Jökull - 31.12.2001, Side 89

Seismicity in Iceland 1991-2000 monitored by the SIL seismic system
Steinunn S. Jakobsdóttir, Gunnar B. Guðmundsson and Ragnar Stefánsson
Department of Geophysics, lcelandic Meteorological Office, Bústaðavegur 9, 150 Reykjavík, Iceland;
ssj@vedur.is, gg@vedui:is, ragnar@vedur.is
Abstract — The SIL seismic system was designed and installed in the South Iceland Lowland in 1989-1990
and wasfully operational in automatic mode in June 1991. In 1994 the system was expanded to the northeastern
part oflceland and has been gradually covering larger parts oflceland since 1996. During thefirst decade of
operation some 160,000 events were recorded and processed. In that period 4 volcanic eruptions and 2 large
earthquak.es (Ms=6.6) have occurred that have all been monitored by the system.
INTRODUCTION
Iceland is situated on the Mid-Atlantic Ridge where
the ridge is shifted towards east in the vicinity of a
mantle plume, centered under the glacier Vatnajökull
(Tryggvason et al., 1983; Stefánsson et al., 1988;
Wolfe et al., 1997; Allen et al., 2002). Earthquakes
with magnitudes up to 7.1 have occurred within the
two fracture zones in Iceland (Einarsson, 1979; 1991;
Stefánsson and Halldórsson, 1988), the Tjörnes Frac-
ture Zone (TFZ) which connects the Northern Vol-
canic Zone (NVZ) to the Kolbeinsey Ridge and the
South Iceland Seismic Zone (SISZ) which connects
the Eastern Volcanic Zone (EWZ) to the Western Vol-
canic Zone (WVZ) and Reykjanes Peninsula. The
TFZ is mostly offshore NE-Iceland, but the SISZ lies
within a relatively densely populated agricultural area.
In 1983, the Ad Hoc Committee of Experts on
Earthquake Research of the Council of Europe ap-
pointed the SISZ as one of 5 test sites in Europe suit-
able for earthquake prediction research. In 1988 a
group of Nordic scientists received funding from the
Nordic Council, the science foundations in Sweden,
Norway and Denmark as well as the Government of
Iceland for a project aiming at designing and build-
ing a highly automatic seismic system for monitor-
ing earthquake activity and crustal processes and for
collecting high quality data for earthquake prediction
research. As medium size and big earthquakes are
not very frequent in Iceland, some 6-7 earthquakes of
M > 6.0 every 100-150 years in South Iceland Low-
land (SIL) and similar in TFZ, these would provide
very limited information to monitor crustal processes.
Therefore the design criteria for the system were set
to detect and locate small earthquakes in order to get
almost continuous information about the stress condi-
tion in the crust. From the beginning the SIL seismic
system has been able to detect events of magnitudes
less than 0 in the SISZ, but in other areas the detec-
tion threshold is somewhat higher (0.5-1.5) depend-
ing on station density. With this sensitivity the sys-
tem records from ~20 to almost 2000 earthquakes a
day, giving approximately 160,000 earthquakes in the
database for the first 10 years.
THE SIL SYSTEM
The SIL system is a network of 3-component digi-
tal seismic stations and a data processing system, run
by the Icelandic Meteorological Office (IMO). It con-
sists of automatic event detection and locations and
can thus be used to monitor changes in seismic ac-
tivity, fault dimensions and other seismic parameters
in search of earthquake precursors for prediction re-
search (Stefánsson et al., 1993). The number of sta-
tions has grown from 8 stations in 1990 in the South
Iceland Lowland to 42 stations at the end of year 2000,
mainly covering the seismic zones and the rift zones
through Iceland (Figure 1).
JÖKULLNo. 51 87