Jökull - 01.12.1976, Síða 12

Fig. 2 A topograpliic map of Hekla, showing
the eruptive fissures and lava flows of the 1970
eruption (eruption data taken from Thorarins-
son (1970)). The main Hekla fissure runs along
the crest of the volcanic ridge.
Mynd 2. Hekluhryggur, gossprungur 1970 og
hraunflákar, sem þá runnu.
On May 5, 1970 an eruption started on sev-
eral short fissures on the flanks of Hekla (Fig. 2).
Since only 23 years had passed since the last
eruption, the 1970 eruption was somewhat un-
expected.
The eruption appears to have broken out
nearly simultaneously in two places on the
south and west flanks of the volcano at about
21:23 GMT (Thorarinsson and Sigvaldason,
1972; Thorarinsson, 1970). About an hour later
an eruption started on the northern flank and
shortly before midnight a new fissure opened
on the west flank. The activity on the west and
south flanks died down on May 8 and 10 re-
spectively. The activity on the north flank
ceased on the morning of May 20 but started
again on a new fissure later the same day. This
activity continued until July 5. The total
volume of lava produced during the eruption
is 0.2 km3 (Thorarinsson, 1970).
Considerable seismic activity associated with
the beginning of the eruption was recorded at
the three permanent seismic stations at the dis-
tances of 80—200 km from the volcano. During
the eruption itself most of the observed seis-
micity seemed to be associated with changes in
the behaviour of the eruption.
10 JÖKULL 26. ÁR
On June 22 a network of short period seismo-
meters was installed near the volcano. The net-
work was operated till August 5. These seismo-
meters recorded only very few earthquakes dur-
ing the remaining time of the eruption. When
the volcanic activity ceased, however, a swarm
of small earthquakes was recorded with epi-
centers near the eruption site on the northern
flank.
DATA COLLECTION AND ANALYSIS
Before June 22 seismological data were avail-
able only from the three permanent seismic
stations run by the Icelandic Meteorological
Office. These stations are Sida (SI), a short
period station at 80 km distance from Hekla;
Reykjavik (REY), a short and intermediate
period station at 110 km distance from Hekla;
and Akureyri (AKU), a short and long period
station at the distance of 200 km from Hekla
(Fig. 1). The Akureyri station is part of the
World Wide Network of Standard Seismograph
Stations (WWNSS).
The short period seismometer array installed
near Hekla on June 22 consisted of 6 vertical
geophones of 2 Hz natural frequency. Four
seismometers were spaced 1—2 km apart and
their signals brought by a telephone wire to a
central station where they were amplified and
recorded. Two seismometers were located about
15 km west and northwest of the volcano and
their signals were amplified and transmitted by
an FM radio link to the same central station.
At the central station all 6 signals, together
witli a radio time signal, were recorded on a
seven channel FM magnetic tape recorder (tape
speed 15/160 ips, center frequency 84.4 Hz).
This seismograph system is most sensitive to
ground motion in the frequency range 2—17
Hz. The earthquake data from the magnetic
tape were reproduced on paper with a time
resolution of 5 cm per sec.
For reliable locations of earthquakes, good
knowledge of the crustal structure beneath the
array is necessary. As will be shown later, the
crustal structure near the volcano is somewhat
irregular and can be expected to vary greatly
over the distance to the two stations farthest
from the volcano. These two stations did not
operate continuously and because of their dis-