Jökull - 01.01.2010, Blaðsíða 50

Martens et al.
monitor regional seismicity by computing locations,
magnitudes, and focal mechanisms of seismic events
automatically and in near real-time (Böðvarsson et
al., 1996, 1999; Stefánsson et al., 1993). By the end
of 2007, the network comprised 51, three-component
seismometers deployed across the whole of Iceland
(Jakobsdóttir, 2008). All SIL seismic data is recorded
at 100 samples per second.
The SIL network is operated and maintained by
the Icelandic Meteorological Office (IMO). Seismic
hypocentres computed by the SIL-system’s automated
detection and location software are published on the
IMO website, typically within minutes of an event.
All locations are then manually assessed and re-
vised as necessary (e.g., for poor time picks and
false events). Phase arrival selections are refined in-
teractively and locations are subsequently recomputed
prior to publication on the ’weekly overview’ section
of the IMO website (hraun.vedur.is/ja/viku).
The SIL system also has the capability to derive
relative locations using a double-difference method
(Slunga et al., 1995). The double-difference algorithm
minimises travel time residuals between neighbouring
events recorded at the same station; hence, it alleviates
the need for station corrections for near-surface varia-
tions or deviations from the assumed one-dimensional
velocity model away from the hypocentral region,
with the potential for some loss in absolute location
accuracy.
References to SIL in this manuscript imply the
processing of SIL data using IMO techniques, unless
explicitly stated otherwise.
In summer 2007, Cambridge University deployed
an independent, dense network of seismometers in
the vicinity of Mt. Upptyppingar to monitor lower
crustal seismicity around Askja volcano (Soosalu et
al., 2010). The temporary network was active be-
tween 6 July and 22 August 2007 and consisted of
21, three-component Güralp 6TD broadband seis-
mometers (Figure 1). For the analysis detailed in
this manuscript, we have also included data from six
nearby SIL stations (MKO, HVA, BRU, KRE, ADA,
and VSH). Deployed in 2004 to monitor the forma-
tion of the nearby Hálslón water reservoir, the six
SIL stations provide additional coverage east of Mt.
Upptyppingar. This collective network of 21 Cam-
bridge University instruments plus six SIL stations
will henceforth be referred to as the ASN, for Askja
Seismic Network (Figure 1). All instruments in the
ASN record at 100 samples per second.
The Upptyppingar swarms represent the most in-
tensive and persistent seismic activity ever recorded
in Iceland’s lower crust. The unique spatial morphol-
ogy and temporal progression of the seismicity have
the potential to shed light on crustal formation and the
dynamics of dyke propagation in ductile crust. Here
we examine a subset of events from the Upptypping-
ar dyke intrusion using data acquired by the ASN and
compare our location results to those obtained using
SIL data alone. We conclude with discussions on SIL
system performance, the value of dense, local net-
works, and a geophysical interpretation of the Upp-
typpingar swarms.
SEISMIC OBSERVATIONS OF ACTIVE
DYKES
The movement of melt within the crust, delineated by
the spatial and temporal progression of seismicity, is
frequently observed in Iceland. Some of the clear-
est examples of dyke intrusion are associated with the
deflation of the Krafla volcano in northern Iceland in
the late 1970s. Earthquakes were observed to migrate
away from the Krafla volcanic centre at rates as high
as 0.5 m s−1, progressing laterally along narrow ver-
tical channels interpreted as dykes (Brandsdóttir and
Einarsson, 1979; Einarsson and Brandsdóttir, 1980).
Similar events have been observed along the
rift zone in Afar (Wright et al., 2006) and near
Mt. Kilauea volcano in Hawaii (Rubin and Gillard,
1998). All of these observations, however, involve
the migration of seismicity through the brittle, near-
surface crust. Seismicity suggestive of melt move-
ment through deeper crust is less common, but has
been observed on several occasions.
In addition to Mt. Upptyppingar, seismicity delin-
eating a planar structure in ductile crust was observed
beneath Lake Tahoe in the western United States in
2003–2004 (Smith et al., 2004). The Tahoe swarms
occurred at 29–33 km depth, well below the local
50 JÖKULL No. 60