Jökull - 01.12.1976, Page 16

Fig. 4. Earthquakes near
Hekla recorded by the
AKU station. Magnitude
plotted versus time of oc-
currence. Filled circles
denote earthquakes locat-
ed with data from two or
more seismic stations.
Open circles show earth-
quakes inferred to be
from Hekla by compar-
ing the S—P time and
wave form on the AKU
records to that of earth-
quakes known to be from
Hekla. Large symbols in-
dicate more than one
earthquake. There is evid-
ence that some earth-
quakes of magnitude less
than 3.4 were missed.
JUNE 2-30
N0 EARTHQUAKES
RECORDED
M
Mynd 4. Jarðskjálftar frá Heklu skráðir á mceli á Akureyri. Myndin sýnir stcerð skjálfta og hve-
ncer þeir urðu. Svartir deplar tákna skjálfta sem unnt var að staðsetja með aðstoð tveggja eða
fleiri skjálftamcela. Hringir tákna skjálfta, setn talið var að kœmu frá Heklu. Var þá höfð hlið-
sjón af S—P tíma og útliti skjálftaritsins og það borið saman við rit af skjálftum sem vitað var
að komu frá Heklu. Vitað er að nokkrir skjálftar minni en M 3,4 komu ekki fram á Akureyri.
seismograms alone comparing S-P time and
wave form to that of events known to be from
the Hekla region. This method is fairly reliable
although contamination of the data sample by
earthquakes from elsewhere cannot be totally
ruled out.
All the seismic activity recorded at AKU
occurred during three time periods separated
by periods of no earthquakes of magnitude
larger than about 3.3.
Tlie first period of seismic activity, from May
5 to May 20, coincides with a period when the
eruption was undergoing rapid changes. On
May 8 and May 10 the craters on the southwest
and southern flanks respectively ceased their
activity. This change seems to be reflected by
high seismicity during the period May 8—11.
After May 10 irregular volcanic activity con-
tinued on the northern flank, accompanied by
considerable seismic activity that culminated
on May 16. During that time the eruption di-
minished steadily. On May 19 and the morning
of May 20 a swarm of earthquakes occurred
14 JÖKULL 26. AR
that appears to mark the cessation of volcanic
activity on the northern flank.
This repose turned out to be only temporary.
In the late afternoon of May 20 the eruption
broke out on a new fissure on the northern
flank. This new outbreak of the eruption was
not accompanied by any conspicuous seismic
activity. No premonitory harmonic tremors
were recorded nor did any earthquakes occur
as was the case when the eruption started on
May 5. On the contrary, this event was followed
by a six days period of no earthquakes strong
enough to be recorded at AIÍU.
The significance of the seismic activity dur-
ing the time period May 26 to June 1 (Fig. 4)
is not clear. No particular event or change in
the volcanic activity is reported to have taken
place during this time.
Between June 2 and July 3 the seismicity
near Hekla was very low. No earthquake was
recorded by the AKU station and only 3 small
events were recorded by the temporary short
period stations installed near Hekla on June 22.