Jökull - 01.12.1976, Page 10

Seismic Activity Associated with the 1970
Eruption of Volcano Hekla in Iceland
PÁLL EINARSSON,1 2
LAMONT-DOHERTY GEOLOGICAL OBSERVATORY“
OF COLUMBIA UNIVERSITY IN THE CITY OF NEW YORK
SVEINBJÖRN BJÖRNSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ICELAND, REYKJAVÍK
ABSTRACT
The eruption that started on May 5, 1970 on
the flanks of volcano Hekla in south central
Iceland was accompanied by considerable seis-
mic activity. Continuous volcanic tremors were
recorded by the seismograph stations in Iceland
for about 25 minutes before the eruption broke
out. It is considered likely that the eruption
could have been predicted a few liours in ad-
vance if a sensitive short period seismograph
with visible recording had been in operation
in the vicinity of the volcano. The beginning
of the eruption was followed by a swarm of
earthquakes (M^4.5) that lasted 15 days and
appeared to be associated with changes in the
volcanic activity. During periods of no changes
the seismicity was generally low. On June 22 a
temporary array of short-period seismometers
was set up near the volcano. The array record-
ed volcanic tremors with predominant fre-
quencies of 1.5—3.0 Hz during the eruption.
These tremors, too weak to be recorded by the
permanent seismograph stations, died down as
the eruption approached its end. Around July 5
when the eruption stopped a swarm of shallow
earthquakes (M^3 3.9) occurred near the last
active fissure. This seismic activity spread out in
space as it decayed with time.
Irregularities in P-wave travel times were ob-
served near Hekla and can be explained by a
dip of the crustal layers above layer 3 (Vp —
6.5 km/sec). The most probable dip is 10° to
the SE.
1) Now at Science Institute, University of Iceland,
Reykjavík.
2) Lamont-Doherty Geological Contribution No. 2245.
INTRODUCTION
This paper describes tlre results of a seismo-
logical study of the 1970 eruption of the vol-
cano Hekla in Iceland.
The Hekla volcano is situated at the western
edge of the eastern volcanic zone in southern
Iceland (Fig. 1). The volcanic edifice is an
elongated ridge (Fig. 2) with a fissure running
along its crest. Major eruptions of Hekla take
place on this fissure, thus maintaining the ridge
shape of the volcano.
The eastern volcanic zone is one of the active
zones marking the trace of the mid-Atlantic
ridge through Iceland. The main fissure of
Hekla is frequently thought of as a clear in-
dicator of the tension across the zone associated
with the process of sea floor spreading. In
several respects, however, Hekla is atypical. The
volcano is situated near the intersection be-
tween the eastern volcanic zone and the zone
of large earthquakes (Fig. 1) that extends to
the west across southern Iceland (see Tryggva-
son (1973), Björnsson and Einarsson (1974)).
This earthquake zone was interpreted as a
transform fault by Ward et al. (1969) and Ward
(1971). Tlius Hekla is near the junction be-
tween a spreading segment of the mid-Atlantic
ridge and a shear zone where the stress field
may by very complex. Furthermore, the Hekla
fissure is anomalous among volcanic fissures in
Iceland as it has erupted repeatedly in historic
times. This anomalous behaviour may be re-
lated to Hekla’s peculiar tectonic setting.
With its 15 historical eruptions (Thorarinsson,
1970) Hekla ranks by volume as the second most
productive volcano in Iceland for the last 1000
8 JÖKULL 26. ÁR