Jökull - 01.01.2005, Side 109

Seismic activity in Iceland during 2004
energy released by earthquakes in 2004 was compa-
rable to the annual energy release since 2001. Addi-
tionally, 315 icequakes and 275 confirmed explosions
were detected. The largest event recorded had a body-
wave magnitude mb=4.5 (USGS/NEIC, 2005). It oc-
curred 80 km offshore on the Reykjanes Ridge (RR)
in September, followed by a short-lived swarm that
lasted only ten hours. The earthquake was not felt
due to its distance from land. Although no hazardous
earthquakes occurred, the dataset indicates height-
ened activity in some regions.
Reykjanes Ridge and Reykjanes Peninsula
Reykjanes Ridge and Reykjanes Peninsula Besides
the mb 4.5 earthquake in September, another event
of similar magnitude occurred during a short-lived
swarm in April, located 40 km offshore on the Reykja-
nes Ridge. In addition, numerous small swarms took
place throughout the year on the ridge. Since 1997,
seismic activity has increased steadily on the Reykja-
nes Ridge (RR). Interestingly, only a few earthquakes
have been observed since 1997 in the 20 km wide
gap between the active areas at Eldeyjarboði and
Geirfugladrangur (see location of RR in Figure 1).
No major earthquakes have occurred on the
Reykjanes Peninsula, since a magnitude 5 event oc-
curred in Krísuvík in August 2003 (Vogfjörð et al.,
2004). The most intense activity in 2004 was in July,
when nearly 2000 events were observed in the Fagra-
dalsfjall mountain, west of Krísuvík (Figure 2). The
activity began on 11 July beneath the western slopes
of the mountain and earthquakes migrated eastwards
for the next week on a series of N-NNE striking faults,
where the largest events of the swarm also occurred
(Mlw ∼3.5; local moment magnitude, Slunga et al.,
1984; Rögnvaldsson and Slunga, 1993). On 18 July,
the activity subsided, but then increased again in the
western part of the area two days later; at this time,
the activity was mainly concentrated on an approxi-
mately 3.5-km-long, NE-striking fault (green events
in Figure 2). Earthquakes are common near to Fagra-
dalsfjall and swarms of similar intensity occurred in
1998 on the same NE-striking fault, and again nearby
in 2000. Furthermore, Fagradalsfjall is highly frac-
tured and a series of N-NE striking surface faults and
fissures have been mapped in the area (Clifton, 2004).
Hengill and the South Iceland Seismic Zone
At the western margin of the South Icelandic Seis-
mic Zone (SISZ), in the Hengill-Ölfus area, seismic
activity was slightly higher during 2004 and 2003 as
compared to 2002 and 2001. But the present regime
pales in comparison to the period 1994-1998; during
this time, magma intruded into the Hengill region, re-
sulting in sustained earthquake activity (Rögnvalds-
son et al., 1996; Vogfjörð et al., 2005a). The largest
earthquake observed in the Hengill-Ölfus area in 2004
was a Mlw 4.2 event that occurred at Dalafjall, 3 km
NW of the town Hveragerði (Figure 3). This location
is near the center of crustal uplift, caused by the in-
trusion (Feigl et al., 2000). The earthquake was felt
widely in southern Iceland (Table 1).
Following the two Mw=6.5 earthquakes in June
2000, seismic activity in the SISZ increased greatly
(Hjaltadóttir and Vogfjörð, 2005b). Since 2000, activ-
ity has decreased significantly and in 2004 only mod-
erate seismic activity was observed, with neither large
swarms nor large events detected. A number of af-
tershocks were located on the two June-2000 faults.
Small swarms occurred at the southern and northern
ends of the 21 June Hestvatn fault, and one event
of magnitude 2.5 occurred on the 17 June Holt fault
at the end of June. The largest event in the SISZ,
Mlw 3.2, occurred in Áshverfi, south of the main tran-
sition zone (Á in Figure 3). It was followed by a
short-lived swarm, located at depths between 9 and
10 km, which is significantly deeper than the 4–8
km commonly observed in the SISZ (Hjaltadóttir and
Vogfjörð, 2005a). Except for the heightened activity
in 2000, earthquakes seldom occur in Áshverfi, but
these source depths are common in this area.
Mýrdalsjökull
The Katla volcano, located under the Mýrdalsjökull
ice cap, has been under constant observation since
1999 when a jökulhlaup issued from Sólheimajökull
(Sigurðsson et al., 1999; Roberts et al., 2003) and a
new cauldron formed in the ice cap (Guðmundsson et
al., 2000; Vogfjörð, 2002). Other existing cauldrons
within the caldera deepened (Guðmundsson et al.,
2000), suggesting a widespread increase in geother-
mal activity in the volcano. Seismicity under the ice
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