Jökull - 01.12.2003, Síða 51

Data report
Seismicity in Iceland 2002
Bergþóra S. Þorbjarnardóttir and Gunnar B. Guðmundsson
Department of Geophysics, Icelandic Meteorological Office, Bústaðavegur 9, 150 Reykjavík, Iceland;
begga@vedur.is, gg@vedur.is
Abstract — Nearly 14,000 earthquakes were located by the SIL seismic system in 2002, on a network of 42
stations. The largest earthquake episode took place within the Tjörnes Fracture Zone, between the islands
of Grímsey and Kolbeinsey. This swarm was initiated by a magnitude 5.5 (mb) earthquake. Seismic activity
beneath the Mýrdalsjökull glacier was greater than usual and prevailed throughout the year. Seismicity beneath
Vatnajökull was also considerable in 2002 with several jökulhlaups accompanied by icequakes, earthquakes
and seismic tremor, an earthquake swarm by the Esjufjöll mountains and a magnitude 4.3 (mb) earthquake plus
aftershocks beneath the Bárðarbunga central volcano. A significant number of events was located along the
June 2000 South Iceland Seismic Zone faults.
INTRODUCTION
The SIL seismic system comprises a network of
three-component digital seismic stations and a data
processing system (Jakobsdóttir et al., 2002). From
1990 to 2001 42 stations were installed along the ac-
tive seismic zones and rift zones in Iceland. No sta-
tion was added to the network in 2002, but consider-
able work was put into improving the quality of the
system.
The SIL system records microearthquakes down
to magnitudes less than zero using a locally de-
rived magnitude scale. As earthquakes exceeding
magnitude 4 tend to be underestimated by the local
scale, body-wave magnitudes (mb) calculated by the
National Earthquake Information Center (NEIC) are
used instead.
Seismicity within the Tjörnes Fracture Zone
(TFZ), beneath the Mýrdalsjökull glacier and the
western Vatnajökull ice cap is emphasized here (Fig-
ure 1). The earthquake activity in these areas was
above average in 2002. The faults of the June 2000
South Iceland Seismic Zone (SISZ) earthquakes were
still fairly active in 2002 but this aftershock activity is
decreasing gradually.
Reports from the public were received concerning
14 earthquakes felt during the year. A list of these
earthquakes is in Table 1.
SEISMICITY WITHIN THE TJÖRNES
FRACTURE ZONE
Most earthquake swarms in 2002 occurred along
the Tjörnes Fracture Zone, offshore northern Iceland
(Figure 1). The largest earthquake episode started
with a magnitude 5.5 earthquake (mb, NEIC) on
September 16. It was located 53 km NNV of the is-
land Grímsey, in the northernmost part of the zone
(Figure 2), and was felt widely along the northern
coast. NEIC calculated the focal planes as either strik-
ing N35◦E and dipping 73◦ to the southeast or a near
vertical plane striking N125◦E. Over 300 aftershocks
were recorded, the largest with magnitude 4.3 (mb) on
September 17. The aftershocks were distributed along
a 15 km long zone, extending SSE from the main-
shock hypocenter (Figure 2). This is the largest swarm
that has been recorded in this area since 1994 when
the SIL seismic network was extended to north Ice-
land. Simultaneously and following this swarm came
two other swarms, about 15 km NNV of Grímsey and
5 km east of the island, with a total of over 200 earth-
JÖKULL No. 53, 2003 49