Jökull - 01.12.1989, Page 52

Fig. 10. The magnetic ano-
maly map, showing the
positive anomalies in the
magnetic map after direc-
tional filtering along the
axis of the Hengill-Lang-
jökull rifting zone. The
filter direction is 40° and its
width is 80° (out of 180°).
The contour lines start at
250 nanotesla and are 250
nanotesla apart.
Mynd 10. Segulkort sem
sýnir jákvœð segulfrávik
stefnusíuð eftir ás gliðnun-
arbeltisins Hengill-Lang-
jökull. Stefna síunnar er 40°
og breiddin er 80° (af
180°). Jafngildislínur byrja
í 250 nanótesla og milli
þeirra eru 250 nanótesla.
temperature, i.e. perpendicular to the constructive
plate boundary. In an oblique rift zone the direction
of such cracks would not be the same as the direc-
tion of plate motion. These cracks are likely to be
preferred locations for off-rift magmatism (Pálma-
son, 1981), leading to anomalous magnetization and
linear magnetic and gravitational anomalies.
THE EARTHQUAKE DATA AND
THE GRAVITY MAP
Figure 12 shows epicenters of earthquakes super-
imposed on the gravity map. The earthquakes are in
the magnitude range 1.5-5.8 and occurred during the
time interval 1974-1987. Parts or all of these data
have been published previously (Einarsson and oth-
ers, 1981; Einarsson and Sæmundsson, 1987;
Foulger, 1988a; Einarsson, 1989). The data are
obtained from a permanent network of short period
seismographs, which has a station spacing of 10-45
km in this area (Einarsson and Bjömsson, 1987).
Most epicenters are located with standard errors
smaller than 2 km, as calculated with the location
program HYPOINVERSE (Klein, 1978).
The epicenters follow the plate boundaries quite
faithfully, i.e. the active volcanic zones and the
South Iceland seismic zone that stretches across the
southem lowlands near 64° N latitude. The seismi-
city is not evenly distributed along these zones,
however. It shows a persistent clustered pattern. The
two westemmost clusters are related to geothermal
areas, the Krísuvík and Hengill areas. The seismic
activity here is quite persistent from year to year,
and in the case of Hengill it has been shown to be
related to the process of heat extraction from the
crust (Foulger, 1988b). Both areas seem to be unre-
lated to any particular features in the gravity map.
50 JÖKULL, No. 39, 1989