Jökull - 01.12.2007, Page 49

Seismicity in Iceland during 2006
SIL SYSTEM
From mid 1991 to 2006 over 250,000 earthquakes
were located using the SIL seismic network. The
system, which has operated continuously since July
1991, consists of a network of three-component digi-
tal seismic stations and a centralized data processing
system. It was designed to detect and locate small
earthquakes in order to obtain nearly continuous infor-
mation about stress conditions in the crust (Stefánsson
et al., 1993; Jakobsdóttir et al., 2002).
At the end of 2006, the SIL network consisted of
50 seismic stations, situated mainly along the plate-
boundary, where seismic activity is highest (Figure 1).
During the year, three new stations were added to the
network: in the southwest, on the Mýrdalsjökull ice
cap, and north of the Vatnajökull ice cap, in the area
of a hydroelectric power plant. Six stations have been
installed in the power plant area since late December
2004. They are in place to monitor possible seismic-
ity induced by the filling of a water reservoir for the
power plant (Jakobsdóttir et al., 2005). With the ad-
dition of these stations, the detection threshold of the
SIL system for earthquakes in the region from north-
west Vatnajökull to the Askja caldera and Herðubreið
has been lowered considerably.
SEISMIC ACTIVITY
In 2006, approximately 9,500 earthquakes were lo-
cated; this is similar to the number of events in 2005,
but slightly less than in the period 2001–2004 (Fig-
ures 1 and 2). Of these events, 480 were icequakes
detected in Skeiðarárjökull glacier and mainly asso-
ciated with intense rainfall (Figures 1 and 8). In ad-
dition, several hundred explosions from construction
sites were recorded.
The largest earthquake in 2006 had a magnitude
of 4.7, located east of Lake Kleifarvatn on the Reykja-
nes Peninsula. Three additional earthquakes occurred
with magnitudes between 4 and 5. One was located
on the Reykjanes Ridge and two occurred within the
Tjörnes Fracture Zone (Figure 1).
Figure 2. Yearly number of earthquakes recorded in
Iceland between 1994 and 2006 in specified magni-
tude ranges. – Árlegur fjöldi mældra jarðskjálfta frá
1994–2006.
Reykjanes Peninsula
Numerous earthquake sequences occurred on the
Reykjanes Peninsula during 2006, particularly in the
vicinity of Lake Kleifarvatn. However, the most un-
usual activity was east of the tip of the peninsula.
Sparse seismicity has previously been recorded there,
though fissures have been mapped north of this recent
activity (Clifton and Kattenhorn, 2006; Einarsson and
Sæmundsson, 1987). Three short-lived earthquake
swarms took place on 31 May - 1 June, 9–10 July and
27–28 September (Figure 3). Half of the earthquakes
were recorded in the July swarm, but the largest earth-
quake occurred during the September swarm, yielding
a magnitude of 3.
Seismicity is very common in the area surround-
ing Lake Kleifarvatn. In 2006, eight earthquake se-
quences were recorded, most of them small, short-
lived swarms. From 25 February to 10 March, a
foreshock-mainshock-aftershock sequence took place
east of the lake. About 10 foreshocks were recorded
between 25 February and 1 March. The mainshock
occurred on 6 March and had a magnitude of 4.7. It
was felt at distances of up to 70 km from the epicenter
(Table 1) (Bird et al., 2008). Several aftershocks were
located between 6 and 10 March (Figure 4). After
relocating the earthquakes using a double-difference
JÖKULL No. 57 47