Jökull - 01.01.2005, Síða 116

Sigurlaug Hjaltadóttir et al.
were observed in a swarm, with the two largest events
(Mlw=3.9) occurring at the end of September and the
beginning of October. The earthquakes have all been
relocated using a double-difference method (Slunga
et al., 1995) and are shown in orange in Figure 7. For
comparison, the single event locations are plotted in
green in the background. After relocation, the epicen-
tral clusters revealed two distinct features. Hypocen-
ters of the first swarm form a N-S lineament, while the
hypocenters in the latter swarm are more concentrated
in a dense cluster, which is elongated in a NNW direc-
tion. Most earthquakes are located at fairly shallow
depths, between 1.5 and 5.5 km. Faults and fissures
striking NE have been mapped on the surface in the
vicinity of Hveravellir (south and east of hve, at ap-
proximately 64.85◦–64.87◦N) (Hjartarson and Ólafs-
son, 2005). North-westerly striking faults have also
been observed, but they are less distinct. Furthermore,
there are surface structures with a NNW strike in Belj-
andatungur, SW of Guðlaugstungur, similar to the ap-
parent trend of the latter swarm, but none have been
observed in the exact area of the seismicity.
The Tjörnes Fracture Zone
The Tjörnes Fracture Zone (TFZ) is a transform zone
connecting the Northern Volcanic Zone to the ridge
segments offshore. Seismicity in the TFZ is mainly
concentrated on two WNW-trending lineaments, start-
ing at the shore near the towns of Húsavík and
Kópasker. The northern Grímsey lineament (GL)
stretches as far as 110 km offshore to the north-
west, while the southern Húsavík-Flatey fault (HFF)
reaches 80 km to the WNW and then bends north-
wards (Figure 1).
In 2004 the largest swarm in the TFZ occurred in
June on the HFF, at the mouth of the fjord Eyjafjörður,
when around 600 earthquakes were located. The
earthquakes, which were all small (Mlw<2.5), have
been relocated, using the double-difference method
(Slunga et al., 1995). The event distribution shows
a NW-striking lineament at a depth of 9–11 km. Seis-
mic activity is very common in the area and the largest
swarms recorded over the past few years occurred in
1997 and 2001. Several other smaller swarms also oc-
curred on the Grímsey Lineament. None of the larger
events (Mlw ∼3) occurring in 2004 were followed
by further activity, which is unusual. They may have
been caused by increased strain in the TFZ, which led
to an earthquake of magnitude ∼5 (M l; EMSC, 2005)
in the centre of the GL in January 2005.
In November and December, two small swarms
occurred under the mountain Kaldbakur in Eyjaförður
(K in figure 1); a total of 65 earthquakes, all with
Mlw ≤2.5. This is quite an unusual location and since
the installation of the SIL system, only one swarm
has been observed in its vicinity. It occurred in Oc-
tober, 1999, when 24 events were located ∼1 km east
of the November 2004 swarm. Kaldbakur is situated
on the Dalvík seismic Lineament (DL), south of the
HFF (Rögnvaldsson et al., 1998). This lineament has
shown much less seismic activity than GL and HFF,
although large earthquakes are known to occur on the
DL, the most notable being the ML 6.2 Dalvík earth-
quake in 1934.
CONCLUSIONS
1. Seismic activity in Iceland during 2004 was
moderate in comparison to the number of events
recorded between 2001 and 2003. Over 12.000
earthquakes were located, and 315 of these
were icequakes located in Skeiðarárjökull in
connection with jökulhlaups and heavy rain-
fall. In addition, 275 reported explosions were
recorded, many of which were located in the
Kárahjúkar construction area.
2. The largest recorded earthquakes occurred
offshore on the Reykjanes Ridge in April
(Mlw=4.5) and September (mb=4.5). Both
events were a part of short-lived swarms, lasting
between one and two days. No volcanic tremor
was detected on the ridge in connection with
the swarms, but seismicity has been increasing
steadily on the ridge during the past few years.
3. Seismic activity in Mýrdalsjökull decreased
substantially during 2004 and now shows simi-
lar intensity as observed before 2001.
4. Heightened seismic activity has been widely
observed in northwest Vatnajökull since mid-
116 JÖKULL No. 55