Jökull - 31.12.2001, Page 91

Seismicity in Iceland 1991-2000
Figure 2. Seismicity in SW-Iceland between 1991 and 1993. The red dots represent earthquakes. The green
stars show earthquakes greater than magnitude 4. Black triangles show the location of the SIL stations dur-
ing this period. - Jarðskjálftavirkni á Suðvesturlandi árin 1991 til 1993. Rauðu deplarnir tákna jarðskjálfta.
Grænu stjörnurnar tákna jarðskjálfta sem eru stœrri en 4. Svörtu þríhyrningarnir tákna SIL mœlistöðvar á
þessu tímabili.
letin within a few minutes of its occurrence and even
before the waveform data have been saved to files on
the hard disk. The ringbuffer is overwritten after 10-
30 days. A more detailed description of the SIL sys-
tem can be found in Stefánsson et al. (1993), Böð-
varsson et al. (1996) and Böðvarsson et al. (1999).
The local magnitude scale M/, used in the SIL sys-
tem, is an empirical scale using maximum amplitude
and calibrated to give the same magnitudes as those
calculated from the older analog instruments. This
method gives a fairly good fit to the old local mag-
nitude scale for earthquakes of magnitudes less than
3, but for bigger events it probably underestimates the
magnitude. Some work is yet to be done to improve
the local magnitude scale. For events larger than 5 the
Ms magnitudes from the National Earthquake Infor-
mation Center (NEIC) are used in this paper.
SEISMICITY IN SW-ICELAND DURING
1991-1993
The SIL database officially started on July 1, 1991.
However, some data exist from 1990 and the first half
of 1991. The volcanic Hekla eruption on January 17,
1991 was the first big event monitored by the SIL sys-
tem. The system was still under development at the
time and the onset of the eruption was not foreseen,
JÖKULLNo. 51 89