Download Seismicity in Iceland: 1994–2007

Reviewed research article
Seismicity in Iceland: 1994–2007
Steinunn S. Jakobsdóttir
Physics Department, Icelandic Meteorological Office, Bústaðavegur 9, Reykjavík, IS-150, Iceland [email protected]
Abstract — Since the end of 1993, the digital, automatic seismic system, known as the SIL-system, has acted
as the national seismic network in Iceland. The number of stations in the network has increased gradually
over the 14 years from 1994 to 2007. The detection capability of the system has increased at the same pace,
especially along the volcanic zones. The general seismicity of Iceland, as monitored by the growing network, is
presented in this paper. The main activity is, as expected, along the plate boundary, but seismicity has also been
observed at intraplate locations. Instead of setting a lower limit to the size of earthquakes to be considered in
this paper, changes in the sensitivity are discussed along with the seismic activity. Several major events have
been observed during the observation period. In June 2000, two magnitude 6.6 earthquakes occurred in the
South Iceland Seismic Zone, followed by a few earthquakes larger than magnitude 5. A few episodes caused by
magma injections were recorded and four volcanic eruptions monitored. In addition to the large earthquakes
in June 2000, seven earthquakes larger than magnitude 5 occurred in this period, two in Bárðarbunga in the
years before the 1996 Gjálp eruption, two near the triple junction between the Reykjanes Peninsula, the Western
Volcanic Zone and the South Iceland Seismic Zone, two in the Tjörnes Fracture Zone and one on the Reykjanes
Peninsula. Intraplate earthquakes were recorded in the northwestern part of Iceland, Vestfirðir, in 1994 and
2006, and in Guðlaugstungur, between Langjökull and Hofsjökull, in 2004. Additionally, earthquakes are
located near Surtsey every year. Two episodes of activity have been recorded in the Esjufjöll central volcano in
the southeastern part of Vatnajökull, and several swarms have been detected under Öræfajökull. All the main
ice caps cover seismically active central volcanoes, explaining the high seismicity beneath them.
INTRODUCTION
In 1991, a new era of seismic monitoring started in
Iceland, when a new digital seismic system, the SIL
system, was upgraded to fully automatic operation.
The SIL seismic system is a highly automated threecomponent data acquisition and monitoring system,
consisting of the SIL network and automatic processing software. It was designed and initiated within
the Nordic SIL project in 1988 - 1994 (Stefánsson
et al., 1993, Böðvarsson et al., 1996, Böðvarsson
et al., 1999). The motivation for the project was
the imminent occurrence of large earthquakes in the
South Iceland Lowland (SIL). The design goal for the
system was to automatically detect and locate earthquakes as small as magnitude Ml 0 or less for monitoring purposes, and in order to get high quality data
for earthquake prediction research. The network was
originally installed and operated in SW-Iceland in the
South Iceland Seismic Zone (Figure 1). It was expanded to the Tjörnes Fracture Zone in 1993, whereby
it became a regional network for all Iceland, covering the main seismic zones. In the following years,
the network was expanded gradually into the highland
along the rift zone. In 1994 the number of stations
was 18, but at the end of 2007 the seismic network
consisted of 51 stations (Figure 1).
JÖKULL No. 58, 2008
75
S. S. Jakobsdóttir
Figure 1. The SIL network. Stations are marked with triangles and color coded according to age. Inverted
triangles represent stations that are no longer operated. Boxes mark the areas plotted in Figures 4 and 8.
Abbreviations are as follows: A: Askja, AF: Arnarfjörður, BB: Bárðarbunga, DE: Dyrhólaey EE: Eldey, EF:
Esjufjöll, EJ: Eyjafjallajökull, EVZ: Eastern Volcanic Zone, GE: Grímsey, GJ: Geitlandsjökull, GR: Grímsvötn/Grímsfjall, GS: Geirfuglasker, GT: Guðlaugstungur, H: Hekla, HT: Herðubreiðartögl, HV: Hveravellir,
KF: Kistufell, KR: Kolbeinsey Ridge, KvF: Kverkfjöll, LH: Lokahryggur, MJ: Mýrdalsjökull, NVZ: Northern
Volcanic Zone, RF: Reykjarfjörður, RP: Reykjanes Peninsula, RR: Reykjanes Ridge, SISZ: South Iceland Seismic Zone, SE: Surtsey, TFZ: Tjörnes Fracture Zone, TJ: Torfajökull, UT: Upptyppingar, WVZ: Western Volcanic Zone ÞJ: Þórisjökull, ÖJ: Öræfajökull. – SIL-netið. Jarðskjálftastöðvarnar eru sýndar með þríhyrningum
og aldur þeirra er táknaður með litum. Þríhyrningar með eitt horn niður tákna stöðvar sem ekki eru lengur í
rekstri. Opnir ferningar skilgreina svæðin sem myndir 4 og 8 byggja á. Skammstafanir eru útskýrðar í enska
textanum.
This paper is based on data recorded by the SIL
system during the years 1994 to 2007. Its aim is to
give an overview of the seismic activity during this
period. The focus is kept on the seismicity, both the
everyday activity and on anomalous swarms and sequences. Less attention is paid to structures derived
from earthquake studies or other studies of the seis-
76 JÖKULL No. 58, 2008
mic zones. The depth distribution of earthquakes is
not discussed in detail, as more work is needed to obtain well-constrained depth determinations. In general, the focal depth of earthquakes in Iceland is less
than 8–12 km, indicating that this is the thickness of
the brittle crust. Focal depths down to 25–30 km are
observed, but they are not always well-constrained as
Seismicity in Iceland: 1994–2007
the closest stations in many cases are too far from the
epicenter, and the one-dimensional velocity models
do not show enough detail. The occurrence of fairly
well-constrained deep earthquakes is discussed in this
paper and some results are given from other studies,
including depth determinations obtained by mapping
of active faults by relative location of swarms.
During the study period there have been four
volcanic eruptions in Iceland and a few magmatic
episodes that did not reach the surface. The seismic
manifestation of the eruptions and magma movements
is also described here. Over ∼250,000 earthquakes in
the magnitude range -2 to 6.6 were recorded and located during this period.
TECTONIC SETTING
In summary, the tectonic setting of Iceland is characterized by its position on the Mid-Atlantic Ridge
(MAR) and by the Icelandic Hotspot (Tryggvason
et al.1983; Wolfe et al., 1997; Allen et al., 2002;
Bjarnason 2008). The MAR is shifted to the east in
the vicinity of the mantle plume, which is centered
under Vatnajökull ice cap (Figure 1). The largest
earthquakes, of magnitudes up to 7.2 (Stefánsson
and Halldórsson, 1988), occur in the South Iceland
Seismic Zone (SISZ), between the Western Volcanic
Zone (WVZ) and the Eastern Volcanic Zone (EVZ),
and in the Tjörnes Fracture Zone (TFZ), connecting
the Northern Volcanic zone (NVZ) to the Kolbeinsey
Ridge (KR). The earthquake faults in the SISZ strike
N-S, lining up parallel, almost perpendicular to the
underlying plate boundary. The TFZ is more complex, composed of three WNW-ESE trending segments (Sæmundsson 1974; Einarsson 1991; Rögnvaldsson et al., 1998a). Volcanic eruptions are frequent in the NVZ and the EVZ. Eruptions are generally preceded by seismic activity that differs from
one volcano to another. Since 1970, Hekla volcano is
known to be seismically quiescent until about an hour
before an eruption while most other volcanoes have a
longer preparation period. For a more detailed study
of the tectonics of Iceland, see Einarsson (2008).
THE SIL SYSTEM
At each station, the incoming data-stream is preprocessed by a computer at the site. Transients in
the data flow are detected and defined by onset time,
amplitude and duration. Different parameters are calculated, such as apparent velocity, azimuth and spectral parameters. This information is packed into short
messages and sent to the central computer in Reykjavík, where an automatic phase association process defines events and sends requests for wave-form data
to the stations (Böðvarsson et al., 1996). Automatic
locations and magnitudes of earthquakes are usually
available within 1-3 minutes of their occurrence and
focal mechanisms are determined for every earthquake using spectral amplitudes (Slunga, 1981; Rögnvaldsson and Slunga, 1993, 1994). All automatic
locations are manually checked, and both locations
and focal mechanisms are re-run with corrected phase
picks. A one minute average in three frequency intervals is calculated automatically for all three components of the seismic data and plotted continuously
on-line in so called tremor plots (Böðvarsson et al.,
1999). These plots are used to monitor changes in
background noise and seismic activity. A relative location method is available in the system. It is used
to obtain precise locations, within tens of meters in
relative accuracy, thereby making it possible to map
active faults (Rögnvaldsson and Slunga, 1994; Slunga
et al., 1995).
There are two types of automatic magnitude calculations used in the SIL-system. The local magnitude, Ml , is empirical, based on the maximum peakto-peak amplitude in a 10 seconds interval around the
S-wave at all stations:
Ml = log10 A + 2.1 * log10 D - 4.8
where A is the maximum velocity amplitude of highpass filtered waveforms with a cut off frequency at 2
Hz and scaled to the response of Lennartz 1 Hz sensor and Nanometrics RD3 digitizer, D is the distance
between the earthquake and the station in km. It is calibrated to give similar magnitude values as the magnitudes calculated from peak-to-peak readings of the
former analog regional network. The other magnitude
is a local moment magnitude, called Mlw or Ml (Mo ):
JÖKULL No. 58, 2008
77
S. S. Jakobsdóttir
Ml (Mo ) = log10 (Mo ) – 10
where Mo is the seismic moment in Nm. This formula
is used for Mlw <2 and is slightly modified for larger
earthquakes (Stefánsson et al., 2005). In this work the
empirical local magnitude Ml is mostly used.
In 1994 the SIL system was complete in detection
of earthquakes in Iceland down to magnitude ∼Ml 2.
In the SISZ earthquakes down to Ml -1.5 were detected and located, but the magnitude of completeness
in the area was ∼Ml 0 (Wyss and Stefánsson 2006).
Earthquakes down to magnitude ∼Ml 0.5 – Ml 1 were
detected and located in TFZ and down to ∼Ml 1,5 in
the highland. Since 2006 most of the highland is well
within the Ml 1 detection limit and only in the NW
part of the country, at Vestfirðir, is the detection limit
higher than Ml 1.5 (Ágústsson, 2006).
had a magnitude larger than Ml 2, which is similar to the magnitude threshold used in former bulletins, based on analog recordings. Figure 3 shows
the epicenter distribution of earthquakes larger than
Ml 1. The seismic activity is mainly concentrated
in the seismic zones. More than 80% of the epicentres are located in the TFZ and in the area from SISZ
through the Hengill triple junction and to the Reykjanes Peninsula (RP). Earthquakes, detected by the network, occur every week in these zones and every year
in the connecting volcanic zones, mainly at volcanic
centers. In addition, activity is recorded every year
near to Surtsey, south of Iceland.
THE DATASET
Seismicity as presented here is based on automatic detections and locations of earthquakes that have been
manually corrected for errors. In some cases, earthquakes that were not detected automatically have been
manually added. These are usually either earthquakes
that are close in time, or undetected earthquakes that
can be seen on the continuous tremor plots. The earthquakes that escape automatic detection are mostly so
called LP (long period) earthquakes, characterized by
emergent onsets and dominated by low frequency and
can even be difficult to locate manually. They occur
mainly in volcanic areas, e.g. under the western part
of Mýrdalsjökull-Katla. Explosions due to construction work are frequently detected, but care has been
taken to mark them as such and to exclude them in
this study. It is still possible that a few explosions
have passed through as earthquakes, mainly in areas
that are known to be seismically active. Results from
relative locations are not used directly in this work.
GENERAL SEISMICITY IN ICELAND
1994–2007
Several seismic swarms and volcanic events have been
observed during this period. Over 250,000 earthquakes were located within Iceland and the surrounding offshore shelf (Figure 2) and about 10,000 of these
78 JÖKULL No. 58, 2008
Figure 2. Number of earthquakes recorded by the SIL
system during 1994–2007, within the co-ordinates
63◦ N–67◦N and 13◦ W–25◦ W. Number of earthquakes larger than Ml 0, Ml 1 and Ml 2 are shown
with colors. – Árlegur fjöldi jarðskjálfta, sem skrásettir voru í SIL-kerfinu frá 1994 til 2007 innan svæðis
sem afmarkast af hnitunum 63◦ N–67◦ N og 13◦ V–
25◦ V. Fjöldi skjálfta af stærð yfir Ml 0, Ml 1 og Ml
2 er sýndur með mismunandi litum.
In the TFZ the Grímsey lineament and the
Húsavík-Flatey fault are most active (Stefánsson et
al., 2008). Two earthquakes of magnitude ∼ M5–5.5
occurred in the TFZ during the study period. A Mw
5.5 earthquake (Table 1) occurred on 8 February 1994
in the western part of the zone and a Mw 5.8 (Table 1)
in the northern part on 16 September 2002.
Seismicity in Iceland: 1994–2007
Magnitude ∼M 5 earthquakes were recorded
yearly by the SIL system prior to the 1996 eruption in
Vatnajökull, a magnitude b 5.7 earthquake on 5 May
1994, a magnitude mb 4.9 on 11 December 1995 and
a magnitude Mw 5.6 on 29 September 1996 (Table 1),
marking the onset of the eruption.
In the Hengill area around the triple junction between RP, WVZ and SISZ, intense activity was observed in 1994 to 1998. This activity culminated with
two Ml 5 earthquakes in 1998. Two earthquakes of
magnitudes Ms 6.6 (Table 1) occurred in the SISZ in
2000, with thousands of aftershocks in the following
days and months. On 23 August 2003 a magnitude
Mw 5.2 was located at Sveifluháls, near Kleifarvatn
on the Reykjanes Peninsula.
Figure 3 shows many events located in Skeiðarárjökull, an outlet glacier south of Vatnajökull. These
events are classified as icequakes, as they are shallow
and are associated with increased water flow through
the glacier (Roberts, 2005). The icequakes will not be
discussed in this paper; for further details, see Roberts
(2005).
Comparing the map in Figure 3 with the seismicity map of Einarsson and Sæmundsson (1987) shows
that all areas that were active in the former period
(1982–1985) are also active in the study period, ex-
Figure 3. Map of earthquakes larger than Ml 1 recorded by the SIL system in 1994–2007. Earthquakes larger
than M 5 are marked with stars. – Kortið sýnir jarðskjálfta stærri en Ml 1, sem skráðir voru í SIL-kerfinu á
árunum 1994 til 2007. Stjörnurnar tákna skjálfta stærri en M 5.
JÖKULL No. 58, 2008
79
S. S. Jakobsdóttir
Table 1. Earthquakes with magnitude larger than 5 from the NEIC website. Locations and magnitudes from
the SIL system are in the last four columns. – Skjálftar sem mælst hafa yfir 5 stig samkvæmt alþjóðlegu neti
skjálftamæla (NEIC). Staðsetningar og reiknaðar stærðir samkvæmt SIL-netinu eru í fjórum seinustu dálkunum.
YEAR MO DA TIME
LAT
LONG
km Mb Me Ms
Mw
SIL-LAT SIL-LONG Mlw Ml
1994
1994
1995
02
05
12
08
05
11
032754.90 66.51
051449.77 64.58
052246.63 64.55
-19.22
-17.48
-17.79
10 5.3
8 5.7
10 4.9
5.3
5.2
4.5
5.50 HRV
66.451
64.634
64.671
-19.249
-17.430
-17.504
4.7
4.6
4.6
4.8
4.2
3.9
1996
1998
1998
2000
09
06
11
06
29
04
13
17
104818.97
213654.19
103834.50
154041.73
64.78
63.993
63.882
63.966
-17.57
-21.290
-21.595
-20.487
10
10
10
10
5.3
5.1
4.9
5.7
5.4
5.1Z
4.4Z
6.6Z
5.60 HRV
5.40 HRV
5.10 HRV
6.80 CSEM
6.50 HRV
64.687
64.036
63.963
63.973
-17.495
-21.290
-21.352
-20.367
4.8
5.0
5.1
5.5
4.4
4.7
4.9
5.7
2000
2000
2002
2003
06
06
09
08
17
21
16
23
154252.63
005146.88
184826.72
020011.89
63.965
63.980
66.94
63.92
-20.633
-20.758
-18.46
-22.26
10
10
10
10
5.7
6.1 6.8 6.6Z
5.5
5.7 5.80 GS
4.8
4.5 5.20 HRV
63.943
63.972
66.975
63.905
-20.460
-20.711
-18.437
-22.085
5.8
5.8
5.3
5.0
5.0
5.4
5.0
4.3
cept for a unique swarm occurring about 20 km south
of Dyrhólaey in July 1985. Several new active areas are seen in 1994–2007. A few earthquakes were
recorded at Vestfirðir in 1994 and in 2006. A few sequences have been located north of Langjökull and
earthquake swarms were recorded at Guðlaugstungur,
NE of Langjökull, in 2004. These earthquakes are
all outside the present rift zones and transform faults.
Hofsjökull has three distinct seismic areas that are
active at different times and Eyjafjallajökull has had
some earthquake episodes. Activity on the Reykjanes Ridge near Geirfuglasker and Eldeyjarboði has
increased since 2000 with several Ml 4 earthquakes
(Höskuldsson et al., 2007). The last big earthquake
swarms occurred on the Reykjanes Ridge in February
and April 2007.
The station density has increased considerably
during the period discussed in this paper. An overview
of the evolution of the network with time is seen in
Figure 1, where the time of installation is color coded.
SEISMIC ACTIVITY IN THE
HIGHLANDS
Most of the observed activity in the highlands is under or in the vicinity of ice caps. Each ice cap covers
80 JÖKULL No. 58, 2008
one or more central volcanos. The Askja central volcano and its surroundings is the only seismically active area that is not close to an ice cap. Earthquakes in
the highland are of magnitudes up to Ml 3–4, except
under Bárðarbunga in Vatnajökull where earthquakes
of magnitudes up to Ml 4.4 (Mw 5.6, Table 1) were
observed before the Gjálp eruption in 1996 (Figure
4h).
Langjökull
The main seismic activity in the western part of the
highlands is the area of the Langjökull ice cap in the
WVZ. In the first half of the period, before 2000,
most of the activity was beneath the southern part of
the ice cap, with swarms occurring in Geitlandsjökull and in Þórisjökull (Figures 3 and 4). A swarm
was recorded at Kaldidalur, west of Geitlandsjökull,
in 1994. The most active period started with a swarm
in Geitlandsjökull in March 1998 and proceeded with
swarms in Þórisjökull in September 1998 and July
1999. The Mw 6.6 earthquake in Holt in June 2000
was followed by seismic activity in other areas near
Langjökull. Earthquakes were recorded near Geysir,
due north of the mainshock, and the activity continued north of Sandfell in a ∼15 km long zone south
of the ice cap (Hjaltadóttir and Vogfjörð, 2005). Fur-
Seismicity in Iceland: 1994–2007
Figure 4. Magnitudes greater than Ml 1 as a function of time for areas shown by boxes in Figures 1, 5 and 6.
Note that scales are different on different plots. a: Langjökull, b: Hofsjökull, c: Torfajökull, d: Goðabunga, e:
Katla caldera, f: Lokahryggur, g: Grímsvötn, h: Bárðarbunga, i: Kistufell, j: Kverkfjöll, k: Herðubreiðartögl,
l: Askja. For Goðabunga and Katla caldera magnitudes larger than 1.8 are plotted. The gray line shows the
accumulated number of earthquakes during the period. – Stærð jarðskjálfta stærri en Ml 1 sem fall af tíma.
Svæðin eru merkt inn á myndir 1, 5 og 6 og eru eftirfarandi: a: Langjökli, b: Hofsjökli, c: Torfajökli, d:
Goðabungu, e: Kötluöskjunni, f: Lokahrygg, g: Grímsvötnum, h: Bárðarbungu, i: Kistufelli, j: Kverkfjöllum, k:
Herðubreiðartöglum, l: Öskju. Athugið að ekki er sami skali á öllum myndunum. Einnig eru einungis sýndir
skjálftar stærri en Ml 1,8 í Goðabungu og Kötluöskjunni. Gráa línan sýnir uppsafnaðan fjölda skjálfta yfir
tímabilið.
JÖKULL No. 58, 2008
81
S. S. Jakobsdóttir
82 JÖKULL No. 58, 2008
Seismicity in Iceland: 1994–2007
JÖKULL No. 58, 2008
83
S. S. Jakobsdóttir
84 JÖKULL No. 58, 2008
Seismicity in Iceland: 1994–2007
ther north, at the NE corner of Langjökull and NW of
Hveravellir, another area was activated. These areas
have all been active since 2000. There is decreased
activity in Geitlandsjökull and Þórisjökull after the 17
June 2000 earthquake, but in 2007 the activity at the
NE flank of Langjökull has increased. In addition,
small swarms have occasionally occurred both north,
west and south of Langjökull during the study period.
Hofsjökull
Beneath the Hofsjökull ice cap three separate areas
were seismically active during the study period (Figures 3 and 4b). Small swarms occur regularly beneath the western part of the ice-filled caldera in the
western part of Hofsjökull and this is the most persistent activity beneath the ice cap. In the northern
part, the activity is more sporadic. A dense swarm of
30 earthquakes was located in the NE part of the ice
cap in November 1999. Other activity in the NE part
has been observed as single earthquakes, some events
might be icequakes.
Torfajökull
There is persistent seismic activity within the Torfajökull volcanic center with magnitudes up to Ml ∼3
(Figures 3 and 4c). Earthquakes are recorded regularly with occasional occurrences of small swarms.
Soosalu et al. (2006b) and Lippitsch et al. (2005)
studied the seismicity within Torfajökull area with
data from a temporary local network they operated
in 2002, in addition to SIL data. They found that
high frequency earthquakes occur at shallow depths
(less than 6 km) in the western part of the Torfajökull caldera while low frequency earthquakes with
emergent onsets cluster in the southern part, near the
glaciers (Figures 1 and 3), at depths reaching from
close to the surface down to 14 km.
Mýrdalsjökull-Katla
The seismic activity in Mýrdalsjökull is mostly at
two sites, within the Katla caldera and at Goðabunga
on the western flank of the ice cap (Figure 5). The
number of earthquakes located beneath Goðabunga is
about 5 times the activity within the caldera (Figures
4d and e) (Jakobsdóttir et al. 2002; Þorbjarnardóttir et
al., 2003). The seismicity in Goðabunga shows seasonal variation and is most intense in the second half
of the year (Einarsson and Brandsdóttir, 2000; Jónsdóttir et al., 2007). The earthquakes are shallow with
emergent onsets and can be difficult to locate with
precision. Einarsson and Brandsdóttir (2000) argue
that the seasonal variation is mainly due to change
in hydrostatic pressure associated with the seasonal
melting of the ice cap. An episode of unrest was
observed beneath Goðabunga from the end of 2001
to the end of 2004. The seasonal variation in seismicity in this period was overlain by constant activity resulting in four times higher seismic activity than
the years before and after (Þorbjarnardóttir and Guðmundsson, 2003; Guðmundsson et al., 2004; Hjaltadóttir et al., 2005a). Earthquakes within the caldera
are of tectonic origin and are more uniform in time.
JÖKULL No. 58, 2008
85
S. S. Jakobsdóttir
Figure 5. Earthquakes in Eyjafjallajökull and Mýrdalsjökull larger than Ml 1 in 1994–2007. The boxes represent (from left to right) Eyjafjallajökull, Goðabunga and Katla caldera. – Kortið sýnir jarðskjálfta stærri en Ml
1 undir Eyjafjallajökli og Mýrdalsjökli á árunum 1994 til 2007. Ferningarnir afmarka svæðin sem skilgreind
eru sem Eyjafjallajökull, Goðabunga og Kötluaskja á myndum 4 og 8.
Uplift of the Katla caldera was observed by geodetic surveying during the period of unrest, but uplift at
Goðabunga is less prominent (Sturkell et al., 2006a;
Sturkell et al., 2008). Soosalu et al. (2006a) suggested a cryptodome under Goðabunga as the cause
of the activity. Since 2005 the number of earthquakes
per year is again similar to the activity in 1998–1999.
Jónsdóttir et al. (2007) observe that the characteristics
of earthquakes in the caldera are similar to volcanic
areas, while the activity under Goðabunga is highly
anomalous. They find annual variation in both areas,
but it is more prominent under Goðabunga where the
maximum has 2–3 months time delay compared to the
time of maximum melting of the ice cap. Jónsdóttir et
al. (2008) reach the conclusion that the LP events at
Goðabunga are icequakes. Pinel et al. (2007) verified that the horizontal velocities measured by GPS in
1999–2004 cannot be explained only by the unloading from the melting of the ice cap and argue that
magma inflow is needed to explain the observed inflation of the Katla volcano. A few deep-seated earthquakes (∼23 km) occurred near Hjörleifshöfði, southeast of Mýrdalsjökull in December 2007 in the magni-
86 JÖKULL No. 58, 2008
tude range Ml 0.7–1.6 (Icelandic Meteorological Office web site: hraun.vedur.is/ja/viku/2007/vika− 49).
NW-Vatnajökull
The NW part of Vatnajökull, above the hotspot, is
one of the most active areas in Iceland, both seismically and volcanically. Other parts of Vatnajökull are not as active. The seismic activity can be divided into five main sites, each with different characteristics. These are Lokahryggur, Grímsvötn, Bárðarbunga, Kistufell and Kverkfjöll. Lokahryggur is
located to the west of Gjálp/Grímsvötn (Figure 6).
Two cauldrons, known as Skaftárkatlar, exist in the
ice cover above Lokahryggur. Seismic activity on
Lokahryggur is rather uniform in time with 40–50
earthquakes of magnitude Ml >1 recorded every year
except before and after the Gjálp eruption in 1996–
1997, when the activity was much greater (Figures 4f
and 6). The geothermal areas under the Skaftárkatlar cauldrons do not show more seismic activity than
elsewhere on Lokahryggur.
Seismicity in the Grímsvötn volcanic center is
closely related to the eruptive activity (Figures 4g
Seismicity in Iceland: 1994–2007
and 6), increasing as the pressure increases under the
caldera, it is most intensive just before the onset of an
eruption and almost stops after an eruption (see also
in the section: Seismic signature of eruptions).
In the three years before the Gjálp eruption in
1996 the Bárðarbunga volcanic center was fairly active with a ∼Ml 5 earthquake recorded every year.
The activity was concentrated in and around the Bárðarbunga caldera (Figure 6), but until August 1996 the
closest SIL-station was about 120 km away (Figure
1) and locations were not so precise (see subsequent
section on the seismic signature of eruptions). After the Gjálp eruption the activity decreased and the
Grímsvötn eruption in 1998 is not seen clearly in the
Bárðarbunga data (Figure 4h). In November 2002 a
swarm occurred in the NE part of Bárðarbunga with
a Ml 4 earthquake and since then the area has again
been more active, with earthquakes of magnitudes up
to Ml 3–4. Earthquakes at Bárðarbunga are usually
difficult to detect at Grímsfjall, the site of the closest
station, but stations that were installed around Hálslón reservoir in 2004–2006 (Figure 1) enhance the detectability considerably.
North of the Bárðarbunga volcanic center and just
south of Kistufell is another concentration of earthquakes. The seismic activity in this area increased
prior to the Gjálp eruption, especially in the last 6
months preceding and during the eruption (Figures 4i
and 6). The largest earthquakes observed were ∼Ml 3.
Only a few earthquakes were located in this area after
the Gjálp eruption until April 2003. Since then, continuous activity has been recorded with a few swarms
Figure 6. Earthquakes in NW-Vatnajökull larger than Ml 1 in 1994–2007. Location of the Gjálp 1996 volcanic
fissure is shown in green and the two Grímsvötn eruptions in 1998 and 2004 are shown with yellow and stars.
The boxes show areas plotted in Figure 4. – Kortið sýnir jarðskjálfta stærri en Ml ∼1 í norðvestanverðum
Vatnajökli á árunum 1994–2007. Gjálp gossprungan er sýnd með grænum lit og Grímsvatnagosstöðvarnar frá
1998 og 2004 eru sýndar með gulri og blárri stjörnu. Ferningarnir afmarka svæði sem notuð eru við gerð 4.
myndar.
JÖKULL No. 58, 2008
87
S. S. Jakobsdóttir
and earthquakes up to magnitude Ml 3–4. The observed activity has been increasing during the last few
years, but that may be biased by the fact that several
stations have been added to the network (Figure 1).
Observed seismic activity at Kverkfjöll has increased
very much in pace with the increasing number of stations in the area (Figures 4j and 6). The largest event
recorded there was magnitude Ml 3.5 earthquake in
November 1994. A few swarms have been detected
since 2005 the largest one, comprising 20 earthquakes
in the magnitude range Ml 1–3, occurred in August
2005. Some 50–70 earthquakes larger than Ml 1 are
now located in the area every year (Figure 4j).
Askja/Herðubreiðartögl
The most active area in the highland outside the ice
caps is Askja volcano and Herðubreiðartögl. Swarms
have been recorded at Herðubreiðartögl since 1996
(Figures 3 and 4k). The largest recorded swarm occurred in June-July 1998 with earthquakes up to magnitude Ml 4. This swarm occurred just before two
new stations were added to the network at about 50
km distance (Figure 1). The activity has been gradually increasing since late 2003. To some extent, this
coincides with the increased number of stations, but
not all the increased activity can be thus accounted
for as no stations were added near Herðubreiðatögl
from September 1998 until December 2004 when the
stations around Hálslón reservoir were deployed.
The earthquakes at the Askja volcanic center are
in general small (Figures 3 and 4l). The largest earthquake recorded in this period was of magnitude Ml
3.5 and occurred in January 2004. The activity is
mostly concentrated in the SE part of the volcanic center, but small swarms are repeatedly recorded north
and northwest of Askja. The activity is steady and in
recent years, with a denser network (Figure 1), earthquakes are recorded every year. The Askja caldera
has been subsiding, at least since 1983 (Sturkell et
al., 2006b). This is interpreted as a deflation of two
magma chambers at different depths (Sturkell et al.,
2006b; Pagli et al., 2006). Soosalu et al. (2008) studied the seismicity near Askja and Herðubreiðartögl
with data from temporary local networks operated in
2006 and 2007, in addition to data from the SIL system. They located most of the earthquakes at depths
88 JÖKULL No. 58, 2008
down to 7–8 km (Soosalu et al., 2008). In addition
they detected small, low frequency earthquakes occurring at depths between 15–34 km under most of the
area. Some of these can be found in the SIL database.
EARTHQUAKE SEQUENCES IN LESS
ACTIVE AREAS
In 1994–2007 earthquake swarms have occasionally
occurred at a few intraplate locations and in central
volcanoes and fissure swarms that generally show little activity. The magnitude of earthquakes in these
swarms have not exceeded Ml 3.5 and they are mostly
very small (Figures 7 and 8).
Vestfirðir
In February 1994 an earthquake of magnitude Ml 2.3
occurred in the northern part of Vestfirðir (Figure 7).
It was felt in the town of Ísafjörður, some 35 km away.
Only one earthquake was recorded, at a similar location as in 1964, when a single earthquake was located
in a swarm of felt earthquakes. In October 1994 six
earthquakes were located south of Arnarfjörður, some
85 km to the southwest from the February earthquake.
The swarm lasted nine days and the earthquakes were
in the magnitude range Ml 2–3. No more activity was
recorded in the NW part of Vestfirðir, but in September 2006 a swarm of earthquakes occurred close to an
extinct central volcano at Reykjarfjörður in the eastern part of Vestfirðir. Altogether 22 earthquakes in the
magnitude range Ml 1.5–3.5 were located in a period
of five days. (Þorbjarnardóttir et al., 2007). In 1996
and 1997 earthquakes were recorded offshore, about
25 km northeast of the Reykjarfjörður swarm.
Guðlaugstungur
Two seismic swarms were observed at Guðlaugstungur, about 20 km NNE of Langjökull, in 2004
(Figures 1 and 3). The first one lasted from the end
of February to April with most earthquakes occurring in March (Figure 8a). Over 50 earthquakes of
magnitudes Ml 1–2 were recorded in this period. In
September the activity resumed and four sequences
were recorded until the beginning of October. A couple of earthquakes reached Ml 3.5, both occurring towards the end of the swarm on 29 September and 1
Seismicity in Iceland: 1994–2007
October, respectively. Hjaltadóttir et al. (2005a) studied the swarms and found the first one grouped along
a N-S lineament while the latter was concentrated in
a NNW elongated dense cluster. Most of the earthquakes in both clusters were located at depths of 2-6
km, but in both sequences a few earthquakes reached
a focal depth of 13 km.
Eyjafjallajökull
Before the SIL system started, no earthquakes were
located beneath Eyjafjallajökull. In the first years,
1991–1993, some 70 earthquakes of magnitudes Ml
0–Ml 2.5 were located, mainly at the northern edge
and near the summit crater (Figure 5). Only three
earthquakes were detected in 1991, but at the end of
April 1992 the activity increased. At the end of May
1994 an intense swarm started at the northern flank
with 120 earthquakes recorded in one month (Figure
8b). The following September a swarm of 20 earthquakes occurred about 5 km north of the ice cap. In
the next months earthquakes were measured in both
areas and near the summit crater, but only a few earth-
quakes were detected in the southern part of the ice
cap. At the end of January 1995 the activity decreased
again. In February–April 1996 activity was again observed, starting with a swarm in the area north of the
ice cap, then a few small swarms near the crater and
ending with a small swarm south of the crater. The
next two and a half years were rather quiet. In December 1998 one more episode started with activity
concentrated at the northern flank of the ice cap. On
5 July 1999 a swarm was observed in the SE part of
the ice cap for the first time. From 10 August to 12
September and from 19 October to 29 November all
earthquakes were located in the southern part. Earthquakes occurred both in the south and the north for
the rest of the year. The last sequence in this episode,
with activity mainly at the northern margin of the ice
cap, lasted from 11 March to 30 May 2000. Since
then a few small swarms have occurred, mainly in the
northern part and north of Eyjafjallajökull.
The largest earthquake in Eyjafjallajökull, of magnitude Ml 3.6, occurred in a swarm on 1 March 1999.
Figure 7. Intra plate earthquakes located in Vestfirðir (Þorbjarnardóttir et al., 2008). – Jarðskjálftar, sem staðsettir hafa verið á Vestfjörðum.
JÖKULL No. 58, 2008
89
S. S. Jakobsdóttir
Figure 8. Magnitudes greater than Ml 1 as a function of time for areas shown by boxes in Figures 1 and 5. Note
that scales are different on different plots. a: Guðlaugstungur, b: Eyjafjallajökull, c: Öræfajökull, d: Esjufjöll,
e: Upptyppingar, f: Hengill. The gray line shows the accumulated number of earthquakes during the period. –
Stærð jarðskjálfta stærri en Ml 1 sem fall af tíma. Svæðin eru merkt inn á myndir 1 and 5 og eru eftirfarandi:
a: Guðlaugstungur, b: Eyjafjallajökull, c: Öræfajökull, d: Esjufjöll, e: Upptyppingar, f: Hengill. Athugið að
ekki er sami skali á öllum myndunum. Gráa línan sýnir uppsafnaðan fjölda skjálfta yfir tímabilið.
Sturkell et al. (2003a) and Pedersen and Sigmundsson
(2004, 2006) have demonstrated two periods of uplift
under Eyjafjallajökull. From tilt-, GPS- and InSAR
data they find an uplift of the order of 15 cm centered
at the southern margin of the ice cap in the time interval from 29 September 1993 to 19 September 1994
90 JÖKULL No. 58, 2008
and a second uplift starting between 19 July and 20
August 1999 and ending between February and May
2000. No uplift was found in relation to the swarms in
1996. Pedersen and Sigmundsson modeled the uplifts
as sill formations at 4.5 and 6.3 km depths respectively with a variable amount of opening. This is in
Seismicity in Iceland: 1994–2007
JÖKULL No. 58, 2008
91
S. S. Jakobsdóttir
good agreement with Dahm and Brandsdóttir (1997)
who found the best located earthquakes during the
swarm in May and June 1994 at depths of 4.7–6.3 km.
Hjaltadóttir et al. (2007) relocated earthquakes in the
SIL database and the relocated hypocentres, both in
1994 and 1999, formed horseshoe-shaped clusters under the northern margin at depths between 9 and 11
km. This is well below the proposed sill intrusion and
probably represents a feeding channel. They located
the 1996 activity at 20–23 km depth and suggested
that the deep origin of the earthquakes might explain
the lack of uplift associated with that activity. They
also mapped two smaller clusters in 1999, one situated at 6 km depth near the summit crater about 2.5
km west of the main activity and the other at 10 km
depth, 2 km to the south. A small cluster in 2006 was
mapped to a depth of 16 km beneath the easternmost
of the two northward going outlet glaciers.
Öræfajökull
Some 50 earthquakes in the magnitude range Ml 0.5–
2.7 have been recorded under Öræfajökull since 1995,
mostly in two swarms occurring in October 2002 and
December 2005 and also during March to September
2007 (Figure 8c). In the 2002 swarm 8 earthquakes of
magnitudes up to Ml 2 were detected. The swarm in
2005 included 10 earthquakes, the largest one again
of magnitude Ml 2. A Ml 2.7 earthquake occurred in
March 2005. In 2007, 11 earthquakes were recorded,
all smaller than Ml 2.
92 JÖKULL No. 58, 2008
Esjufjöll
Two swarms have been observed in the southern part
of the Esjufjöll volcanic center during the study period. The first swarm started in late November 1996
with two Ml 2 earthquakes (Figure 8d). No seismicity was detected in the following weeks or until the
swarm resumed on 21 December. During the next 5
days, 15 earthquakes were recorded in the magnitude
range Ml 1–2.5 and on January 3 the swarm was completed with a magnitude Ml 2 earthquake. The closest
stations at that time were at ∼60 and ∼95 km distance, both installed in October 1996.
The second swarm occurred in 2002 (Figure 8d).
It started in a similar way with a single ∼Ml 2
earthquake detected on 25 September. More than
three weeks later the activity resumed and over 90
earthquakes in the magnitude range Ml 0.8–3.5 were
recorded in the second half of October. The two
largest earthquakes, Ml 3–3.5, occurred on 21 October. A few earthquakes were recorded a week later
and finally about a month later, on 16 December, six
more earthquakes with magnitudes up to Ml 2.5 occurred within a couple of hours. At this time the two
closest stations were at about 40 km distance from Esjufjöll. Since 2005, when stations were added north
of Vatnajökull, earthquakes have been detected every
year in the Esjufjöll volcanic centre.
Upptyppingar
In February 2007 seismic activity began near Upptyppingar in the Kverkfjöll fissure swarm (Figures 3
Seismicity in Iceland: 1994–2007
and 8e). The seismic activity consisted of many small
swarms and was increasing in the first months with a
few periods of low activity in between. It culminated
in July–August and then calmed down again. A new
burst of swarms started on 16 October and lasted for
a month. The most intensive swarm by end of 2007
began on 7 December with over 1000 earthquakes
recorded in five days. It is still ongoing although a
little calmer at the time of writing (Jakobsdóttir et al.,
2008).
Since February 2007, almost 5000 earthquakes
have been detected in the magnitude range Ml -0.1–
2.2, but only a couple of earthquakes have reached
magnitude Ml 2. The earthquakes are deep, 12–20
km, and epicenter locations are concentrated to small
areas that migrate in time. The hypocenters do not define a single plane, but form an elongated cluster that
dips ∼45◦ toward the south. Since August the activity has migrated to the northeast to the shallower end
of the cluster (depth range 12–17 km). Such a long
lasting swarm of deep earthquakes has never been observed before in Iceland, but with the station spacing
prior to 1998 it would probably not have been detected. It is suggested here that this episode might
be a formation of a magma chamber or a pluton with
repeated injections of sills (Kavanagh et al., 2006).
LARGER EVENTS
The June 2000 earthquake sequence
On 17 June 2000 at 15:40 h an earthquake that was
widely felt in SW-Iceland struck in the middle of the
South Iceland Lowland (SIL). The magnitude of this
earthquake is given by NEIC as Ms 6.6 (Table 1).
The earthquake was expected, as long term statistical forecast stated a ∼90% probability of an earthquake larger than magnitude 6 to occur in the area
within 20 years and for various reasons it was expected to occur in the seismic gap observed at the
epicenter area of the 17 June earthquake (Stefánsson
and Halldórsson, 1988). It had also been pointed out
that the gap coincided with a long-term concentration
of microearthquake activity in the seismic zone (Stefánsson et al., 1993) and although it was assumed more
likely that the microearthquakes indicated high stress
in preparation of a large earthquake it could not be
excluded that they reflected aseismic strain release.
At least three earthquakes were triggered by the S
wave of the main shock in SISZ and on the Reykjanes Peninsula (RP) during the first 30 seconds (Figure
3) (Árnadóttir et al., 2003; Vogfjörð, 2003; Vogfjörð
et al., 2005a; Anatoli et al., 2006). The magnitudes
were in the range 3.5–5.5. Some additional M5 earthquakes occurred along the zone in the next minutes
(Table 1) and thousands of aftershocks were measured
along the 100 km long belt in the following days. The
area around Geysir, some 35 km north of the main
shock, was activated and showed increased seismicity
for about a year. One of the aftershock sites in the
SISZ was south of the lake Hestvatn. As the activity
continued there and based on the knowledge of former
strain release in SISZ, a forecast was given to the Civil
Protection Department that an earthquake of similar
size, centered near Hestvatn, might strike at any time.
The warning was issued 25 hours prior to the Ms 6.6
earthquake which struck early on 21 June, within the
box given by the forecast as the most likely location (Stefánsson et al., 2000, 2003). The June 2000
earthquakes have been studied by several authors and
they were the backbone of the EU project PREPARED
(Stefánsson 2006; Stefánsson et al., 2006a,b). Hjaltadóttir and Vogfjörð (2004, 2005) mapped the main
faults with relative locations of the aftershocks. They
found them composed of several segments, reaching
to 10 km depth. The faults have the overall strike of
7◦ and 179◦ , respectively. They are both near vertical except that the northern part of the Hestvatn fault
dips 77◦ east. This is in good agreement with fault
plane solutions obtained by Harvard, that give strike
4◦ , dip 87◦ and rake -164◦ for the 17 June earthquake
and strike 2◦ , dip 85◦ and rake -167◦ for the 21 June
event. The CSEM solutions give strike 16◦ , dip 68◦ ,
rake -158◦ and strike 358◦, dip 85◦ , rake -175◦ , respectively. The faults have also been modeled to fit
various types of data with similar results. Pagli et
al. (2003) and Pedersen et al. (2001, 2003) modeled
the faults on the Reykjanes Peninsula and in SISZ using InSAR data and GPS data whereas Suhadolc used
strong motion data to constrain his model (Hjaltadóttir
et al., 2005b; Stefánsson et al., 2006b). Árnadóttir et
JÖKULL No. 58, 2008
93
S. S. Jakobsdóttir
al. (2001, 2003, 2004, 2005) have studied the crustal
deformation before, during and after the earthquakes
using GPS data and Jónsson et al. (2003) compared
post-earthquake ground movement with water level in
boreholes. Lindman et al. (2006) studied the physics
of the aftershock sequences.
No casualties were caused by the earthquakes, but
some buildings had to be abandoned due to damages.
The Hengill-Ölfus episode in 1994–1998
In August 1994 a swarm of more than 5000 earthquakes occurred near Hrómundartindur north of
Hveragerði. A slight increase in seismicity had been
observed already a month earlier. The swarm started
on 13 August with a magnitude Ml 3.5 earthquake
and 6 more earthquakes of magnitude >Ml 3.5 occurred during the next 8 days. The swarm lasted 10
days. The largest earthquake in the swarm reached
magnitude Ml 4 and occurred on 20 August. This
swarm marked the beginning of an unusually active
period at the triple junction near Hengill (Figure 8f)
(Rögnvaldsson et al., 1998b,c). It lasted for about
five years and culminated in two ∼Ml 5 earthquakes
in June and November 1998. Over 90,000 earthquakes were located altogether in this episode, which
makes 36% of the earthquakes in the SIL data base
for the whole period studied in this paper. The activity was mainly concentrated north of Hveragerði,
but increased seismicity was simultaneously observed
on a WSW-ENE trending belt southwest of Hveragerði and west of SISZ. The Ml 4.7 earthquake (NEIC
5.40 Mw , Table 1), that occurred on 4 June 1998, ruptured the area between these two concentrations and
the Ml 4.9 (NEIC 5.10 Mw ) earthquake on 13 November 1998 originated in the southern belt. Vogfjörð et
al. (2005b) mapped in detail the faults that were active in 1996–1998. They find near vertical NE and
E striking faults most common with right lateral slip
on NE striking faults and left lateral slip on E striking
faults. Normal faulting is also seen. An uplift of ∼7.5
cm centered south of Hrómundartindur volcanic center was observed from interferograms covering the period from July 1993 to September 1998, using InSAR
(Feigl, 2000). Uplift was also seen in leveling data
from 1992–1995 (Sigmundsson et al., 1997). The uplift was interpreted as magma injection and modeled
94 JÖKULL No. 58, 2008
with a Mogi source at a depth of 6.5–7 km. Neither of
these uplift studies included the November 1998 rupture.
Other earthquakes greater than M 5.
The NEIC bulletin reports 10 earthquakes with magnitude greater than 5 in the time interval 1994–2007
(Figure 3, Table 1). Two of these are the 1998 earthquakes in the Hengill episode and three more belong
to the June 2000 episode in SISZ. The first ∼M 5
earthquake occurred on 8 February 1994 some 35 km
off the north coast in the western part of the TFZ. The
local magnitude Ml is 4.8 while the NEIC bulletin
gives Mw 5.5 (Table 1). This was the first earthquake
in a swarm that lasted for 3 days with more than 800
recorded earthquakes on a NW trending extension of
the Húsavík-Flatey fault.
The next big earthquake occurred in Bárðarbunga
on 5 May 1994. The local magnitude was Ml 4.2
while the NEIC magnitude was mb 5.7. Only 2 aftershocks were detected. The following year, on 11
December 1995, a Ml 3.9 earthquake (NEIC mb 4.9)
struck Bárðarbunga and again on 29 September 1996
a Ml 4.4 (NEIC Mw 5.6) earthquake occurred (Table
1). This event marked the onset of seismic unrest
under Bárðarbunga that culminated in an eruption in
Gjálp, SE of Bárðarbunga, 1 1/2 days later (see also
the section: Seismic signature of eruptions). No earthquakes M>4.5 have been reported by NEIC in Bárðarbunga since the Gjálp eruption.
On 16 September 2002 a Ml 5.0 (NEIC Mw 5.8)
was recorded in the northern part of the TFZ, 53 km
NNV of Grímsey (Þorbjarnardóttir and Guðmundsson 2003). More than 450 aftershocks were observed
in the following 5 days. The location of the swarm
is near the submarine mountain Stóragrunn that was
mapped by Brandsdóttir et al. (2002).
The last earthquake larger than 5 reported by
NEIC was a Mw 5.2 earthquake with Ml 4.3 that occurred on 23 August 2003 (Table 1). The epicenter location was at Sveifluháls on the Reykjanes Peninsula,
3 km north of Krísuvík. The aftershock distribution
was along a 6.5 km long belt striking NS and along a
4 km belt striking ∼60◦ to ENE from the main shock.
A north-south oriented strike slip fault is in agreement
with the optimum fault plane solution (Guðmundsson
Seismicity in Iceland: 1994–2007
et al., 2004). Vogfjörð et al. (2004, 2005a) mapped
the fault using aftershocks and GPS data. The result
reveals a vertical strike-slip event striking N-S.
SEISMIC SIGNATURE OF ERUPTIONS
The buildup of pressure under active volcanoes is often manifest as an increase in seismic activity. The
earthquakes associated with this increased pressure
and eventual magma movement are often very small.
With a more sensitive seismic network in the vicinity
of volcanoes, the possibility of detecting the preparation phase of an eruption is enhanced.
Gjálp 1996
When Gjálp, in northwestern Vatnajökull (Figure 6),
erupted in October 1996 the SIL network consisted
mostly of two network clusters, one at the SISZ and
another at the TFZ. Two highland stations, some 100
and 50 km west of Bárðarbunga, were implemented a
month before and the day after the onset of the eruption. Nevertheless some increase in seismic activity
was observed at Bárðarbunga from the end of August 1995 and the most intensive swarm recorded by
the SIL system at Lokahryggur occurred in February
1996. No earthquakes were recorded at the location
of the Gjálp fissure in the months prior to the eruptions. The volcanic episode started with a Mw 5.6
earthquake in Bárðarbunga at 10:48 on 29 September
(Table 1). Nettles and Ekström (1998) find the fault
mechanism consistent with reverse faulting propagating, as a series of subevents, around the surface of
a cone. The intensive aftershock activity shifted to
the Gjálp location some hours before the onset of the
subglacial eruption in the evening of 30 September
(Einarsson et al., 1997; Vogfjörð 1999). The eruption lasted 2 weeks and has been described by several authors (Guðmundsson et al., 1997; Einarsson et
al., 1997). Seismic activity continued at Gjálp and
Lokahryggur until June 1997.
Grímsvötn 1998 and 2004
The seismic behavior of the two Grímsvötn eruptions
in 1998 and 2004 was in many ways similar (Figures 4g and 6). Increased seismic activity was detected some months before the eruptions and the area
became quiet for a while after the eruptions. The
first earthquakes prior to the eruption in December
1998 were recorded in March the same year and then
one event was recorded every month until November
when three earthquakes were detected. Early morning
on 18 December an earthquake swarm was observed
and the eruption started at about 9:30. No earthquake
was located in Grímsvötn for months after the eruption. The two closest stations at that time were about
55 km away. Sturkell et al. (2003b) observed a deflation of a magma chamber associated with the eruption. Before the eruption they had measured uplift of
the caldera rim, Grímsfjall. The uplift started again
after the eruption implying a new inflation period.
In 2001 a seismic station was installed on the
caldera rim on Grímsfjall and five more stations were
within a distance of 110 km around the Vatnajökull
ice cap. As a result, earthquakes were now measured
every month in Grímsvötn. In July 2003, 15 months
prior to the second eruption, an increase was observed
in the seismicity and on 17 October 2004 the seismic activity was again seen to increase. Monitoring
of icequakes and microseismicity revealed a glacial
flood starting from Grímsvötn caldera on 29 October
and bursting out through an outlet glacier the following day. Two days later, in the early morning of 1
November, a magnitude Ml 3 earthquake occurred in a
swarm that lasted for a few hours. At 19:30 the swarm
resumed as the magma started to move towards the
surface marking the onset of the eruption (Vogfjörð et
al., 2005c). Very few earthquakes were recorded in
the area during the next year (Figure 4g).
Hekla 2000
Hekla volcano is known to be seismically quiescent
between eruptions. Only a few earthquakes are detected and they generally have magnitudes Ml <1. The
first earthquake recorded by the SIL system before the
Hekla eruption on 26 February 2000 occurred at 17:05
h with magnitude Ml 0.3. The biggest earthquake in
the swarm that followed had magnitude Ml 2.2 and
occurred at 18:17 h, which is two minutes before the
onset of the eruption. After the first hours of the eruption no locatable earthquakes were observed, but continuous tremor was recorded. This is typical behavior,
similar to what was observed prior to and during the
eruption in Hekla in 1991. Further studies of Hekla,
JÖKULL No. 58, 2008
95
S. S. Jakobsdóttir
based on SIL data and data from a permanent analog network (Einarsson and Björnsson, 1976, 1987)
are found in Soosalu and Einarsson (2004, 2006) and
Soosalu et al. (2005).
DISCUSSION AND CONCLUSIONS
The seismicity in Iceland during 1994–2007 has been
described above. The detection threshold for different areas has been changing in this period, but instead of limiting the study to earthquakes of magnitudes higher than the magnitude of completeness in
1994, an effort has been made to compare differences
in observed seismic activity at different times. As
the network becomes denser, new areas of activity are
detected and the information obtained becomes more
detailed. One of the details that are emerging in the
last years is the occurrence of deep earthquakes. It
is now confirmed that deep earthquakes of a tectonic
type occur near Eyjafjallajökull (Hjaltadóttir et al.,
2007) and at Upptyppingar, while deep earthquakes
observed near Askja are LP earthquakes with emergent onsets (Soosalu, 2008). It has also been possible to detect an imminent volcanic eruption at an earlier stage with lower detection thresholds. Intraplate
earthquakes are occasionally observed, both as single
earthquakes and as swarms.
There are 250,000 recorded earthquakes in the
SIL database for the period 1994-2007. Of these
over 200,000 were recorded in the seismic zones:
TFZ (48,800), SISZ (35,200), Hengill triple junction
(99,200) and RP (18,900), but these areas have had the
lowest detection threshold during the whole period.
On Reykjanes Ridge, around Eldey and Geirfuglasker,
some 2000 earthquakes were detected. 16,600 earthquakes were observed in Mýrdalsjökull and 3,700 in
NW-Vatnajökull. About 8,000 earthquakes have been
recorded in the highland north of Vatnajökull, where
5,000 of them occurred at Upptyppingar in 2007.
Bárðarbunga has resumed its previous activity after a
four-year-period of quiet following the Gjálp eruption
and seismicity near Kistufell seems to be increasing.
Seismic activity in Herðubreiðartögl has been increasing in the last four years.
96 JÖKULL No. 58, 2008
Acknowledgements
This paper is based on the work of the monitoring team at the Physics Department of the Icelandic Meteorological Office over the 14 year period
from 1994 to 2007. Reynir Böðvarsson and Ragnar Slunga are the main designers of the SIL-system.
Gunnar B. Guðmundsson and Matthew J. Roberts
contributed most of the figures. Gunnar, Matthew
and Bergþóra Þorbjarnardóttir critically read and improved the manuscript. All maps were made with
GMT software (Wessel and Smith, 1998). The graphs
were made using R (R Development Core Team,
2008). Insightful comments by the reviewers, Björn
Lund and Sveinbjörn Björnsson, helped to improve
the paper.
ÁGRIP
Jarðskjálftavirkni á Íslandi á árunum 1994 til 2007
Seinni hluta árs 1993 voru settar upp fimm stafrænar
jarðskjálftamælistöðvar í kringum Tjörnesbrotabeltið til viðbótar 11 stöðvum á Suðurlandi og stöð á
Akureyri. Þar með varð hið sjálfvirka jarðskjálftamælikerfi, SIL-kerfið, að landsneti jarðskjálftamæla.
Sumarið 1994 var sett upp stöð í Borgarfirði. Á næstu
árum óx kerfið úr 18 stöðvum, árið 1994, í 51 stöð í
árslok 2007. Stöðvarnar eru að mestu leyti staðsettar
í kringum flekaskilin (1. mynd). Eftir því sem stöðvum fjölgaði jókst næmni kerfisins, sérstaklega á hálendinu. Hér er fjallað um jarðskjálftavirkni á Íslandi
frá ársbyrjun 1994. Á tímabilinu hafa flestir skjálftar á landinu stærri en 2 verið staðsettir, en á sumum svæðum hafa mun smærri skjálftar verið skráðir.
Þannig hafa nánast allir skjálftar niður í stærð ∼0 verið skráðir á Suðurlandsundirlendi og skjálftar stærri
en ∼1 í Tjörnesbrotabeltinu. Í lok tímabilsins voru
stöðvar orðnar það margar að flestir skjálftar á landinu stærri en ∼1–1,5 voru skráðir, nema skjálftar á
Vestfjörðum. Í stað þess að takmarka umfjöllunina í
þessari grein við skjálfta stærri en þá sem mælanlegir
voru árið 1994, er gerð grein fyrir aukinni næmni á
mismunandi svæðum samhliða umræðunni um virknina. Töluvert er fjallað um smáskjálftavirkni, bæði
á vel þekktum jarðskjálftasvæðum sem og á svæðum
þar sem jarðskjálftar eru óalgengir.
Seismicity in Iceland: 1994–2007
Um 250.000 jarðskjálftar voru skráðir í SILkerfinu á þessu tímabili (2. og 3. mynd). Þar af
mældust um 48.800 skjálftar í Tjörnesbrotabeltinu,
um 35.200 í Suðurlandsbrotabeltinu og um 18.900 á
Reykjanesskaga. Mynd 4. sýnir skjálftavirkni sem fall
af tíma fyrir helstu jarðskjálftasvæðin á hálendinu og
undir jöklunum. Svæðin sem um er að ræða eru merkt
á 1., 5. og 6. mynd.
Tvær skjálftahrinur hafa mælst á Vestfjörðum,
annars vegar við Arnarfjörð árið 1994 og hins vegar
við Reykjarfjörð árið 2006 (7. mynd).
Á 8. mynd má sjá skjálftavirkni sem fall af tíma
fyrir nokkur svæði þar sem jarðskjálftavirkni er stopulli og fyrir Hengilssvæðið, sem er eitt virkasta svæði
landsins. Á Hengilssvæðinu - á mótum vestra gosbeltisins, Suðurlandsbrotabeltisins og Reykjanesskaga mældust ∼99.200 skjálftar á tímabilinu. Á þessu
svæði var sérstaklega mikil skjálftavirkni á árunum
1994 til 1998 og náði hún hámarki með tveimur
skjálftum af stærð ∼5 í júní og nóvember 1998. Á
sama tíma lyftist land á svæðinu og hefur atburðurinn
verið túlkaður sem uppsöfnun kviku á um 7 km dýpi.
Á Hengilssvæðinu hefur næmnin verið mikil á öllu
tímabilinu og mælast þar skjálftar allt niður í stærð -2.
Kortið á 5. mynd sýnir skjálfta stærri en 1 undir Eyjafjallajökli og Mýrdalsjökli. Virkni undir Eyjafjallajökli mældist mest í hrinum á árunum 1994, 1996
og 1999 (mynd 7b) og er talin tengjast innskotavirkni.
Undir Mýrdalsjökli er virknin mest við Goðabungu
(mynd 4d). Flestir skjálftar mælast þar síðsumars og
fram á vetur. Á árunum 2001 til 2005 var virknin mikil allt árið, en þó með árstíðabundinni aukningu. Tvær
kenningar eru um orsakir virkninnar, annars vegar að
hraungúll mjakist í átt að yfirborði undir Goðabungu
og hins vegar að virknin sé í raun ísskjálftar tengdir
jöklinum. Töluvert minni jarðskjálftavirkni er undir Kötluöskjunni og hún er ekki eins greinilega tengd
árstíðum (mynd 4e). Landlyfting mældist undir öskjunni á árunum 2001 til 2004, en við Goðabungu hefur
bæði mælst lyfting og sig.
Tveir stórir skjálftar urðu á Suðurlandi í júní
2000. Bylgjan frá fyrri skjálftanum, þann 17. júní,
setti af stað röð skjálfta af stærð 3.5–5.5 vestan við
stóra skjálftann, allt vestur að Kleifarvatni. Þúsundir
skjálfta mældust á Suðurlandi á næstu vikum og mán-
uðum á eftir.
Yfirvofandi eldgos má yfirleitt sjá út frá skjálftavirkni. Mismunandi eldfjöll sýna mismunandi aðdraganda. Þannig hefst skjálftavirkni undir Heklu
rúmlega klukkutíma fyrir gos, en almennt má sjá
aukna jarðskjáftavirkni dögum og jafnvel árum fyrir eldgos. Þannig mátti sjá lengri aðdraganda að
Gjálpar- og Grímsvatnagosunum í Vatnajökli. Fyrir gosið í Grímsvötnum 2004 mátti sjá aukna virkni
meira en ári fyrir gos (mynd 4g) og skjálftavirknin jókst enn meira um tveimur vikum fyrir gos. Í
báðum gosunum í Grímsvötnum á tímabilinu mátti
greina tilsvarandi aukningu fyrir gos og eftir goslok
datt skjálftavirknin alveg niður í marga mánuði á eftir
í báðum tilfellum (mynd 4g). Mun fleiri mælistöðvar voru í nágrenni gosstöðvanna árið 2004 en í gosinu
1998. Í Gjálpargosinu hófust forskjálftar við Bárðarbungu og skjálftavirknin færðist síðan að gosstöðvunum rétt fyrir gos. Eftir gosið hélt skjálftavirkni áfram
í nokkra mánuði í Bárðarbungu (mynd 4h) og á Lokahrygg (mynd 4f). Mesta jarðskjálftavirknin á Lokahrygg á tímabilinu mældist í kjölfar gossins.
REFERENCES
Allen, R., G. Nolet, W. J. Morgan, K. S. Vogfjörð, M. Netles, G. Ekström, B. H. Bergsson, P. Erlendsson, G. Foulger, S. S. Jakobsdóttir, B. Julian, M. Pritchard, S. Ragnarsson and R. Stefánsson 2002. Plume driven plumbing
and crustal formation in Iceland. J. Geophys. Res. 107(B8),
doi:10.1029/2001JB000584.
Antonioli, A., M. E. Belardinelli, A. Bizzarri and K. S. Vogfjörð
2006. Evidence of instantaneous dynamic triggering during
the seismic sequence of year 2000 in south Iceland. J. Geophys. Res. 111(B03302), doi:10.1029/2005JB003935.
Ágústsson, K. 2006. Mat á næmni SIL jarðskjálftamælinetsins:
Hugmyndir um framtíðaruppbyggingu. Icelandic Meteorological Office-Report 06014, 22 pp.
Árnadóttir, Þ., S. Hreinsdóttir, G. B. Guðmundsson, P. Einarsson,
M. Heinert and C. Völksen 2001. Crustal deformation measured by GPS in the South Iceland Seismic Zone due to two
large earthquakes in June 2000. Geophys. Res. Lett. 28(21),
4031–4033.
Árnadóttir, Þ., S. Jónsson, R. Pedersen and G. B. Guðmundsson
2003. Coulomb stress changes in the South Iceland Seismic
Zone due to two large earthquakes in June 2000. Geophys.
Res. Lett. 30(5), doi:10.1029/2002GL016495.
Árnadóttir, Þ., H. Geirsson and P. Einarsson 2004. Coseismic
stress changes and crustal deformation on the Reykjanes
Peninsula due to triggered earthquakes on 17 June 2000. J.
Geophys. Res. 109(B09307), doi:10.1029/2004JB003130.
JÖKULL No. 58, 2008
97
S. S. Jakobsdóttir
Árnadóttir, Þ., S. Jónsson, F. F. Pollitz, W. Jiang and K. L.
Feigl 2005. Postseismic deformation following the June 2000
earthquake sequence in the South Iceland Seismic Zone. J.
Geophys. Res. 110(B12308), doi:10.1029/2005JB003701.
Bjarnason, I. Th. 2008. An Iceland hotspot saga. Jökull 58, this
issue.
Brandsdóttir, B., R. Detrick, G. Helgadóttir, E. Kjartansson, B.
Richter, K. Gunnarsson, S. Þ. Guðlaugsson, N. Driscoll
and G. Kent 2002. A new perspective of the tectonics of
the Tjörnes Fracture Zone, offshore northern Iceland, from
EM300 Multibeam Bathymetry, High Resolution MCS and
CHIRP Sonar Profiles. Eos Trans. AGU 83, F731.
Böðvarsson, R., S. Th. Rögnvaldsson, S. S. Jakobsdóttir, R.
Slunga and R. Stefánsson 1996. The SIL data acquisition and
monitoring system. Seism. Res. Lett. 67, 35–46.
Böðvarsson, R., S. Th. Rögnvaldsson, R. Slunga and E. Kjartansson 1999. The SIL data acquisition system-at present and
beyond year 2000. Phys. Earth Planet. Inter. 113, 89–101.
Dahm, T. and B. Brandsdóttir 1997. Moment tensors of microearthquakes from the Eyjafjallajökull volcano in South Iceland. Geophys. J. Int. 130, 183–192.
Einarsson, P. 1991. Earthquakes and present-day tectonism in Iceland. Tectonophysics 189, 261–279.
Einarsson, P. 2008. Plate boundaries, rifts and transforms in Iceland. Jökull 58, this issue.
Einarsson, P. and S. Björnsson 1976. Seismic activity associated
with the 1970 eruption of Volcano Hekla in Iceland. Jökull
26, 8–19.
Einarsson, P. and S. Björnsson 1987. Jarðskjálftamælingar á
Raunvísindastofnun Háskólans. In: Þ. I. Sigfússon (editor),
Í hlutarins eðli. Festschrift for Þorbjörn Sigurgeirsson, Menningarsjóður, Reykjavík, 251–278.
Einarsson, P. and K. Sæmundsson 1987. Earthquake epicenters
1982–1985 and volcanic systems in Iceland (map). In: Þ. I.
Sigfússon (editor), Í hlutarins eðli. Festschrift for Þorbjörn
Sigurgeirsson, Menningarsjóður, Reykjavík.
Einarsson, P., B. Brandsdóttir, M. T. Guðmundsson, H. Björnsson, K. Grönvold and F. Sigmundsson 1997. Unrest near the
center of the Iceland hotspot: The October 1996 subglacial
eruption in Vatnajökull. Eos Trans. AGU 78, 369, 373–375.
Einarsson P. and B. Brandsdóttir 2000. Earthquakes in the
Mýrdalsjökull area, Iceland, 1978–1985: Seasonal correlation and connection with volcanoes. Jökull 49, 59–73.
Feigl, K., J. Gasperi, F. Sigmundsson and A. Rigo 2000. Crustal
deformation near Hengill volcano, Iceland 1993–1998: Coupling between magmatic activity and faulting inferred from
elastic modelling of satellite radar interferograms. J. Geophys. Res. 105, 25655–25670.
Guðmundsson, G. B., S. S. Jakobsdóttir and B. S. Þorbjarnardóttir
2004. Seismicity in Iceland 2003. Jökull 54, 67–74.
Gudmundsson, M. T., F. Sigmundsson and H. Björnsson 1997.
Ice-volcano interaction of the 1996 Gjálp subglacial eruption,
Vatnajökull, Iceland. Nature 389, 954–957.
Hjaltadóttir, S. and K. S. Vogfjörð 2004. WP5.1. Mapping
subsurface faults in southwestern Iceland with the microearthquakes induced by the June 17th and June 21st
98 JÖKULL No. 58, 2008
earthquakes. In: PREPARED-first periodic report. February
1, 2003-January 31, 2004. Icelandic Meteorological OfficeReport 04014, 80–84.
Hjaltadóttir S. and K. S. Vogfjörð 2005. Subsurface fault mapping
in Southwest Iceland by relative location of aftershocks of
the June 2000 earthquakes. Icelandic Meteorological OfficeResearch Report 21, 18 pp.
Hjaltadóttir, S., H. Geirsson and Þ. Skaftadóttir 2005a. Seismicity
in Iceland 2004. Jökull 55, 107–119.
Hjaltadóttir, S., K. S. Vogfjörð, Þ. Árnadóttir, P. Einarsson and
P. Suhadolc 2005b. A model of the release of the two June
2000 earthquakes based on all available observations. Icelandic Meteorological Office-Report 05020, 9 pp.
Hjaltadóttir, S., K. S. Vogfjörð and R. Slunga 2007. Relocated
earthquakes in Eyjafjallajökull between 1991 and 2006: Does
deep seismicity indicate intruding material from below? In:
ESC Working Group “Earthquakes and Volcanoes" Meeting:
"Hot and Cold: Seismicity associated with geothermal areas
and ice-covered volcanoes”, abstract, Nesjavellir, Iceland.
Höskuldsson, Á., R. Hey, E. Kjartansson and G. B. Guðmundsson
2007. The Reykjanes Ridge between 63◦ 10’N and Iceland.
J. Geodynamics 43, 73–86.
Jakobsdóttir, S. S., G. B. Guðmundsson and R. Stefánsson 2002.
Seismicity in Iceland 1991–2000 monitored by the SIL seismic system. Jökull 51, 87–94.
Jakobsdóttir, S. S., M. J. Roberts, G. B. Guðmundsson, H. Geirsson and R. Slunga 2008. Earthquake swarms at Upptyppingar, North-east Iceland: a sign of magma intrusion? Studia
Geophysica et Geodaetica, accepted.
Jónsdóttir, K., A. Tryggvason, R. Roberts, B. Lund, H. Soosalu
and R. Böðvarsson 2007. Habits of a glacier covered volcano: Seismicity and structure study of the Katla volcano,
South Iceland. Annals of Glaciology 45, 169–177.
Jónsdóttir, K., R. Roberts, A. Tryggvason, B. Lund, V. Pohjola,
S. S. Jakobsdóttir and R. Böðvarsson 2008. Local lp-events
study in a glaciated volcanic environment in south Iceland.
In: Abstract of the EGU General Ass., Vienna, Austria.
Jónsson, S., P. Segall, R. Pedersen and G. Björnsson 2003. Postearthquake ground movements correlated to pore-pressure
transients. Nature 424, 179–183.
Kavanagh, J. L., T. Menand and R. S. J. Sparks 2006. An experimental investigation of sill formation and propagation in
layered elastic media. Earth Planet. Sci. Lett. 245, 799–813.
doi:10.1016/j.epsl.2006.03.025.
Lindman, M., B. Lund, R. Roberts and K. Jónsdóttir 2006.
Physics of the Omori law: Inferences from interevent time
distributions and pore pressure diffusion modeling. Tectonophysics 424, 209–222.
Lippitsch, R., R. S. White and H. Soosalu 2005. Precise hypocenter relocation of microearthquakes in a high-temperature
geothermal field: the Torfajökull central volcano, Iceland.
Geophys. J. Int. 160, 371–388.
Nettles, M. and G. Ekström 1998. Faulting mechanism of anomalous earthquakes near Bárdarbunga Volcano, Iceland. J. Geophys. Res. 103, 17,973–17,983.
Seismicity in Iceland: 1994–2007
Pagli, C., R. Pedersen, F. Sigmundsson and K. L. Feigl 2003. Triggered fault slip on June 17, 2000 on the Reykjanes Peninsula,
SW Iceland captured by radar interferometry. Geophys. Res.
Lett. 30(6), 1273, doi:10.1029/2002GL015310.
Pagli, C., F. Sigmundsson, Þ. Árnadóttir, P. Einarsson and E.
Sturkell 2006. Deflation of the Askja volcanic system: contraints on the deformation source from combined inversion of
satellite radar interferograms and GPS measurements. Journ.
Volc. Geotherm. Res. 152, 97–108.
Pedersen, R., F. Sigmundsson, K. L. Feigl and Þ. Árnadóttir 2001.
Co-seismic interferograms of two Ms = 6.6 earthquakes in
the South Iceland Seismic Zone, June 2000. Geophys. Res.
Lett. 28, 3341–3344.
Pedersen, R., S. Jónsson, Þ. Árnadóttir, F. Sigmundsson and K. L.
Feigl 2003. Fault slip distribution of two Mw = 6.5 earthquakes in South Iceland estimated from joint inversion of
InSAR and GPS measurements. Earth Plan. Sci. Lett. 213,
487–502.
Pedersen, R. and F. Sigmundsson 2004. InSAR based sill model
links spatially offset areas of deformation and seismicity for
the 1994 unrest episode at Eyjafjallajökull volcano. Iceland.
Geophys. Res. Lett. 31, L14610.
Pedersen, R. and F. Sigmundsson 2006. Temporal development
of the 1999 intrusive episode in the Eyjafjallajökull volcano,
Iceland, derived from InSAR images. Bull. Volc. 68, 377–
393.
Pinel, V., F. Sigmundsson, E. Sturkell, H. Geirsson, P. Einarsson,
M. T. Guðmundsson and Þ. Högnadóttir 2007. Discriminating volcano deformation due to magma movements and variable surface loads: application to Katla subglacial volcano,
Iceland. Geophys. J. Int. 169(1), 325–338.
R Development Core Team 2008. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0. URL
http://www.R-project.org.
Roberts, M. J. 2005. Aerial and seismic observations of the August 2005 jökulhlaup from Grænalón. Icelandic Meteorological Office-Report 05022, 18 pp.
Rögnvaldsson, S. Th. and R. Slunga 1993. Routine fault plane solutions for local and regional networks: a test with synthetic
data. Bull. Seism. Soc. Am. 11, 1247–1250.
Rögnvaldsson, S. Th. and R. Slunga 1994. Single and joint
fault plane solutions for microearthquakes in South Iceland.
Tectonophysics 237, 73–86.
Rögnvaldsson, S. Th., Á. Guðmundsson and R. Slunga 1998a.
Seismotectonic analysis of the Tjörnes Fracture Zone, an
active transform fault in north Iceland. J. Geophys. Res.
103(B12), 30,117Ů30,129.
Rögnvaldsson, S. Th., G. B. Guðmundsson, K. Ágústsson, S. S.
Jakobsdóttir, R. Slunga and R. Stefánsson 1998b. Overview
of the 1993–1996 seismicity near Hengill. Icelandic Meteorological Office-Research Report VÍ-R98006, 16 pp.
Rögnvaldsson, S. Th. Þ. Árnadóttir, K. Ágústsson, Þ. Skaftadóttir, G. B. Guðmundsson, G. Björnsson, K. S. Vogfjörð, R.
Stefánsson, R. Böðvarsson, R. Slunga, S. S. Jakobsdóttir,
E. Kjartansson, B. S. Þorbjarnardóttir, P. Erlendsson, B. H.
Bergsson, S. Ragnarsson, P. Halldórsson, B. Þorkelsson and
M. Ásgeirsdóttir 1998c. Skjálftahrina í Ölfusi í nóvember
1998. Icelandic Meteorological Office-Report VÍ-G98046,
19 pp.
Sigmundsson, F., P. Einarsson, S. Th. Rögnvaldsson, G. R. Foulger, K. M. Hodgkinson and G. Þorbergsson 1997. The 1994–
1995 seismicity and deformation at the Hengill triple junction, Iceland: Triggering of earthquakes by minor magma injection in zone of horizontal shear stress. J. Geophys. Res.
102, 15,151Ů15,161.
Slunga, R. 1981. Earthquake source mechanism determination
by use of body-wave amplitudes-an application to Swedish
earthquakes. Bull. Seism. Soc. Am. 71, 25–35.
Slunga, R., S. Th. Rögnvaldsson and R. Böðvarsson 1995. Absolute and relative locations of similar events with application to microearthquakes in southern Iceland. Geophys. J. Int.
123, 409–419.
Soosalu H. and P. Einarsson 2004. Seismic constraints on magma
chambers at Hekla and Torfajökull volcanoes, Iceland. Bull.
Volc. 66, 276–286.
Soosalu, H., P. Einarsson and B. S. Þorbjarnardóttir 2005. Seismic activity related to the 2000 Hekla eruption, Iceland. Bull.
Volc. 68, 21–36.
Soosalu H. and P. Einarsson 2006. Seismic characteristics of the
Hekla volcano, Iceland. Jökull 55, 87–106.
Soosalu, H., K. Jónsdóttir and P. Einarsson 2006a. Seismicity crisis at the Katla volcano, Iceland-signs of a cryptodome? J.
Volc. Geotherm. Res. 153, 177–186.
Soosalu, H., R. Lippitsch and P. Einarsson 2006b. Low-frequency
earthquakes at the Torfajökull volcano, South Iceland. J.
Volc. Geotherm. Res. 153, 187–199.
Soosalu, H., C. Knox, R. S. White, P. Einarsson, S. S. Jakobsdóttir and E. Sturkell 2007. Seismicity at the Askja volcano
and its surroundings, North Iceland. In: Abstracts of the Volcanic and Magmatic Studies Group, Winter Meeting, Oxford,
United Kingdom.
Soosalu, H., R. S. White, A. J. Key, C. Knox, P. Einarsson and
S. S. Jakobsdóttir 2008. Lower-crustal earthquakes reflect
magma movements beneath the north Iceland rift near Askja.
In: Abstracts of the EGU General Assembly, Vienna, Austria.
Stefánsson, R. and P. Halldórsson 1988. Strain build-up and strain
release in the South Iceland seismic zone. Tectonophysics
152, 267–276.
Stefánsson, R., R. Böðvarsson, R. Slunga, P. Einarsson, S. S. Jakobsdóttir, H. Bungum, S. Gregersen, J. Havskov, J. Hjelme
and H. Korhonen 1993. Earthquake prediction research in the
South Iceland seismic zone and the SIL project. Bull. Seism.
Soc. Am. 83, 696–716.
Stefánsson, R., G. B. Guðmundsson and P. Halldórsson 2000.
The two large earthquakes in the South Iceland seismic zone
on June 17 and 21, 2000. Icelandic Meteorological OfficeReport 00010, 9 pp.
Stefánsson, R., G. B. Guðmundsson and P. Halldórsson 2003. The
South Iceland earthquakes 2000 - a challenge for earthquake
prediction research. Icelandic Meteorological Office-Report
03017, 21 pp.
JÖKULL No. 58, 2008
99
S. S. Jakobsdóttir
Stefánsson, R., F. Bergerat, M. Bonafede, R. Böðvarsson, S.
Crampin, P. Einarsson, K. L. Feigl, C. Goltz, Á. Guðmundsson, F. Roth, R. Sigbjörnsson, F. Sigmundsson, P. Suhadolc,
M. Wyss, J. Angelier, Þ. Árnadóttir, M. E. Belardinelli, A.
Clifton, L. Dubois, G. B. Guðmundsson, P. Halldórsson, S.
Hjaltadóttir, B. Lund, S. Ólafsson, S. Richwalski, C. SensSchönfelder, R. Slunga, A. Tryggvason, K. S. Vogfjörð and
B. Þorkelsson 2005. PREPARED - second periodic report.
February 1, 2004 - January 31, 2005. Icelandic Meteorological Office-Report 05007, 107 pp.
Stefánsson, R. 2006. PREPARED - final report. Icelandic Meteorological Office-Report 06009, 65 pp.
Stefánsson, R., M. Bonafede, F. Roth, P. Einarsson, Þ. Árnadóttir
and G. B. Guðmundsson 2006a. Modelling and parameterizing the Southwest Iceland earthquake release and deformation process. Icelandic Meteorological Office-Report 06005,
49 pp.
Stefánsson, R., F. Bergerat, M. Bonafede, R. Böðvarsson, S.
Crampin, P. Einarsson, K. L. Feigl, C. Goltz, Á. Guðmundsson, F. Roth, R. Sigbjörnsson, F. Sigmundsson, P. Suhadolc,
M. Wyss, J. Angelier, Þ. Árnadóttir, M. E. Belardinelli, G.
Björnsson, A. Clifton, L. Dubois, G. B. Guðmundsson, P.
Halldórsson, S. Hjaltadóttir, Á. R. Hjartardóttir, G. Jónsson,
M. Khodayar, B. Lund, B. Ófeigsson, S. Ólafsson, S. Richwalski, R. Slunga, P. Theodórsson, K. S. Vogfjörð, B. S. Þorbjarnardóttir and B. Þorkelsson 2006b. PREPARED - third
periodic report. February 1, 2005-July 31, 2005. Icelandic
Meteorological Office-Report 06008, 131 pp.
Stefánsson, R., G. B. Guðmundsson and P. Halldórsson 2008.
Tjörnes Fracture Zone. New and old seismic evidence
for the link between the North Iceland Rift Zone and
the Mid-Atlantic Ridge. Tectonophysics 447, 117–126,
doi:10.1016/j.tecto.2006.09.019.
Sturkell, E., F. Sigmundsson and P. Einarsson 2003a. Recent unrest and magma movements at Eyjafjallajökull and Katla volcanoes, Iceland. J. Geophys. Res. 108(B8), 2369.
Sturkell, E., P. Einarsson, F. Sigmundsson, S. Hreinsdottir and
H. Geirsson 2003b. Deformation of Grímsvötn volcano, Iceland:1998 eruption and subsequent inflation. Geophys. Res.
Lett. 30, 1182.
Sturkell, E., P. Einarsson, F. Sigmundsson, H. Geirsson, H. Ólafsson, R. Pedersen, E. de Zeeuw-van Dalfsen, A. L. Linde, I. S.
Sacks and R. Stefánsson 2006a. Volcano geodesy and magma
dynamics in Iceland. J. Volc. Geotherm. Res. 150, 14–34.
Sturkell, E., F. Sigmundsson and R. Slunga 2006b. 1983–2003
decaying rate of deflation at Askja caldera: Pressure decrease
in an extensive magma plumbing system at a spreading plate
boundary. Bull. Volc. 68, 727–735.
Sturkell, E., P. Einarsson, M. J. Roberts, H. Geirsson, M. T. Guðmundsson, F. Sigmundsson, V. Pinel, G. B. Guðmundsson,
H. Ólafsson and R. Stefánsson. Seismic and geodetic insights
into magma accumulation at Katla subglacial volcano, Iceland: 1999 to 2005, 2008. J. Geophys. Res. 113(B03212),
doi:10.1029/2006JB004851.
100 JÖKULL No. 58, 2008
Sæmundsson, K. 1974. Evolution of the axial rifting zone in
northern Iceland and the Tjörnes Fracture Zone. Geol. Soc.
Am. Bull. 85, 495–504.
Tryggvason, K., E. Huseby and R. Stefánsson 1983. Seismic image of the hypothesized Icelandic hot spot. Tectonophysics
100, 97–118.
Vogfjörð, K. S. 2003. Triggered seismicity in SW Iceland after
the June 17, Mw = 6.5 earthquake in the South Iceland Seismic Zone: The first five minutes. In: Abstracts from the EGSAGU-EGU Joint Assembly, Nice, France.
Vogfjörð, K., S. Th. Rögnvaldsson, R. Slunga, J. Morgan, G. Nolet, R. Allen, P. Erlendsson, S. Ragnarsson, R. Stefánsson,
B. Julian and G. R. Foulger 1999. Propagation of seismicity during the September/October 1996 sub-glacial eruption
episode near Bárðarbunga Volcano, Iceland. In: Abstracts of
the SSA Annual Meeting, Seattle, Washington.
Vogfjörð, K. S., H. Geirsson and E. Sturkell 2004. Krísuvíkurhrinan í ágúst 2003: kortlagning brotflata með eftirskjálftum og
GPS mælingum. In: Abstracts of the Spring Meeting, Geoscience Society of Iceland 2004.
Vogfjörð, K. S., S. Hjaltadóttir and R. Slunga 2005a. The M∼5
triggered events in the South Iceland Seismic Zone on June
17, 2000: Determination of fault plane magnitude and mechanism. In: Abstract of the EGU General Ass., Vienna, Austria.
Vogfjörð, K. S., S. Hjaltadóttir and R. Slunga 2005b. Volcanotectonic interaction in the Hengill Region, Iceland during
1993–1998. In: Abstracts, EGU General Ass., Vienna, Austria.
Vogfjörð, K. S., S. S. Jakobsdóttir, G. B. Guðmundsson, M. J.
Roberts, K. Ágústsson, Þ. Arason, H. Geirsson, S. Karlsdóttir, S. Hjaltadóttir, U. Ólafsdóttir, B. Þorbjarnardóttir, Þ.
Skaftadóttir, E. Sturkell, E. B. Jónasdóttir, G. Hafsteinsson,
H. Sveinbjörnsson, R. Stefánsson, and T. Jónsson 2005c.
Forecasting and Monitoring a Subglacial Eruption in Iceland.
Eos Trans. AGU 86, 245 and 248.
Wessel, P. and W. H. F. Smith 1998. New, improved version of
Generic Mapping Tools Released. Eos Trans. AGU 79, 579.
Wolfe, C.-J., I. Th. Bjarnason, J. C. VanDecar and S. C. Solomon
1997. Seismic structure of the Iceland mantle plume. Nature
385, 245–247.
Wyss, M. and R. Stefánsson 2006. Nucleation points of recent
main shocks in southern Iceland mapped by b-values. Bull.
Seism. Soc. Am. 96, 599–608, doi:10.1785/0120040056.
Þorbjarnardóttir, B. S., G. B. Guðmundsson and S. S. Jakobsdóttir
2003. Seismicity in Iceland during 2001. Jökull 52, 55–60.
Þorbjarnardóttir, B. S. and G. B. Guðmundsson 2003. Seismicity
in Iceland 2002. Jökull 53, 49–54.
Þorbjarnardóttir, B. S., G. B. Guðmundsson, S. Hjaltadóttir and
M. J. Roberts 2007. Seismicity in Iceland during 2006.
Jökull 57, 45Ů59.