Jökull - 01.12.1989, Blaðsíða 53

Two clusters coincide with gravity lows in the
westem volcanic zone. Both are characterized by
swarm activity. The southemmost one, at the shield
volcano Skjaldbreiður, had substantial swarms in
1984 with the largest event reaching magnitude 4.5.
The northemmost cluster, near Langjökull, is to a
large degree due to a swarm in 1985, with the
strongest earthquakes exceeding magnitude 4.
In contrast to the volcanic zone, the E-W
transform zone in South Iceland does not show any
obvious correlation with lineaments in the gravity
map. The South Iceland seismic zone cuts across a
positive gravity anomaly, the highest one in the
map. A pronounced epicentral cluster occurs near
the apex of the anomaly. This cluster is a persistent
feature of the seismicity, and may represent an area
of stress concentration around an asperity in the
crust. The gravity anomaly indicates irregularity in
the crustal structure, possibly the source of the stress
concentration.
The eastemmost earthquake cluster all belongs to
a single sequence of events in May 1987. The
Fig. 11. The cross-grain structures. The
hatched areas show filtered gravity
anomalies exceeding 4 milligals while the
- shaded areas show filtered negative mag-
netic anomalies below -200 nanotesla.
Both data sets were passed through a nar-
row directional filter oriented 135° with a
bandwidth of 40°. Also shown by a heavy
broken line is the contour line drawn by
_ Pálmason (1971) encompassing an area
with depth less than 2.5 kilometers down
to seismic layer 3.
Myndll. Meginlínur þvert á gliðnunar-
beltið. Strikuðu svœðin sýna síuð þyngd-
atfrávik yfir 4 milligal en gráu svœðin
- sýna síuð segulfrávik undir -200 nanó-
tesla. Bœði þyngdar- og segulkortið voru
síuð með þröngri stefnusíu sem sneri 135°
og var 40° breið. Brotna línan umlykur
svœði þar sem dýpi á hljóðhraðalag 3 er
minna er 2,5 kílómetrar (Guðmundur
Pálmason 1971).
sequence was a typical foreshock-mainshock-
aftershock sequence, with a mainshock of magni-
tude 5.8. The event was associated with right-lateral
strike-slip on a vertical, N-S striking fault (Bjama-
son and Einarsson, in press). The mode of faulting is
thus similar to that of previous large earthquakes in
the transform zone as interpreted from surface
features (Einarsson and Eiríksson, 1982).
ANOMALIES IN THE VOLCANIC ZONE
Figure 13 shows a composite map of the earth-
quakes and the gravity low. and magnetic high
observed in the volcanic zone. The gravity anomaly
presumably is generally deep-seated, with roots
extending at least 5-10 kilometers downward. One
must bear in mind, however, that a part of the ano-
maly may be a fictitious creation of the Bouguer
correction, due to unsuitable density values. Most of
the magnetic anomaly has a much shallower origin,
probably no more than 2 kilometers. The earth-
quakes plotted here all have their origin at depths
less than 10-15 kilometers.
JÖKULL, No. 39, 1989 51