Jökull - 01.12.1976, Page 14

the blasts it is possible to calculate the thickness
of layer 0. This thickness was found not to be
significantly different from Pálmason’s results
on the profile east of Hekla. A significant dif-
ference was found, however, for the thickness
of layer 1. Since apparent velocity correspond-
ing to the velocity in layer 1 was measured
across the array the cross-over distance between
the head wave arrivals from layer 1 and layer 2
must be greater than 12 km. Thus a lower limit
of 2.0 km is found for the thickness of layer 1.
As described earlier the crustal velocity model
in Table 1 was approximated by a model with
layers of constant velocity gradients. The latter
model was then used as an input for the loca-
tion program.
Using the location program to locate the ex-
plosions it was found that the distances and
depths were well within the error limits, but
the azimuths were systematically wrong by 15—
18°. This systematic deviation can be corrected
by assuming a time delay of 0.05 sec on the
southeasternmost station in the array. A dip of
the interface between layer 0 and layer 1 of
10° to the SE would cause the necessary time
delay.
Many different combinations of relative time
delays can produce the observed error in azi-
muth. But since the apparent velocity of the
explosions NE of the array was not significantly
in error the azimuth error is most likely caused
by a time delay on the south-easternmost sta-
tion. Furthermore it can be explained by a
dipping interface with a strike parallel to the
structural grain of the region.
Fortunately most of the earthquakes occurred
close to the lake where the explosions were
made. Thus the azimuth corrections applied to
most of the earthquakes were nearly identical
to those applied to the explosions and are
therefore independent of any assumptions about
irregularities in the crustal structure.
During the period of seismic recording at
Hekla a swarm of earthquakes occurred near
Sandvatn about 50 km to the NNW of Hekla
(Fig. 1). Numerous earthquakes of that swarm
were recorded by the Hekla array. Judging
from the crustal structure, the distance and the
probable depth of these earthquakes the first
P-wave should arrive at the Hekla array with
an apparent velocity of about 6.5 km/sec, which
is the P-velocity in layer 3. It was found, how-
12 JÖKULL26. ÁR
ever, that the apparent velocities were signi-
ficantly lower, averaging about 5.5 km/sec. As
before it is possible to explain this difference
by assuming a time delay on tlie southeastern-
most station. The necessary delay is of the same
magnitude as the one necessary to explain the
discrepancy in tlie azimuth of the explosions.
If interpreted in terms of dipping layers this
observation implies that all interfaces down to
and including the layer 2 — layer 3 interface
have the same dip of 10° to the SE.
THE SEISMIC ACTIVITY
Before the eruption
An examination of the records from the
WWNSS seismograph station at Akureyri and
the Preliminary Seismograph Readings of the
Meteorological Office do not reveal any un-
usual seismic activity around Hekla during
several weeks prior to the eruption. Two earth-
quakes of magnitudes 2.8 and 3.3 were recorded
on April 16 and 18 respectively with epicenters
possibly near Hekla. These events would by no
means awaken any suspicion of an impending
eruption. It is possible, however, that significant
activity occurred but was too weak to be re-
corded by the permanent seismic stations. A
considerable number of weak earthquakes oc-
curred prior to the 1973 Heimaey eruption
(Fig. 1) and was recorded by two short-period
seismometers located at the distances of 60 and
90 km (Björnsson and Einarsson, paper in pre-
paration). This activity was not observed by the
permanent seismic network.
The beginning of the eruption
During the last half hour before the start of
the eruption, earth tremors were recorded by
all three permanent seismic stations in Iceland.
These tremors were continuous and their ampli-
tude increased steadily until 21:18 when there
was a sudden burst of tremors (Fig. 3). After
that time numerous bursts occurred superim-
posed on the continuous tremors. These bursts
are probably S-waves and surface waves of
earthquakes. For some of them the correspond-
ing P-waves could be found on the records
with S-P time appropriate for earthquakes near
Hekla. These events were of magnitudes be-
tween 3.6 and 4.0. No particularly outstanding