Jökull - 01.01.2017, Page 6

Reviewed research article
A peculiar cluster of microearthquakes on the eastern flank of
Katla volcano, southern Iceland
Zeinab Jeddi,1,? Giulia Sgattoni,1,3,4 Ólafur Guðmundsson,1,2
Ari Tryggvason,1 and Björn Lund1
1. Department of Earth Science, Geophysics, Uppsala University, Sweden
2. School of Science and Engineering, Reykjavík University, Reykjavík, Iceland
3. Department of Biological, Geological and Environmental Sciences, University of Bologna, Bologna, Italy
4. Institute of Earth Sciences, Science Institute, University of Iceland, Reykjavík, Iceland
?Corresponding author, zeinab.jeddi@gmail.com
Abstract — A peculiar cluster of seismicity near the tip of Sandfellsjökull on the eastern flank of Katla volcano
in southern Iceland has been analyzed in detail using data from a temporary seismic network. A total of 300
events were detected between July 2011 and August 2013, most of them from a swarm between December 4th
and 12th, 2011. The sparser permanent network detected a small fraction of these events, but also a larger
swarm in November 2010. When seismic activity started in this area is uncertain because of changes in the
detection capability of the network over time. The events are of low magnitude (-0.5 < ML < 0.5) and the
b-value of their magnitude distribution is high (1.6±0.1). Based on their frequency content (4–25 Hz) and
clear P and S arrivals, the events are classified as volcano-tectonic. Two multiplets probably with different
source mechanism are identified in their population. The events locate at approximately 3.5 km depth. Most
of them are tightly clustered according to double difference relative locations in a volume that is only about
400 m in diameter in all directions. Several events are scattered up to 800 m beneath this volume. There is
some suggestion of elongate structure in the cluster with a NNE/SSW strike and a dip of 60 degrees. We argue
that these events cannot be due to a glacial or a broad tectonic process. Possibly, a localized source of fluid
pressure, e.g., a small magma body at depth may be the source of these events.
INTRODUCTION
Seismic signals are almost always recorded before
and during eruptions. This makes real-time moni-
toring of the seismicity around an active volcano an
important tool both for developing forecasting meth-
ods and as part of an early warning system. Further-
more, the seismicity provides information about the
volcano’s processes (e.g., magma movement, pressure
changes, and subsidence) as well as its internal struc-
ture. At sub-glacial volcanoes, the analysis of seismic
signals becomes even more important, as direct obser-
vations may be problematic because of the ice cover.
At the same time, the interpretation of seismic sig-
nals can be difficult because waveforms originating
from glacial and volcanic sources may have similar
features.
The Katla volcanic system in southern Iceland
(Figure 1) is a prime example of an active glacier cov-
ered volcano in need of real-time monitoring. Katla is
known to be one of the most active volcanoes in the
country’s written history, with volcanic activity dom-
inated by phreatomagmatic eruptions (the last occur-
ring in 1918) accompanied by tephra fallout as well as
glacial floods (jökulhlaup).
Katla is characterized by intense seismicity that
has been recorded since the first sensitive seismome-
ters were installed in Iceland (Tryggvason, 1973). A
seismic network was installed in South Iceland in
1973–1974 that included 5 stations within 50 km of
the volcano (Einarsson and Björnsson, 1987). The
current, permanent seismic network, the SIL net-
work, operated by the Icelandic Meteorological Office
JÖKULL No. 67, 2017 1