Jökull - 01.12.2007, Page 47

Reviewed research article
Seismicity in Iceland during 2006
Bergþóra S. Þorbjarnardóttir, Gunnar B. Guðmundsson,
Sigurlaug Hjaltadóttir and Matthew J. Roberts
Icelandic Meteorological Office, Physics Department,
Bústaðavegur 9, 150 Reykjavík, Iceland;
begga@vedur.is, gg@vedur.is, slauga@vedur.is, matthew@vedur.is
Abstract— Approximately 9,500 earthquakes were detected in Iceland during 2006 by the SIL seismic network
of the Icelandic Meteorological Office. This is similar to the number of earthquakes registered in 2005, but
slightly less than in preceding years. The largest earthquake occurred on 6 March, east of Lake Kleifarvatn
on the Reykjanes Peninsula. The event, which was preceded by foreshocks, had a local moment magnitude
of 4.7 (Mlw); additionally, several aftershocks were registered. Three earthquake swarms were recorded a
few kilometers east of the tip of the Reykjanes Peninsula. Sparse activity has previously been recorded in this
area. In the Hengill region, the largest earthquake sequence was a swarm of 80 earthquakes on 29 May.
No major activity was recorded in the South Iceland Seismic Zone. Seismicity beneath Goðabunga in the
Mýrdalsjökull ice cap followed a seasonal pattern, as in the last few years and preceding 2002. North of
Goðabunga, beneath Entujökull, a short-lived swarm of high-frequency earthquakes took place in November.
The largest earthquake in the Vatnajökull region was a mainshock-aftershock sequence in the northwestern
sector of the ice cap in September. The majority of earthquakes in the Askja-Herðubreið area, north of the
Vatnajökull ice cap, were recorded in two earthquake swarms: one northwest of Herðubreiðartögl in April and
the other north of Herðubreið in May. The most intense earthquake sequence of the year was a mainshock-
aftershock sequence within the Tjörnes Fracture Zone. The sequence occurred southeast of the island Flatey at
the beginning of November, initiated by a mainshock of magnitude 4.2. Over a seven-day period in September,
a swarm of intraplate earthquakes was detected in the Western Fjords, within a Tertiary fault zone.
INTRODUCTION
In Iceland seismicity takes place mainly within two
transform zones and three interconnecting volcanic
zones. These zones are the South Iceland Seismic
Zone (SISZ), the Tjörnes Fracture Zone (TFZ), the
Western Volcanic Zone (WVZ), the Eastern Volcanic
Zone (EVZ) and the Northern Volcanic Zone (NVZ)
(Figure 1).
In this article, earthquake activity in Iceland in
2006 is described and compared to that of previous
years. Some earthquake sequences have been relo-
cated using a double-differencemethod (Slunga et al.,
1995) to increase their positional accuracy. With im-
proved data on hypocenter and epicenter locations,
fault patterns become better resolvable (Hjaltadóttir
and Vogfjörð, 2005). The focal mechanisms of several
notable earthquakes are discussed. With these two
methods we are able to define common fault planes
and determine their slip direction. The magnitude
scale used here is a local moment magnitude scale,
Mlw (Slunga et al., 1984).
JÖKULL No. 57 45