Jökull - 01.12.2007, Page 59

Seismicity in Iceland during 2006
seismicity was triggered by the injection of water into
a new borehole.
4. Seismic activity beneath the western part of
Mýrdalsjökull ice cap showed a seasonal pattern as
in the last few years and preceding 2002. Also, an un-
usual swarm of high-frequency earthquakes was ob-
served at 7 km depth beneath the Entujökull glacier
in November. Focal mechanisms suggest right-lateral
movement on a northwest striking plane.
5. The addition of seismometers north of the
Vatnajökull ice cap in the last few years has increased
the number of earthquakes detected from northwest
Vatnajökull to the Askja caldera and Herðubreið. The
largest earthquake sequence recorded in the Vatna-
jökull region in 2006 was a mainshock-aftershock se-
quence in the northwestern sector of the ice cap. The
magnitude 3.7 mainshock, together with aftershocks,
clustered along a near-vertical plane striking slightly
to the northeast, at depths of 10–13 km.
6. Recorded seismicity in the Askja-Herðubreið
region in 2005 and 2006 was predominantly
swarm activity in the vicinity of Herðubreið and
Herðubreiðartögl. Earthquakes in three swarms were
located at 3–6 km depth on northeasterly striking
near-vertical fault planes.
7. Earthquake sequences were common within
the TFZ, the largest a mainshock-aftershock sequence
southeast of Flatey Island. The magnitude 4.2 main-
shock and subsequent aftershocks were located at a
depth of about 11 km on a WNW-striking fault plane,
dipping to the northeast.
8. Seismicity, primarily swarm activity, beneath
Kaldbakur has increased in the last few years. In 2006
the largest swarm yet recorded occurred in August
and a smaller swarm was located in the same area in
November.
9. A swarm of intraplate earthquakes was de-
tected south of Reykjarfjörður in the Western Fjords
in September. Two additional swarms have been
recorded in the Western Fjords: one in 1964 and the
other in 1994, plus the occasional isolated earthquake.
Acknowledgements
This paper is based on data from weekly reports made
by scientists from the Physics Department of the Ice-
landic Meterological Office. The monitoring team in
2006 comprised Bergþóra S. Þorbjarnardóttir, Gunnar
B. Guðmundsson, Halldór Geirsson, Helgi Gunnars-
son, Hjörleifur Sveinbjörnsson, Kristín S. Vogfjörð,
Kristján Ágústsson, Matthew J. Roberts, Sigurlaug
Hjaltadóttir, Sigþrúður Ármannsdóttir, Steinunn S.
Jakobsdóttir and Þórunn Skaftadóttir. The team con-
tributed to the operation of the SIL system and to the
expansion of the earthquake database. Jósef Hólmjárn
and Sighvatur K. Pálsson are responsible for main-
taining the SIL stations and Reynir Böðvarsson and
Ragnar Slunga are the main designers of the sys-
tem. Gratitude is expressed to Bryndís Brandsdótt-
ir for providing bathymetric data (Figure 10) and to
Amy Clifton for providing survey measurements of
fractures on the Reykjanes Peninsula (Figure 3). We
also thank Gunnar Geir Pétursson for deriving the b–
value data in Figure 6. Thanks are expressed to Páll
Einarsson and another reviewer for comments on an
earlier version of this article. Most figures were made
with GMT software (Wessel and Smith, 1998).
ÁGRIP
Skjálftavirkni á Íslandi árið 2006
Í lok árs 2006 voru 50 jarðskjálftastöðvar í SIL jarð-
skjálftamælaneti Veðurstofu Íslands. Þrjár stöðvar
bættust við netið á árinu, í Kaldárseli við Hafnar-
fjörð, á Goðabungu í Mýrdalsjökli og í Kreppuhrauni
á Kárahnjúkasvæðinu. Um 9.500 skjálftar voru stað-
settir með SIL kerfinu á árinu, sem er svipaður fjöldi
og árið áður en heldur minni en árin þar á undan. Um
480 þessara skjálfta voru ísskjálftar sem mældust í
Skeiðarárjökli og stöfuðu að mestu leyti af mikilli úr-
komu. Auk þess mældust mörg hundruð sprengingar.
Margar skjálftaraðir urðu á Reykjanesskaga, sér-
staklega í nágrenni Kleifarvatns. Stærsta röðin hófst
með skjálfta 4,7 að stærð austan við Kleifarvatn í
mars. Hann fannst víða um Suðvesturland. Þrjár
skjálftahrinur mældust austan við Reykjanestá, en lítil
virkni hefur áður mælst þar.
Mesta skjálftaröðin á Hengilssvæðinu var skjálfta-
hrina 29. maí, nokkrum kílómetrum vestur af Hvera-
gerði, en þá mældust 80 skjálftar. Einnig mældust
skjálftar í kjölfar niðurdælingar í borholu við Hellis-
heiðarvirkjun. Á Suðurlandsundirlendinu var virkni
JÖKULL No. 57 57