum
in central
and N55”W.
These directions
compressive stress (N78”W)
Svalbard
(Hast,
compressive stress (N 11X”W)
of maximum
(Bungum and Kristoffersen,
found
1969) and do not differ found for a
1980). The rough consistency of
these observations over widely separated regions of Svalhard suggests that the stress field in that region of the Barents shelf is of plate
tectonic origin.
ACKNOWLEDGMENTS
We are grateful to the Norwegian
Polar Institute for providing information
location of huts in northern Svalbard and for providing Zollweg of the University
of Washington
and Karl Chauff of Saint Louis University
operated two of the field instruments and read many This research was sponsored by the National Programs. under Grant
on the
logistical assistance. James
of the recorded
Science Foundation,
arrival
times.
Division of Polar
DPP-8121478.
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