Jökull - 01.01.2010, Side 58

Martens et al.
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o
rt
h
in
g
[k
m
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437
438
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a
s
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g
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20Jul 12:00 21Jul 12:00 22Jul 12:00 23Jul 12:00 24Jul 12:00
13
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D
e
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th
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Date in 2007
Figure 8. ASN double-difference relative locations (red) with respect to SIL-system double-difference relative locations
(blue). The locations have been re-datumed to depth below sea level. Although the hypocentres are no longer as offset in
depth relative to the single-event locations, an offset of ∼0.5 km in easting has appeared (Table 1). All hypocentres, however,
are clustered considerably tighter than the single-event and catalogue locations. – Samanburður á afstæðum staðsetningum
(double-difference) úr ASN netinu (rauðir punktar) og SIL-kerfinu (bláir punktar). Dýpið er miðað við sjávarmál í báðum
tilfellum. Munur á dýptarákvörðunum er minni en áður, en í staðin er um 0,5 km hliðrun til austurs, sem ekki sást áður
(tafla 1). Upptök skjálftanna sýna nú mun minni dreifingu en þegar hver skjálfti er staðsettur fyrir sig.
at N085◦E±17◦. Furthermore, for SIL catalogue lo-
cations that are routinely published on the IMO web-
site, the hypocentres are fit by a plane dipping at
53.5◦±21.8◦ and striking at N133E◦±79◦. The ori-
entation of the best-fit plane to the catalogue loca-
tions is relatively unconstrained due to the consid-
erable scatter in the hypocentres. Uncertainties are
obtained through Monte Carlo simulations, in which
each hypocentre is varied in a normal distribution
about its initial location to the extent of the location
errors. Standard deviations are then calculated from
the distribution of best-fit planes. Since error esti-
mates for individual events are not published with the
on-line catalogue locations, we used error estimates
provided directly by the IMO.
ASN double-difference relocations have a root-
mean-square (RMS) misfit to the best-fit plane of
only 135 m (114 m if the two off-axis points between
16.5 and 17.0 km depth are removed). This value
is less than half of the SIL RMS misfit of 293 m
and approximately twice the expected precision of
relocations derived from ASN time picks at the 95%
confidence level (i.e. 60 m). The RMS misfit for the
on-line catalogue locations is 538 m. Characteristics
of the best-fit planes are summarised in Table 2.
58 JÖKULL No. 60