Jökull - 01.01.2005, Page 110

Sigurlaug Hjaltadóttir et al.
−22.4˚
−22.4˚
−22.3˚
−22.3˚
−22.2˚
−22.2˚
63.85˚ 63.85˚
63.9˚ 63.9˚
2005 Oct 11 15:09:08 SH: Fagrdfj−juli04
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FAGRADALSFJALL JULY 2004
Mlw 1 2 3 4
RP
Figure 2. The July 2004 swarm in Fagradalsfjall, RP. The earthquakes are colour coded according to age. –
Smáskjálftahrina í Fagradalsfjalli í júlí 2004. Skjálftarnir hafa mismunandi lit eftir því hvenær í mánuðinum
þeir urðu.
cap is mainly concentrated in two locations, west of
the caldera, in western Goðabunga, and within the
caldera. Earthquakes in Goðabunga have shown a sea-
sonal pattern for the last four decades, with increased
activity between late summer and December (Einars-
son and Brandsdóttir, 2000). In 2001 seismic activity
increased substantially and remained high throughout
the year (Figure 4). Continuous GPS measurements
south of the caldera and campaign measurements at
the northern caldera rim have also shown uplift and
outward displacements from the caldera, further sup-
porting magma accumulation in the volcano (Geirs-
son et al., 2005; Sturkell et al., 2003). Soosalu (2004)
argued that the concentrated and shallow activity at
Goðabunga might indicate an intruding cryptodome
beginning to propagate toward the base of the ice cap
in 1999. At the end of 2004, activity decreased again
and now shows similar intensity as before 2001 (Fig-
ure 4). An event of magnitude 3.9 occurred in the
caldera during a swarm in June and July (Table 1).
The earthquake was felt in Langidalur, Þórsmörk, a
few km west of the ice cap. Similar to Goðabunga,
seismicity in the caldera has decreased.
110 JÖKULL No. 55