Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 57-60 |
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57
Earthquake activity in Greenland has been registered and
mapped since 1907 (Larsen et al . 2006) and thus a long
(albeit relatively sparse) record of seismic activity is available
for evaluation of seismic hazard and risk. Seismic hazard
assessment is carried out by judging the probability of future
earthquakes in a given region and is based on statistic treat-
ment of earthquake data. The determination of the seismic
hazard is the first step in an evaluation of seismic risk, i.e. the
possible economic costs and loss of human life after an earth-
quake. The motivation for this seismic hazard study is the
registration of four significant earthquakes in Greenland in
2005. The Geological Survey of Denmark and Greenland
(GEUS) received reports of all four earthquakes from resi-
dents who had felt the shaking. The 2005 earthquakes were
located at or near Qeqertarsuaq on 30 March, Sisimiut on 23
July, Station Nord on 30 August and Attu on 23 October
(Fig. 1), with magnitudes on the Richter scale of 4.3, 4.1, 5.1
and 2.5, respectively. The earthquake in Attu led to the
inhabitants fleeing in their boats.
mapped since 1907 (Larsen et al . 2006) and thus a long
(albeit relatively sparse) record of seismic activity is available
for evaluation of seismic hazard and risk. Seismic hazard
assessment is carried out by judging the probability of future
earthquakes in a given region and is based on statistic treat-
ment of earthquake data. The determination of the seismic
hazard is the first step in an evaluation of seismic risk, i.e. the
possible economic costs and loss of human life after an earth-
quake. The motivation for this seismic hazard study is the
registration of four significant earthquakes in Greenland in
2005. The Geological Survey of Denmark and Greenland
(GEUS) received reports of all four earthquakes from resi-
dents who had felt the shaking. The 2005 earthquakes were
located at or near Qeqertarsuaq on 30 March, Sisimiut on 23
July, Station Nord on 30 August and Attu on 23 October
(Fig. 1), with magnitudes on the Richter scale of 4.3, 4.1, 5.1
and 2.5, respectively. The earthquake in Attu led to the
inhabitants fleeing in their boats.
Earthquake activity
Seismic hazard is just one of many natural hazards in Green -
land. Other natural hazards include: continuous permafrost
that constitutes a serious obstacle to development over the
northern two-thirds of Greenland; strong katabatic winds
that occur along the edge of the ice cap; very low wind chill
and cold sea water, and; landslides and landslide-generated
tsunamis (e.g. Dahl-Jensen et al . 2004). Offshore geohazards
have been studied by GEUS for the 2002 licensing round off
the Greenland west coast (Christiansen et al . 2002).
land. Other natural hazards include: continuous permafrost
that constitutes a serious obstacle to development over the
northern two-thirds of Greenland; strong katabatic winds
that occur along the edge of the ice cap; very low wind chill
and cold sea water, and; landslides and landslide-generated
tsunamis (e.g. Dahl-Jensen et al . 2004). Offshore geohazards
have been studied by GEUS for the 2002 licensing round off
the Greenland west coast (Christiansen et al . 2002).
A first estimate of the seismic hazard of Greenland was
presented during the Global Seismic Hazard Assessment
Program (GSHAP; Giardini et al . 1999). For Greenland
GSHAP used the very sparse data set that was collected prior
to the mid-1990s. Since that time the data set has been much
improved. The earthquake information used in this seismic
hazard study has been extracted from the GEUS earthquake
database and constitutes 227 events that occurred from
November 1971 tOFebruary 2006 (Figs 1, 2). The majority
of these events had a magnitude between 3.0 and 5.0, but
since 2005 improved analytical methods have lowered the
detection threshold from 3.0 to 1.0 in some areas. The earth-
Program (GSHAP; Giardini et al . 1999). For Greenland
GSHAP used the very sparse data set that was collected prior
to the mid-1990s. Since that time the data set has been much
improved. The earthquake information used in this seismic
hazard study has been extracted from the GEUS earthquake
database and constitutes 227 events that occurred from
November 1971 tOFebruary 2006 (Figs 1, 2). The majority
of these events had a magnitude between 3.0 and 5.0, but
since 2005 improved analytical methods have lowered the
detection threshold from 3.0 to 1.0 in some areas. The earth-
quakes were primarily shallow; 93% have been located
between 0 and 40 km depth.
between 0 and 40 km depth.
The location of the earthquakes in the GEUS earthquake
database is determined primarily by using measurements
from the network of permanent and temporary seismic sta-
from the network of permanent and temporary seismic sta-
Seismic hazard assessment of Greenland
Peter Voss, Stine Kildegaard Poulsen, Sebastian Bjerregaard Simonsen and Søren Gregersen
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 57-60. Available at:
www.geus.dk/publications/bull
Fig. 1. Map showing the nine seismic source zones chosen for seismic
hazard assessment of Greenland and the location and magnitude of earth -
quakes registered in the period November 1971 tOFebruary 2006 used
in this study. In zones bordered by dashed lines the number of earth-
quakes is too low to provide input values to hazard assessment (see
Table 1).
tions. By the end of 2006, the network of broadband, seismic
stations in Greenland included four permanent and 14 tem-
porary stations (Larsen et al . 2006). If an earthquake in
Greenland has been recorded by a seismic network in a neigh-
bouring country, these recordings are included in the data-
base. The neighbouring networks that have provided most
data are operated by the Geological Survey of Canada, the
Norwegian Seismic Array (NORSAR) and the University of
Bergen, Norway.
stations in Greenland included four permanent and 14 tem-
porary stations (Larsen et al . 2006). If an earthquake in
Greenland has been recorded by a seismic network in a neigh-
bouring country, these recordings are included in the data-
base. The neighbouring networks that have provided most
data are operated by the Geological Survey of Canada, the
Norwegian Seismic Array (NORSAR) and the University of
Bergen, Norway.
Data analysis
The input for this seismic hazard study is the location, time
and magnitude of the observed earthquakes. The location
and time are determined using a 1D Earth model and an iter-
ative linear inversion scheme (Lienert & Havskov 1995;
Havskov & Ottemöller 2003). The magnitude used is the
body wave magnitude m
and magnitude of the observed earthquakes. The location
and time are determined using a 1D Earth model and an iter-
ative linear inversion scheme (Lienert & Havskov 1995;
Havskov & Ottemöller 2003). The magnitude used is the
body wave magnitude m
b
, and if that is not available, the
local magnitude M
L
(Gregersen 1982). The attenuation of
seismic L
g
waves is known at a few seismic stations as de -
scribed by Gregersen (1982), but a full attenuation model for
Greenland is at present not available. We have therefore
applied the global reference model SEA96 by Spudich et al .
(1997) that describes attenuation from normal faults in hard-
rock conditions. Focal plane solutions have been estimated
for only five earthquakes in Greenland (e.g. Gregersen 2006),
and have thus not been included in this study. Palaeoseismic
information could be of interest if available, but has not been
included since palaeoseismic data are of little influence for
return periods less than 1000 years in intraplate settings (Ata -
kan et al . 2001). Offshore reflection seismic profiles along the
west coast of Greenland (Chalmers & Pulvertaft 2001) show
the presence of major fault systems that should be taken into
account for return periods longer than 1000 years.
Greenland is at present not available. We have therefore
applied the global reference model SEA96 by Spudich et al .
(1997) that describes attenuation from normal faults in hard-
rock conditions. Focal plane solutions have been estimated
for only five earthquakes in Greenland (e.g. Gregersen 2006),
and have thus not been included in this study. Palaeoseismic
information could be of interest if available, but has not been
included since palaeoseismic data are of little influence for
return periods less than 1000 years in intraplate settings (Ata -
kan et al . 2001). Offshore reflection seismic profiles along the
west coast of Greenland (Chalmers & Pulvertaft 2001) show
the presence of major fault systems that should be taken into
account for return periods longer than 1000 years.
The coastal area of Greenland has been divided into eight
seismic source zones that were chosen according to geological
structures and the seismicity of the area (Fig. 1). The central
part of the ice cap is represented by a single zone. Input val-
ues for the hazard assessment for each seismic source zone
were determined from b-value and magnitude estimates (see
Table 1 and Fig. 3). As an example, the computation of the
b-value for seismic source zone 4 is shown in Fig. 3. The haz-
ard computation is in the form of estimated maximum accel-
eration for a return period of 475 years, and is described in
detail by Poulsen & Simonsen (2006).
structures and the seismicity of the area (Fig. 1). The central
part of the ice cap is represented by a single zone. Input val-
ues for the hazard assessment for each seismic source zone
were determined from b-value and magnitude estimates (see
Table 1 and Fig. 3). As an example, the computation of the
b-value for seismic source zone 4 is shown in Fig. 3. The haz-
ard computation is in the form of estimated maximum accel-
eration for a return period of 475 years, and is described in
detail by Poulsen & Simonsen (2006).
Ice cap seismic hazards
Earthquake measurements on the Greenland ice cap have
shown that the seismic energy is transmitted through the ice
for both local and teleseismic earthquakes. The attenuation
model we have applied to estimate the seismic hazard in Green -
land is based on hard-rock conditions; thus, the approach
used in this study and the results obtained do not apply for
conditions on the ice cap. Ground shaking from the newly
discovered glacial earthquakes, or icequakes, (Ekström et al .
2003, Larsen et al . 2006) has not been taken into account
either. Only a very few earthquakes have been located below
the ice cap (see Fig. 1), and seismic hazard in the interior of
Greenland is therefore considered to be even lower than the
lowest hazard in any coastal area.
shown that the seismic energy is transmitted through the ice
for both local and teleseismic earthquakes. The attenuation
model we have applied to estimate the seismic hazard in Green -
land is based on hard-rock conditions; thus, the approach
used in this study and the results obtained do not apply for
conditions on the ice cap. Ground shaking from the newly
discovered glacial earthquakes, or icequakes, (Ekström et al .
2003, Larsen et al . 2006) has not been taken into account
either. Only a very few earthquakes have been located below
the ice cap (see Fig. 1), and seismic hazard in the interior of
Greenland is therefore considered to be even lower than the
lowest hazard in any coastal area.
58
Fig. 2. Seismogram for the Richter scale 2.5 Attu earthquake on 23
October, 2005 recorded at the seismic station in Kangerlussuaq. Each
trace shows the ground velocity in the direction indicated. The seismo-
gram is band-pass filtered between 1 and 5 Hz, and the scale of the max-
imum amplitude is given at the end of each trace.
Seismic hazard
The seismic hazard assessment of Greenland is computed for
a return period of 475 years as shown in Fig. 4. The maxi-
mum hazard is found in seismic source zone 4 at a value of
0.051 g (50.37 cm/s
a return period of 475 years as shown in Fig. 4. The maxi-
mum hazard is found in seismic source zone 4 at a value of
0.051 g (50.37 cm/s
2
). From this result we assess the general
seismic hazard in Greenland to be low, following the classifi-
cation of Jiménez et al . (2003) in which low, moderate and
high hazards correspond to peak ground accelerations of
0.0-0.08 g, 0.08-0.24 g and above 0.24 g, respectively, for a
475-year return period.
cation of Jiménez et al . (2003) in which low, moderate and
high hazards correspond to peak ground accelerations of
0.0-0.08 g, 0.08-0.24 g and above 0.24 g, respectively, for a
475-year return period.
Seismic source zone 4 covering the northern and north-
eastern parts of Greenland is the area with the highest seismic
hazard; the seismic hazard is below 0.05 g in the other seis-
mic source zones where the highest hazards are encountered
in the Disko Bugt - Sisimiut area (seismic source zone 8) fol-
lowed by southern Greenland (seismic source zone 1). These
results differ considerably from the GSHAP estimates that
are below 0.02 g for the northern and the north-eastern parts
of Greenland. Along the east coast of the Baffin Bay we find
the seismic hazard to be below 0.024 g, where GSHAP
reported a hazard of up to 0.08 g.
hazard; the seismic hazard is below 0.05 g in the other seis-
mic source zones where the highest hazards are encountered
in the Disko Bugt - Sisimiut area (seismic source zone 8) fol-
lowed by southern Greenland (seismic source zone 1). These
results differ considerably from the GSHAP estimates that
are below 0.02 g for the northern and the north-eastern parts
of Greenland. Along the east coast of the Baffin Bay we find
the seismic hazard to be below 0.024 g, where GSHAP
reported a hazard of up to 0.08 g.
Seismic risk
A full evaluation of seismic risk would include collecting in-
depth knowledge of factors such as infrastructure, building
standards and population density distribution, work that is
beyond the scope of this study. Here we outline only the
expected, relative seismic risk for the four areas of highest
seismic hazard. Though the highest seismic hazard is found in
the northern and north-eastern parts of Greenland (seismic
zone 4), the seismic risk is very low, since the only permanent
residents are the five Danish Air Force personnel at Station
depth knowledge of factors such as infrastructure, building
standards and population density distribution, work that is
beyond the scope of this study. Here we outline only the
expected, relative seismic risk for the four areas of highest
seismic hazard. Though the highest seismic hazard is found in
the northern and north-eastern parts of Greenland (seismic
zone 4), the seismic risk is very low, since the only permanent
residents are the five Danish Air Force personnel at Station
59
Fig. 3. Histogram showing the number and magnitude of earthquakes
determined in seismic source zone 4. It is a global experience in seis-
mology that the number of earthquakes (N) of various magnitudes (M)
can be expressed in a logarithmic relation: log N = a - b
× M (black line).
The a-value is a measure of regional seismicity, whereas the b-value
may be viewed as a regional physical constant.
Fig. 4. Map showing seismic hazard in Greenland for a 475-year return
period, corresponding to the 10% probability of exceeding a given g-
value in a 50-year period (hard-rock conditions). Seismic source zones
from Fig. 1 indicated.
60
Nord and the staff at the Danmarkshavn weather station,
where the buildings are designed to withstand the Arctic cli-
mate. The next two zones in order of decreasing seismic haz-
ard are the Disko Bugt - Sisimiut area (seismic zone 8) and
southern Greenland (seismic zone 1). Larger infrastructure
and denser population in seismic zones 1 and 8 indicate that
these are judged to be at the highest seismic risk in the whole
of Greenland.
where the buildings are designed to withstand the Arctic cli-
mate. The next two zones in order of decreasing seismic haz-
ard are the Disko Bugt - Sisimiut area (seismic zone 8) and
southern Greenland (seismic zone 1). Larger infrastructure
and denser population in seismic zones 1 and 8 indicate that
these are judged to be at the highest seismic risk in the whole
of Greenland.
Concluding remarks
The results presented in this study have completely changed
the seismic hazard assessment of Greenland compared to the
seismic hazard map compiled by GSHAP (Giardini et al .
1999). GSHAP overestimated the seismic hazard of the area
north of Disko Bugt and underestimated the seismic hazard
for the southern, the northern and the north-eastern parts of
Greenland.
the seismic hazard assessment of Greenland compared to the
seismic hazard map compiled by GSHAP (Giardini et al .
1999). GSHAP overestimated the seismic hazard of the area
north of Disko Bugt and underestimated the seismic hazard
for the southern, the northern and the north-eastern parts of
Greenland.
During recent years, the majority of reports received by
GEUS of earthquakes felt by the resident population are
from Tasiilaq (seismic zone 2), but our results show that the
seismic hazard in this area is low because these earthquakes
are small.
from Tasiilaq (seismic zone 2), but our results show that the
seismic hazard in this area is low because these earthquakes
are small.
The overall seismic hazard in Greenland is low compared
to the many other natural hazards. However, large destructive
earthquakes can occur unexpectedly even in areas with low
seismicity, as illustrated by the magnitude 6.3 Latur earth-
quake in central India on 30 September, 1993 (Gupta 1993).
earthquakes can occur unexpectedly even in areas with low
seismicity, as illustrated by the magnitude 6.3 Latur earth-
quake in central India on 30 September, 1993 (Gupta 1993).
Acknowledgements
GeoForschungsZentrum (GFZ) Potsdam, Germany, provides instrumen-
tation and technical support to the seismograph in Danmarkshavn and,
together with the Incorporated Research Institutions for Seismology
(IRIS), USA, to the seismograph in Kangerlussuaq. GFZ has also contri -
buted to the installation and operation of several temporary seismic sta-
tions. The Bureau of Minerals and Petroleum, Government of Greenland,
provided financial support for several temporary installations.
tation and technical support to the seismograph in Danmarkshavn and,
together with the Incorporated Research Institutions for Seismology
(IRIS), USA, to the seismograph in Kangerlussuaq. GFZ has also contri -
buted to the installation and operation of several temporary seismic sta-
tions. The Bureau of Minerals and Petroleum, Government of Greenland,
provided financial support for several temporary installations.
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Authors' addresses
P.V & S.G.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
pv@geus.dk
S.K.P. & S.B.S., Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark.
S.K.P. & S.B.S., Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark.
