Jökull - 01.01.2010, Page 23

Reviewed research article
The May 29th 2008 earthquake aftershock sequence within the
South Iceland Seismic Zone: Fault locations and source
parameters of aftershocks
Bryndís Brandsdóttir1, Matthew Parsons2, Robert S. White2,
Ólafur Guðmundsson1,3,4, Julian Drew2 and Bergþóra S. Thorbjarnardóttir5
1Institute of Earth Sciences, Science Institute, University of Iceland
2Department of Earth Sciences, Bullard Laboratories, University of Cambridge, England, UK
3Department of Science and Engineering, Reykjavík University, Iceland
4Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36 Uppsala, Sweden
5Icelandic Meteorological Office, Reykjavík, Iceland
bryndis@raunvis.hi.is, mdp31@cam.ac.uk, rsw1@esc.cam.ac.uk, ogud@ru.is, drew2@slb.com, begga@vedur.is
Abstract — On May 29th 2008, two Mw ∼6 earthquakes struck the western part of the South Iceland Seismic
Zone. The first event was followed within seconds by a similar size event on a second fault ∼5 km further west.
Earthquakes, detected by a temporary network of 11 seismometers and three permanent SIL-network stations
were located using an automated Coalescence Microseismic Mapping technique. The epicenters delineate two
major and several smaller N-S faults as well as an E-W zone of activity stretching further west into the Reykja-
nes Peninsula Rift Zone. Fault plane solutions show right lateral strike slip mechanisms along the two major
N-S faults and suggest both smaller N-S right-lateral strike slip faults further west as well as an E-W zone of
left lateral strike slip fault. The aftershocks deepen from 3–5 km in the north to 8–9 km in the south, suggesting
that the main faults dip southwards. A brief increase in aftershock seismicity is most likely caused by short-
term static stress buildup on adjacent faults. The faulting is interpreted to be driven by the local stress due to
transform motion between two parallel segments of the divergent plate boundary crossing Iceland.
INTRODUCTION
The mid-Atlantic plate boundary breaks up into a se-
ries of segments across Iceland (Figure1). The South
Iceland Seismic Zone (SISZ) is a complex transform
zone where left-lateral E-W shear between the Reyk-
janes Peninsula Rift Zone and the Eastern Volcanic
Zone is accommodated by bookshelf faulting along
N-S lateral strike-slip faults (Einarsson et al., 1981).
The SISZ is also a transient feature, migrating lat-
erally in response to the southward propagation of
the Eastern Volcanic Zone. Sequences of large earth-
quakes (M > 6) lasting from days to years and affect-
ing most of the seismic zone have occurred repeatedly
in historical time (last 1100 years), separated by in-
tervals of relative quiescence lasting 45–112 years
(Einarsson and Björnsson, 1979). The latest earth-
quake sequences occurred in 1896, 1912, 1987 and
2000. They typically start with a large earthquake
in the eastern part of the SISZ followed by events of
equal or smaller magnitude further west. A major
sequence activating faults through the whole SISZ
region occurred in 1896 with five Ms 6–6.9 events
taking place over a period of two weeks (Einarsson et
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