Jökull - 01.01.2010, Side 25

The 2008 South Iceland Seismic Zone aftershock sequence
A dense network of seismographs encompassing
the earthquake source region lowers detection thres-
holds and reduce location uncertainty. Following the
May 29th 2008 events, a temporary network of eleven
seismometers (Figure 1) was deployed within the re-
gion in order to obtain accurate aftershock locations
for 3-D imaging of active faults. This LOKI network
located thousands of aftershocks during the follow-
ing days, delineating two major and several smaller
faults. We applied a coalescence microseismic map-
ping method (CMM) for detection and localization of
the aftershock sequence of the two May 29, 2008 Ölf-
us earthquakes. The technique is both automatic and
robust. The algorithm performs an exhaustive search
in time and space for events, and incorporates travel-
time inverse theory in imaging earthquake locations
(Drew et al., 2005; Drew, 2010).
Tectonics of the South Iceland Seismic Zone
The SISZ is markedly different from fracture zones
elsewhere along the Mid-Atlantic Ridge plate bound-
ary with minimal surface relief and diffuse tecton-
ics. Seismic and geodetic data show the over-all left-
lateral transform motion within the SISZ to be ac-
commodated by bookshelf faulting along N-S aligned,
right-lateral, strike-slip faults (Einarsson et al., 1981;
Hackman et al., 1990; Einarsson, 1991; Rognvaldsson
and Slunga, 1994; Sigmundsson et al., 1995; Hreins-
dóttir et al., 2001, 2009; Dubois et al., 2008). De-
tailed surface mapping shows the SISZ to be made
up of more than twenty N-N20E oriented, en eche-
lon strike-slip faults (Einarsson and Eiríksson, 1982;
Bjarnason et al., 1993; Bergerat and Angelier, 2003;
Clifton and Einarsson, 2005) which extend westwards
across the Reykjanes Peninsula Rift Zone (Sæmunds-
son and Einarsson, 1980; Sæmundsson 1995; Clifton
and Kattenhorn, 2006).
The SISZ, from the Hengill Triple Junction in the
west, to the Eastern Volcanic Zone, is represented by
a roughly 10 km wide band of seismicity (Einarsson,
1991; Stefánsson, 1993; Bjarnason et al., 1993). Af-
tershocks of the Ms 5.8 1987 Vatnafjöll earthquake
define a 12 km long and 4 km wide zone, between
6 and 13 km depth (Bjarnason and Einarsson, 1991),
whereas aftershocks of the two Mw 6.5 earthquakes
in 2000 delineate approximately 12.5 km and 16.5
long zones down to 10 km depth (Hjaltadóttir and
Vogfjörð, 2005). Geodetic modelling based on joint
inversion of InSAR and GPS measurements indicates
two 15 km long, near vertical faults extending from
the surface to approximately 10 km depth (Pedersen et
al., 2003), in good agreement with the aftershock dis-
tribution. The Vatnafjöll earthquake rupture did not
reach the surface.
The long term average spreading direction across
Southern Iceland, ∼103◦ (NUVEL-1A, DeMets et al.,
1994), was shown by early modelling of GPS mea-
surements to cause accumulation of left-lateral shear
strain across a 20–30 km wide (N-S) zone (e.g. Sig-
mundsson et al., 1995). More recent models show the
SISZ as a complex zone driven by N-S faulting near
the surface and E-W left-lateral shear below 15–20
km depth (Árnadóttir et al., 2006). Mapped surface
faults range in length from less than 1 km at the west-
ernmost tip of the RPRZ to 10–15 km near the centers
of the RPRZ and SISZ (Clifton and Einarsson, 2005;
Clifton and Kattenhorn, 2006).
Aftershocks from the 2008 earthquakes delineate
two parallel N-S trending, segmented faults 4–5 km
apart, with additional activity stretching westward
along an E-W aligned zone (Figure 1). Models of
geodetic observations (GPS and InSAR) yield a total
moment release of Mw 5.8 for the first event and Mw
5.9 for the second event, with a composite moment of
Mw 6.1 for both events (Decriem et al., 2010) in right-
lateral strike-slip motion on two parallel N-S trending
faults. Only minor slip was observed on other struc-
tures illuminated by earthquake activity. High rate (1
Hz) continuous GPS data also indicate that the west-
ern fault event occurred within 3 s of the main event
on the eastern fault (Hreinsdóttir et al., 2009, Decriem
et al., 2010).
DATA ACQUISITION
Within two days of the May 29th events, 11 portable
seismometers were deployed around the two faults
that had ruptured, to augment earthquake locations by
the permanent network. The LOKI network was oper-
ated until July 2nd and recorded over twenty thousand
events.
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