Jökull - 01.12.1976, Page 17

Fig. 5. Á map of the
Hekla region showing
eruptive vents and fissur-
es, the 1970 lava flows,
the location of the tem-
porary seismic network
and the epicenters of
earthquakes that occurred
between June 22 and Aug-
ust 5. The largest circles
indicate the best located
epicenters, the error is
less than about ± 2.5 km.
The calibration explo-
sions for the seismic array
were set off in the small
lake to the north of the
cluster of epicenters.
Mynd 5. Gigar og gos-
sprungur á H'eklusvœði.
Gráir flákar sýna hraun,
sem runnu 1970. Þrihyrn-
ingar sýna skjálftamala,
opnir hringir stað skjálfta. Starstu hringirnir tákna skjálfta, sem nákvcemast voru staðsettir. Óná-
kvcemni í stað þeirra er þó ± 2,5 km. Til að bceta nákvcemni í staðsetningu voru sprengingar gerð-
ar i Sauðafellsvatni rétt norður af upptökum skjálftanna.
The temporary seismic network, on the other
hand, recorded continuous volcanic tremors
with predominant frequency of 1.5 to 3.0 Hz.
These tremors were too weak to be recorded
by the permanent seismograph stations. During
this period the volcanic activity continued on
the northern flank without any major change.
By the end of June the intensity of the erup-
tion had diminished considerably followed by
a corresponding decrease in the amplitude of
the volcanic tremors.
The end of the eruption
On July 4 a swarm of earthquakes occurred
with origin near the last active fissure on the
northern flank. On July 5 the eruption on that
fissure stopped. These events were followed by
a period of some seismic activity. Five earth-
quakes were large enough to be recorded by
the AKU station, the largest (M = 3.9) occurr-
ing on July 6. Thirty-five earthquakes from this
active period could be located with data from
the temporary seismic network. The locations
are shown in Fig. 5. The error in the best loca-
tions (the largest circles in Fig. 5) is considered
to be less than about ± 2 km in horizontal
dimensions. All the events outside of the seis-
mic array were recorded with apparent P-wave
velocity of less than 5.0 km/sec. The depth of
the hypocenters must therefore be less than 3
km. No further specification of the depth is
possible.
All the located events occurred on the NW
side of Hekla, most of them close to the last
active fissure on the northern flank of the
volcano. The epicenters seem to define a zone
of diffuse activity with an E-W trend, but the
number of events is hardly large enough to
draw definite conclusions.
The earthquakes of July 4 occurred in a tight
cluster, whereas the subsequent seismic activity
took place in a wider zone. Fig. 6 demonstrates
this behaviour. The distance of the epicenters
from the center of the initial swarm are plotted
versus the time the earthquake occurred. An
outward migration of seismic activity is sug-
gested.
Both compressional and dilatational first mo-
JÖKULL 26. ÁR 1 5