Jökull - 01.01.2010, Page 37

The 2008 South Iceland Seismic Zone aftershock sequence
0
100
200
Ea
rth
qu
ak
e
fre
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cy
June 01 June 05 June 09 June 13 June 17 June 21 June 25 June 29
0
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in
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/s
Figure 11. A histogram of aftershocks in one hour bins, from May 31th to June 29nd, using 19449 CMM loca-
tions (red bars). Note the strong 24 hour periodicity as well as enhanced activity May 31–June 1, June 2–4, June
6–7 and June 8–9. Filtered CMM locations (8233 events, yellow bars) show same trend. Blue and green graphs
show wind strength at two weather stations within the epicentral region, Eyrarbakki (blue) and Ingólfsfjall
(green). Black stars denote SIL located earthquakes Mlw 3 and larger (Figure 2). – Fjöldi staðsettra skjálfta
á hverri klukkustund (rauðar og gular súlur) ásamt vindhraða á sjálfvirkum veðurstöðvum við Eyrarbakka og
Ingólfsfjall (bláar og grænar línur). Aukinn vindhraði minnkar næmni jarðskjálftamælanna. Svartar stjörnur
sýna skjálfta M 3 og stærri skv. 2. mynd.
Aftershock distribution
Close to 40 small foreshocks occurred within an hour
prior to the main event at 1545h, on May 29th, the
largest Mlw=3.5 at 1441h (Figure 2). Although the
SIL catalogue of events is incomplete during Julian
days 150–154, the cumulative strain release during
aftershock activity of two similar sized main events
seems to vary considerably between the two faults
(Figure 12). Immediately following the main events,
about 40 aftershocks Mlw=3–4.75 were located along
the western (Reykjafjall) fault whereas only a few af-
tershocks of this magnitude were located along the
eastern (Ingólfsfjall) fault. Triggered earthquake ac-
tivity is also more pronounced westwards along the
SISZ, with nearly twenty events Mlw> 2.5 originating
along the E-W zone, west of the main faults versus
five events Mlw> 2.5 on N-S faults, east of the Ingólfs-
fjall fault. The elevated triggering of earthquakes
westwards is, however, somewhat unexpected as ma-
jor seismic swarms occurred in this region in 1993–
1998, culminating with a Ml∼5.1 event in November
1998 (Jakobsdóttir et al., 2002; Jakobsdóttir, 2008).
Both main events occurred along previously
mapped surface fault zones (Einarsson, this issue).
Temporal and spatial aftershock patterns indicate that
the two main fault zones are made up of many singular
fault strands (Figures 5 and 10), concurrent with sur-
face mapping of en echelon fault segments (Clifton
and Einarsson, 2005). The epicentral zone of the
western fault is divided into two main sections strik-
ing N10–15◦E. Fault plane solutions obtained for 123
aftershocks greater than Mlw=1.3 are mostly strike-
slip. However, a few normal faulting and dip-slip
events occur at the junction of the western (Reykjaf-
jall, Kross) fault with the E-W zone, north of Hver-
agerði and close to 63◦N57.2’ and 21◦W17’, just east
of the SIL station Bjarnastaðir (BJA) (Figures 12–
16) reflecting localized extension, down to about 9
km depth (Figure 5), i.e. the base of the seismogenic
crust in this region (Stefánsson et al., 1993). Both
regions lie at the junction of the SISZ with volcanic
rift zones of the Hengill and Grensdalur volcanic sys-
tems. Geothermal activity is also present.
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