Jökull - 01.01.2010, Side 31

The 2008 South Iceland Seismic Zone aftershock sequence
of the smaller events. Although the SIL–catalogue is
missing some data May 30th and June 2nd (Figure 2),
the overall distribution correlates well with the CMM
locations (Figure 6a-c). Using only SIL data, the
CMM program has difficulties constraining hypocen-
tral depths, with many events at a default depth of 1.5
km. Given that there are only three SIL stations within
the epicentral region and CMM was never designed to
work with far-field stations, it is not surprising that it
has not been very successful. The fact that it has pro-
duced a similar planar view is encouraging.
The SIL located events lie systematically about
500 m east of the CMM locations in the northern epi-
central zone (Figure 7) where hypocenters are shal-
lower. Shallower events (above 4 km) are also system-
atically located deeper by SIL than by CMM (Figure
6). Both SIL and CMM locations are based on a 1-D
velocity model derived from the SIST refraction pro-
file for the SISZ (Bjarnason et al., 1991). In order
to test how slight regional variations in upper crustal
velocities may affect CMM locations we relocated all
events using the 1-D model SUL (Figure 8) derived
by Vogfjörd et al. (2002) for the Hengill-Hellisheiði
region and a 1-D model derived from the RISE refrac-
tion profile which crosses the epicentral region (Weir
et al., 2001). The three models are very similar al-
though the SUL and RISE models have slightly higher
velocities between 3–6 km depth. In addition, the
SUL model has a lower crustal velocity gradient be-
low 6 km depth. Our results showed no marked shift
in the epicentral zone between the SIL and SUL mod-
els (Figure 7, bottom). Locations of about 900 events
on June 2 using the RISE model was identical to the
other two models. Slightly higher velocities at 3–6
km depth may be responsible for deepening the SIL
locations with respect to CMM locations. No station
corrections were derived but localized near surface ve-
locity anomalies could also potentially cause some of
the observed shift in the epicentral zone for the shal-
lower events as the SIL stations are distributed over a
larger area than the 14 stations used for the CMM lo-
cations. A 500 m westward shift of the epicentral zone
also fits GPS and InSAR coseismic deformation mod-
els for the Ingólfsfjall fault (Decriem et al., 2010).
Fault Plane Solutions
Fault Plane Solutions were produced for 123 events
spread over the epicentral area (Figures 9 and 10)
using the program of Reasenberg and Oppenheimer
(1985) which calculates best fit double-couple fault
plane solutions from the polarity data. P and T axes
were extracted using a modified version of FPPLOT
and plotted using StereoNett (Duyster, 2000). The
fault plane solutions were limited to events larger than
Mlw1.3 and are thus generally well constrained, with
strike, dip and rake uncertainties less than 15◦, 25◦
and 20◦, respectively. A total of 114 fault plane solu-
tions had strike uncertainties of 8◦ or less.
The fault plane solution of the first event, from
the Global CMT Harvard Catalogue (2009), is consis-
tent with a right lateral strike slip fault on a N-S plane
(Figure 10, lower right). Our fault plane solutions are
also dominated by NNW-NE aligned strike slip fault-
ing on individual fault strands with variable normal
Figure 5. Top: CMM located aftershocks (7846 events) with SN ratio greater than 3 and calculated epicentral and depth
location errors less than 1 km and less than 2 km, respectively, using 14 LOKI and SIL stations (red triangles). The events
are colored depending on depth. Flanking the epicentral map are two N-S transects along the main N-S faults (within the
region marked by vertical lines) and an E-W transect across the whole zone. Events at the western end of the EW zone are
not well constrained. The western fault borders the village of Hveragerði (H) and the town of Selfoss (S) lies 3 km east of
the eastern fault. Below: for comparison, a total of 13868 CMM located aftershocks with SN ratio greater than 2.5. Also
shown are 5189 SIL located events May 29–July 2 (grey dots) with epicentral and depth errors less than 1 km and 2 km.
Roads are light grey and surface faults are dark grey. – Upptakadreifing 7846 jarðskjálfta sem staðsettir voru með neti ellefu
færanlegra mæla ásamt þremur SIL stöðvum (rauðir þríhyrningar). Jarðskjálftarnir eru litaðir eftir upptakadýpi. Reiknuð
staðsetningaróvissa er innan við 1 km í láréttu plani og 2 km í dýpi. Til hægri eru þversnið af sprungunum við Reykjafjall,
rétt austan Hveragerðis og Ingólfsfjall en fyrir neðan er þversnið yfir allt svæðið. Neðri myndin sýnir upptakadreifingu allra
CMM staðsettra skjálfta, samtals 13868. Til samanburðar eru SIL staðsetningar á 5189 skjálftum (dökkgráir punktar).
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