Jökull - 31.12.2001, Blaðsíða 93

Seismicity in Iceland 1991-2000
jökulhlaup in the river Jökulsá á Sólheimasandi (Ein-
arsson, 2000; Guðmundsson et al., 2000).
In June 2000 two large earthquakes struck near the
center of the SISZ (Pedersen et al., 2001; Árnadóttir et
al., 2001). The first occurred on June 17 with Ms=6.6
and triggered earthquakes of magnitudes near 5.0 to
the west throughout Reykjanes Peninsula. High seis-
micity was also triggered to the north, activating the
area south of Langjökull and near Geysir. The sec-
ond earthquake struck on June 21 with Ms= 6.6. Since
then, seismic activity has remained high on the faults
of both earthquakes.
DISCUSSION AND CONCLUSIONS
The first decade of the SIL system has been a phase
of building up a digital high quality seismic system
to cover the active seismic and volcanic zones in Ice-
land. Large amounts of data have been collected and
processed, which in turn has increased the knowledge
and understanding of crustal structure and crustal pro-
cesses. The data have been used to map active faults,
monitor stress changes in the crust, monitor swarms in
near real-time, develop methods for earthquake pre-
diction and model the velocity structure of the crust
by different methods. Lour volcanic eruptions have
been monitored by the system. All of them have been
discovered from the seismic activity recorded by the
analog highland network and the SIL system and re-
ported by scientists at the Science Institute and IMO.
Some efforts have been made to find and test
methods that might help in predicting earthquakes.
Common to all these methods is their focus on moni-
toring changes in some calculated parameters that are
likely to reveal if the stress approaches the breaking
point of an asperity. Some examples are the hori-
zontal stress component of microearthquakes, shear-
wave splitting (Crampin et al., 1999) and a multipa-
rameter method developed by Slunga (2003). None
of these methods have proved to be a robust single
parameter prediction tool, but they all give hints that
can be included in interpretations leading to predic-
tions. The experience obtained from the SIL system,
monitoring earthquakes down to magnitudes less than
0, has shown that many (most) earthquakes greater
than Mi= 4 are not preceded by heavy seismic activity,
which would be helpful in predicting the time factor
of expected earthquakes. Accordingly the SISZ earth-
quake of June 17, 2000 was not predicted on a short-
term basis, even though it was predicted on a long-
term basis. But the second big earthquake, on June
21, was foreseen and a warning was issued to the Na-
tional Civil Defence 26 hours before it occurred, stat-
ing that an earthquake of magnitude close to Ms=6.5
could be expected any time soon. The expected loca-
tion almost coincided with the actual rupture (Stefáns-
son et al. 2000b) With all the data now available from
these episodes the earthquake prediction research will
continue.
Acknowledgements
This paper is based on the work of people work-
ing at the Department of Geophysics, Icelandic Me-
teorological Office during 1990-2000. These are
Barði Þorkelsson, Bergur H. Bergsson, Bergþóra
S. Þorbjarnardóttir, Einar Kjartansson, Gunnar B.
Guðmundsson, Helgi Gunnarsson, Hjörleifur Svein-
björnsson, Jósef Hólmjárn, Karl L. Sellgren, Krist-
ín Jónsdóttir, Kristín S. Vogfjörð, Kristján Ágústs-
son, Margrét Ásgeirsdóttir, Olafur Rögnvaldsson,
Páll Halldórsson, Pálmi Erlendsson, Ragnar Stefáns-
son, Sigurður Th. Rögnvaldsson, Steinunn S. Jakobs-
dóttir, Sturla Ragnarsson, Sveinn Ólafsson, Vigfús
Eyjólfsson and Þórunn Skaftadóttir. They contributed
in running the SIL system and building up the
database. Heidi Soosalu has also worked on refining
the database. Reynir Böðvarsson and Ragnar Slunga
are the main designers of the SIL system.
ÁGRIP
Jarðskjálftar á íslandi 1991-2000 mældir með SIL
jarðskjálftamælanetinu.
SIL-kerfið er stafrænt og sjálfvirkt jarðskjálftamæla-
kerfi sem safnar og vinnur upplýsingar úr skjálfta-
mælingum. Forsögu að uppbyggingu kerfisins má
rekja aftur til ársins 1979 þegar Evrópuráðið hvatti
til aukinna rannsókna á jarðskjálftahættu og jarð-
skjálftaspám. Árið 1983 útnefndi ráðið að frumkvæði
íslendinga 5 svæði í Evrópu sem tilraunasvæði í
slíkum rannsóknum, og var Suðurlandsundirlendið
JÖKULLNo. 51 91