Jökull - 01.12.2007, Side 55

Seismicity in Iceland during 2006
During 2006, over 460 icequakes were sourced
from the lower ablation zone of Skeiðarárjökull (Fig-
ure 1). Most of these icequakes occurred in seven
short-lived sequences comprising between 12 and 185
events. Icequakes ranged in size from magnitude 0
to 2.7, and sequences coincided either with periods
of intense rainfall over southern Vatnajökull (Figure
8), or high, diurnal variations in air temperature in
the vicinity of the glacier. Additionally, icequakes
were generated during a minor jökulhlaup in Septem-
ber from Grænalón –a lagoon situated at the north-
western flank of Skeiðarárjökull, as observed during
previous floods from the lagoon since 2001 (Roberts,
2005). Using a network of three continuous GPS sta-
tions, Roberts et al. (2006) demonstrated that seismic-
ity from Skeiðarárjökull is associated with short-term
uplift and enhanced, down-glacier displacement of the
ice surface, presumably due to temporary storage of
water beneath the glacier.
Askja–HerðubreiðRegion
The number of earthquakes detected in the area just
north of the Vatnajökull ice cap has increased with the
installation of six additional seismic stations in recent
years (Figure 1). Seismicity in 2006 was mostly con-
fined to two earthquake swarms in the vicinity of the
table mountain Herðubreið and the hyaloclastite ridge
Herðubreiðartögl (Figure 9). This area lies between
the Askja and Kverkfjöll fissure swarms and no sur-
face fissures have been observed there (Jóhannesson
and Sæmundsson, 1998a,b). An earthquake swarm
took place northwest of Herðubreiðartögl from 12 to
18 April and another swarm north of Herðubreið from
27 to 30 May. After relocating the earthquakes with
a double-differencemethod, the majority of hypocen-
ters in both swarms were sourced at depths of 3–6 km
on northeasterly striking near-vertical planes. The
focal mechanisms of the largest earthquakes in each
swarm indicate left-lateral movement along the fault
Figure 8. Timing of icequake activity in Skeiðarárjökull in April and May 2006 and rainfall observed at a mete-
orological station (560 m a.s.l.), sited 40 km west of the glacier. Note the time-lag between the onset of intense
rainfall on 29 April and seismicity from Skeiðarárjökull. See Figure 1 for the epicentral location of icequake
sequences in Skeiðarárjökull during 2006. – Uppsafnaður fjöldi ísskjálfta í Skeiðarárjökli í apríl-maí 2006
og úrkoma mæld á Laufbala, um 40 km vestur af jöklinum. Fylgni milli mikillar úrkomu og skjálftavirkni í
Skeiðarárjökli er greinileg.
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