Jökull - 01.01.2013, Page 109

Seismicity observed under Snæfellsjökull volcano
Figure 4. a) Seismogram of the Ml 1.1
event on 18 July 2011 showing vertical
(Z), radial (R) and transversal (T) com-
ponents as recorded by three different
stations (see Figure 2 for station label-
ing). For a better visualization data
are bandpassed with a filter between
4–12 Hz where the signal-to-noise ra-
tio is best. Picks for the phase onsets
are marked by solid red lines. b) Spec-
trogram of the same event as recorded
on the GLA station after instrument
correction. The data displayed in the
spectrograms was highpass-filtered for
f > 3 Hz so the plot is not dominated
by low frequency noise. – Línurit af
skjálfta 18. júlí 2011, stærð Ml 1,1. a)
Lóðrétt (Z), geislalæg (R) og þverstæð
(T) hreyfing á stöðvum ATA, GLA og
OLA (2. mynd), á tíðnibandinu 4–12
Hz, en á því bili er hlutfallið á milli
merkis og bakgrunnssuðs best. Aflest-
ur P- og S-bylgjufasa er sýndur með
rauðum strikum. b) Tíðnigreining af
sama skjálfta á GLA, eftir leiðréttingu
fyrir tækjasvörun. Tíðnum neðan 3 Hz
var sleppt, þar sem lágtíðnisuð yfir-
gnæfir merkið.
were much more prominent than P-wave arrivals. The
latter are often not well pronounced and pose one of
the major uncertainties in the determination of the
hypocentral depth.
Seismicity at Snæfellsjökull tends to cluster in
time and space. A total of 10 events were recorded
between 19th and 22nd of August 2011 and another 9
events occurred during 12 hours on September 6th.
While seismic activity in July and August is rather
uniformly distributed, the September events predomi-
nantly belong to the north-eastern cluster.
The events recorded near the volcano were too
small to be detected by the fifth seismometer located
60 km away near the Ljósufjöll volcanic complex. On
this distant seismometer we could not identify clear
signals of microseismicity like those measured with
the stations close to the Snæfellsjökull volcano. How-
ever, we did record signals that could be of seismic
origin but due to the lack of nearby seismic stations it
is not possible to identify the nature of these events.
DISCUSSION
We show evidence of ongoing seismic activity be-
neath and around the Snæfellsjökull volcano. We can
not rule out seismic activity in other parts of the Snæ-
fellsnes peninsula, because our network was specifi-
cally designed to detect seismicity around Snæfells-
jökull. The similarity in waveforms for many of the
JÖKULL No. 63, 2013 109