Jökull - 01.06.2000, Page 69

Páll Einarsson and Bryndís Brandsdóttir
10
20
30
40
50
60
70
N
1 2 3 4 5 6 7 8 9 10 11 12
1978-1985
Eastern cluster
Western cluster
10
20
30
40
50
60
70
N
1 2 3 4 5 6 7 8 9 10 11 12
Figure 7. Histograms showing the cumulative
monthly number of earthquakes during 1978-1985 lo-
cated with an accuray of 2 km or less. The seis-
mic activity is highest in both clusters during July-
October. – Samanlagður fjöldi staðsettra skjálfta í
hverjum mánuði á árunum 1978-1985. Óvissa í stað-
setningum er innan við 2 km. Skjálftavirknin undir
vestari (Goðabungu) og eystri (Kötlu) þyrpingunni er
mest frá júlí og fram í október.
DISCUSSION AND CONCLUSIONS
The Mýrdalsjökull earthquakes are spatially re-
lated to volcanoes and do not have a direct associa-
tion with the plate boundary in South Iceland. Several
physical processes active in a volcano can lead to seis-
mic events, including the following:
1. Brittle fracturing of crustal rock due to stress
changes in response to magma movements,
such as inflation or deflation of a magma cham-
ber, or intrusion of a dyke.
2. Brittle fracturing of crustal rocks due to con-
centrations of regional stress around volumes of
magma.
3. Cracking due to thermal stresses, heating
around magma bodies or cooling due to hydro-
thermal circulation into hot crust.
4. Episodic or unstable magma flow in a conduit.
5. Explosions in magma or in hydrothermal sys-
tems.
Let us examine these possibilities. Explosive phe-
nomena must be considered to be most unlikely in
this area. Manifestations of explosions have never
been identified. There are geothermal areas beneath
Mýrdalsjökull but they are relatively modest. Com-
paring the seismicity of Mýrdalsjökull with that of
other, much more powerful geothermal areas where
thermally related seismicity has been identified, s.a.
Hengill (Foulger, 1988) and Torfajökull (Soosalu and
Einarsson, 1997), it seems rather unlikely that thermal
processes are responsible. Intrusive activity cannot be
ruled out, but we point out that no other evidence indi-
cating magma movements is available. Tilt measure-
ments around Mýrdalsjökull in the last 30 years fail to
show significant accumulation of magma (Tryggva-
son, 2000). Episodes of continuous tremor have not
been recorded except in July 1999 (Einarsson, 2000).
Whatever process is responsible for the seismicity,
it must have been at work for almost half a century.
The fact that seasonal variations have triggered seis-
micity all this time implies that a very sensitive bal-
ance is maintained between the loading and trigger-
ing processes. In order for almost all the seismicity to
68 JÖKULL No. 49