Jökull - 01.07.2003, Page 57

Data report
Seismicity in Iceland during 2001
Bergþóra S. Þorbjarnardóttir, Gunnar B. Guðmundsson and Steinunn S. Jakobsdóttir
Department of Geophysics, Icelandic Meteorological Office, Bústaðavegur 9, 150 Reykjavík, Iceland;
begga@vedur.is, gg@vedur.is, ssj@vedur.is
Abstract — Over 14,000 earthquakes were located in 2001 by the SIL seismic system. During the first half
of the year the majority of the located earthquakes were aftershocks of the magnitude 6.6 earthquakes of June
2000. The Mýrdalsjökull region was more seismically active than usual in the first half of the year and the
Reykjanes ridge was fairly active in the latter half of the year. Many small earthquake swarms occurred as
well, the two largest along the Tjörnes Fracture Zone, offshore northern Iceland. The larger of these swarms
originated in Öxarfjörður with a total number of earthquakes greatly exceeding previous swarms in the area,
recorded by the SIL system. Five stations were added to the SIL network and one station was dismantled. At
the end of the year 42 seismic stations comprised the SIL network.
INTRODUCTION
The SIL seismic system is a network of 3-
component digital seismic stations and a data pro-
cessing system (Jakobsdóttir et al., 2002). In 1990
eight stations were installed in the South Iceland Low-
lands. The number has since gradually increased to
42 in 2001 (Figure 1). The stations are concentrated
along the seismic zones and rift zones through Ice-
land. Since its installment the system has recorded
nearly 200,000 events (May, 2003). The density of the
SIL network varies between areas, producing vary-
ing detection levels. In some areas, for example the
Reykjanes peninsula and the South Iceland Lowlands,
the detection threshold is about 0. In other areas,
such as the highlands, the stations are more widely
spaced and only earthquakes greater than 1.5 may be
detected.
At the beginning of 2001, a total of 38 stations
were operating within the SIL network (Figure 1).
Three stations were installed late in the year 2000, but
were not fully operational until 2001. These stations
are; Vestmannaeyjar (ves), offshore southern Iceland,
and Flatey (fla) and Brettingsstaðir (bre) in the north.
Other stations added during the year were Grímsfjall
(grf) on the Vatnajökull ice cap and Eystri Skógar
(esk), southwest of the Mýrdalsjökull glacier. The
Hafnarfjörður (haf) station in southwest Iceland was
dismantled due to increasing traffic in the area. Thus
a total of 42 stations comprised the SIL network at the
end of the year 2001 (Figure 1).
TJÖRNES FRACTURE ZONE
The largest earthquake swarms in 2001 occurred
within the Tjörnes Fracture Zone, offshore northern
Iceland (Figures 1 and 2). The Tjörnes Fracture Zone
is a transform fault zone connecting the rift zone in
north Iceland with that of the Kolbeinsey Ridge. Most
of the seismicity in north Iceland is associated with
this zone (Rögnvaldssson et al., 1998). In Septem-
ber and October 2001 an earthquake swarm occurred
in Öxarfjörður with over 2000 earthquakes. This is
the largest swarm that has been recorded in the Öxar-
fjörður area by the SIL system since it was expanded
in 1994 to cover the northern rift zone (Figure 3). As
the swarm progressed the seismicity migrated to the
southwest.
In December another large swarm was located
within the Tjörnes Fracture Zone, north of Eyjafjörður
(Figures 1 and 2). Over 1200 earthquakes were lo-
cated.
JÖKULL No. 52, 2003 55