Jökull - 01.01.2005, Side 107

Reviewed research article
Seismic activity in Iceland during 2004
Sigurlaug Hjaltadóttir, Halldór Geirsson and Þórunn Skaftadóttir
Icelandic Meteorological Office, Physics Department,
Bústaðavegur 9, 150 Reykjavík, Iceland;
slauga@vedur.is, dori@vedur.is, thorunn@vedur.is
Abstract— Using seismological data from the Icelandic Meteorological Office, this article summarises inland
and offshore earthquake activity in Iceland during 2004. About 12,000 earthquakes were recorded during the
year. The largest earthquake had a body-wave magnitude of 4.5 and it occurred on 25 September 80 km offshore
from the southwest tip of the Reykjanes Peninsula. Two intense earthquake swarms occurred on the Reykjanes
Ridge; one in April and the other in September. The largest swarm on the Reykjanes Peninsula comprised
1500 earthquakes; it occurred in July and was concentrated beneath the Fagradalsfjall mountain. Activity in
the Ölfus-Hengill region paralleled that of the previous three years, with the largest earthquake attaining a
local moment-magnitude (Mlw) of 4.2. In the South Iceland Seismic Zone, residual aftershocks occurred on
the faults produced by two local Mw=6.5 earthquakes in June 2000. Two earthquake swarms were observed
northeast of Langjökull in a region where earthquakes are uncommon. Earthquake activity within the western
flank of the Katla volcano showed strong seasonality, peaking during the autumn as observed for decades. An
earthquake swarm took place within the Katla caldera, including a M lw 3.9 earthquake on 02 July. Seismic
activity at Grímsvötn increased significantly from mid 2003 until late 2004. On 01 November, a swarm of over
180 earthquakes were recorded at Grímsvötn; at the height of this swarm, a volcanic eruption began, lasting for
a week. A public warning was issued one hour ahead of the eruption. Additionally, 29 icequakes were located
in Skeiðarárjökull during a jökulhlaup from Grímsvötn that began two days before the November eruption.
Throughout 2004, many earthquake swarms happened in the Tjörnes Fracture Zone; the largest swarm was
located at the mouth of Eyjafjörður and comprised 600 events. Swarms of similar intensity are commonplace
in this region.
INTRODUCTION
Background and aim
Seismicity in Iceland has been monitored over the
last 15 years by the national seismic network, SIL,
which has been operating automatically since June
1991 (Jakobsdóttir et al., 2002). During this time,
over 230,000 events have been recorded and pro-
cessed daily to provide nearly continuous information
about stress conditions in the Icelandic crust. Addi-
tionally, the near-real-time data has proven invaluable
for predicting and monitoring hazardous events, such
as volcanic eruptions and jökulhlaups.
The main aim of this article is to summarize
the seismicity during 2004 and compare this year’s
seismicty to that of previous years. We will give
an overview of earthquake activity in selected areas,
starting on the Reykjanes Ridge in the southwest, fol-
lowing the plate boundary to the Tjörnes Fracture
Zone in the north.
The SIL seismic network
The SIL (South Iceland Lowland) seismic network
consists of 44 three-component, digital stations, lo-
cated mostly around the tectonic plate-boundary,
where seismic activity is highest (Jakobsdóttir et al.,
2003) (Figure 1). In December 2004, three new sta-
tions were established in the Kárahnjúkar area, north
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