Jökull - 01.01.2004, Page 69

Seismicity in Iceland 2003
Increased earthquake activity has been on the
northern part of Reykjanes Ridge since 2000. At the
end of April, swarm activity was recorded close to
Geirfuglasker with an earthquake of magnitude 4. An
earthquake of magnitude 4 was also recorded about 15
km SW of Eldeyjarboði in a swarm at the end of the
year. An earthquake Mb = 5 (NEIC) was recorded
on 62.9 degrees north, on the Reykjanes Ridge on
July 19.
An earthquake of magnitude Mlw = 3.2 occurred
beneath Nesjavellir in the Hengill area on March 11.
It was felt in Reykjavik and Selfoss. Some small
earthquakes at about 16 km depth were recorded by
Heimaey in the Westmann Islands in August and
November.
REYKJANES PENINSULA
The Reykjanes Peninsula is a continuation of the
oblique spreading ridge, Reykjanes Ridge, and con-
nects it to the South Iceland Seismic Zone (SISZ) and
the Western Volcanic Zone (WVZ).
On August 23, 2003 at 02:00 an Mlw = 5 earth-
quake occurred west of Kleifarvatn on the Reykja-
nes Peninsula at about 4 km depth (Vogfjörð, 2004);
(Figure 3). The earthquake was felt in many parts of
SW–Iceland. More than 1200 aftershocks followed
on the same day and the day after of which 10 were
greater than 3 in magnitude. The aftershock activ-
ity culminated about a week later. The epicenters of
the aftershocks aligned on a NS-line containing the
main shock and also on an ENE-line from the main
shock into Kleifarvatn. The optimum fault plane so-
lution for the main event and the aftershocks indicate
that the fault plane is a N–S right–lateral strike slip
fault. This seismic activity on the Reykjanes Penin-
sula is the greatest since June 2000 when 3 earth-
quakes of magnitude greater than 5 were triggered on
the Reykjanes Peninsula by the main shock in SISZ
on June 17, 2000 (Vogfjörð, 2003).
An earthquake swarm was recorded in Bláfjöll
in early April with the biggest earthquake of mag-
nitude 2.2. A small earthquake swarm took place at
Fagradalsfjall in October.
MÝRDALSJÖKULL
The Katla volcano is located under Mýrdalsjökull. It
has shown signs of unrest since 1999 when a jökul-
hlaup emerged from the southern part of the glacier
and a new cauldron was formed and other cauldrons
showed changes in size (Guðmundsson et al., 2000;
Vogfjörð 2002). GPS measurements in Austmanns-
bunga, at the northeastern rim of the Katla caldera,
have also shown uplift and outward displacements
from the year 2002 (Sturkell et al., 2003a). The seis-
micity under Mýrdalsjökull is mainly concentrated in
two areas, within the Katla caldera and to the west of
the caldera where the seismicity is usually seasonal
with nearly all earthquakes occurring in the second
half of the year (Einarsson and Brandsdóttir, 2000).
In 2002 the pattern of seismicity in the western part
of Mýrdalsjökull changed from seasonal to continu-
ous seismic activity throughout the year (Þorbjarnar-
dóttir et al., 2003). The majority of earthquakes lo-
cated under Mýrdalsjökull in 2003 originated under
the western part, where the seismic activity had the
same continuous pattern as in 2002. The seismicity
within the caldera was mainly under its northern part
(Figure 4). On September 22, an earthquake swarm
occurred beneath the northeastern rim of the caldera,
close to Austmannsbunga. The biggest earthquake in
that swarm had a magnitude Mlw = 3.9.
VATNAJÖKULL
The Vatnajökull ice cap includes several volcanic sys-
tems. The most active is the Grímsvötn volcano,
which erupted last in 1998, and has since been inflat-
ing (Sturkell et al., 2003b). From mid year 2003 the
seismicity at Grímsvötn increased (Figure 5). Most of
the earthquakes have their origin close to the southern
rim of the Grímsvötn caldera.
In the beginning of July 2003 a small earthquake
swarm occurred beneath Bárðarbunga. Some earth-
quakes were recorded on Loki ridge, the ridge striking
eastward from Hamarinn, and also some in Kverk-
fjöll. Icequakes were recorded in Skeiðarárjökull
in relation to jökulhlaups from Grænalón or rainfall
(Roberts et al., 2005).
JÖKULL No. 54, 2004 69