Jökull - 01.12.1976, Page 15

2100 GMT
AKU MAY 5, 1970
Fig. 3. A segment of the AKU seismogram of May 5, 1970 (the short period vertical compon-
ent), displaying the seismic activity before and during the initial phase of the eruption. The
volcanic tremors are first visible at 21:00 GMT. Earthquakes are superimposed on the con-
tinuous tremors. The eruption became visible at 21:23. The record was taken off at 23:05. The
AKU seismograph station is 200 km NNE of Hekla.
Mynd 3. Hlnti af skjálftariti á Akureyri 5. mai 1970, sem sýnir óróa og skjálfta í byrjun gossins.
Orói verður sýnilegur um kl. 21:00. Jarðskjálftar blandast inn i óróann. Eldar sáust fyrst kl. 21:23.
Blaðið var tekið af mœlinum kl. 23:05. Mcelirinn á Akureyri er 200 km NNA frá Heklu.
seismic event occurred at 21:23 when the erup-
tion first became visible on the surface.
The continuous harmonic tremors became
visible first on the seismograms of AKU and
SI at 20:58 and a little later on the REY sta-
tion. At 21:06 the traces on the SI seismogram
became illegible because of tangling. At this
time any alert observer would be alarmed that
something unusual was taking place, but since
all the seismograms were recorded on photo-
graphic paper, nobody became aware of the
tremors until after the eruption had started
and the seismograms were developed.
Because of the large distance to the seismic
stations and the gradual increase of the ampli-
tude of the tremors there is a reason to believe
that they started before the time when they
first appeared on the seismograms. It is likely
that a seismometer, if located in the vicinity
of the volcano, would have recorded tremors
many hours before the start of the eruption.
Thorarinsson (1970) notes that people in the
vicinity of Hekla did not feel any tremors
before the start of the eruption but dogs be-
came very restless and nervous in the afternoon
and evening of May 5. Dogs and some other
animals are known to react very strongly to
weak earth tremors not perceivable by humans.
If tremors actually caused the restlessness of
the dogs, those tremors must have started at
least 4—6 hours before the eruption broke out.
The intense seismic activity during the initial
phase of the eruption soon died down. By mid-
night, or 2y2 hours after the eruption first
broke out, harmonic tremors were hardly visible
on the seismic records and the last earthquake
that can be attributed to the initial phase oc-
curred at 00:55 on May 6.
During the eruption
Fig. 4 is a summary of seismic activity near
Hekla in May, June and July as recorded by
the seismic station AKU at the distance of 200
lcm. Some of these events were located by using
readings from two or more seismic stations,
otlier events had to be located with the AKU
JÖKULL 26. ÁR 1 3