Jökull - 01.01.2017, Síða 8

Microearthquakes on the eastern flank of Katla volcano, S-Iceland
volcanic event classification by McNutt (2005), the
high-frequency events with clear onsets of P and S
phases are classified as volcano-tectonic (VT) events,
while low-frequency events with emergent P and lack
of S phases are classified as long-period (LP). The
classification of the events into similar categories is
however not straight forward at Katla because of their
complex waveforms (Jeddi et al., 2016).
The seismicity on the eastern flank of Katla (here-
after referred to as the eastern events), consists of
high-frequency events and was briefly described by
Jeddi et al. (2016), but was not studied in detail. Our
objective with this paper is to remedy that. We study
waveform characteristics, absolute and relative loca-
tions, magnitudes and discuss the possible source of
the Katla eastern flank seismicity.
THE KATLA VOLCANIC SYSTEM
Katla is located near the southern tip of the propa-
gating active rift in the Eastern Volcanic Zone (Fig-
ure 1). The Katla volcanic system consists of a cen-
tral volcano and a fissure swarm extending 80 km to
the NE (Jakobsson, 1979; Jóhannesson et al., 1990).
The central volcano hosts a large ice-filled summit
caldera (∼100 km2) elongated in the NW-SE direc-
tion (Björnsson et al., 2000). A number of ice caul-
drons (17) located within the caldera and around its
rim represent the surface expression of subglacial
geothermal activity (Guðmundsson et al., 2007).
Katla’s products consist mainly of basalts, with
subordinate silicic lavas and tephra (Lacasse et al.,
2007; Óladóttir et al., 2008). Recent volcanic ac-
tivity at Katla is characterized by frequent basaltic
phreatomagmatic eruptions with at least 300 such
eruptions during the Holocene (Óladóttir et al., 2005).
Large effusive basaltic eruptions along Katla’s fissure
system are the least common eruption type, e.g. the
AD 934–940 Eldgjá Fires (Thordarson et al., 2001).
Furthermore, explosive silicic eruptions from the cen-
tral volcano are evidenced by at least 12 tephra lay-
ers in the Holocene (Larsen, 2000). Several outcrops
around the caldera rims and glacier margins are com-
posed of rhyolitic lavas interpreted as late Quaternary
(middle to late Pleistocene) felsic extrusives (e.g., La-
casse et al., 2007; Sgattoni, 2016).
After the last major eruption in 1918, several
episodes of unrest were recorded at Katla in 1955,
1999 and 2011, characterized by increased earthquake
activity (also tremor in 1999 and 2011) and glacial
floods. These unrest episodes may have been associ-
ated with minor subglacial eruptions not breaking the
ice surface (Thorarinsson, 1975; Guðmundsson et al.,
2007; Sgattoni, 2016).
The eastern seismic activity was not detected until
the seismic monitoring network was densified in 2010
and 2011. Jeddi et al. (2016) analyzed the seismo-
grams from the improved network of permanent and
temporary stations operating between 2011 and 2013
and classified the seismicity at Katla into three cate-
gories: Volcano-tectonic (VT), long period (LP) and
"mixed" (M). The events with more than 75% high-
frequency (>5 Hz) seismograms were defined as VT,
events with no high-frequency recordings as LP and
the remaining events as M. In addition, a notable fea-
ture of Katla’s seismic activity is its seasonal vari-
ation, with the highest seismicity rates in the sum-
mer and early autumn months (Einarsson and Brands-
dóttir, 2000; Jónsdóttir et al., 2007; Jónsdóttir et al.,
2009).
A low P wave velocity zone associated with an S
wave shadow was revealed underneath the caldera by
2-D seismic undershooting and interpreted as a shal-
low magma chamber (Gudmundsson et al., 1994). A
volume with reduced P-velocities, beneath the seis-
micity in the northeastern part of the caldera (∼1 km
thick), was also identified with three-dimensional (3-
D) local earthquake tomography (Jeddi et al., 2016).
Jeddi et al. (2016) suggested this may be due to hot
rhyolitic material in the shallow crust below the re-
located seismic activity. They also identified a high-
velocity core (∼6.5 km/s) at 4 km below sea level (bsl)
beneath the low-velocity zone, interpreted to consist
of mafic cumulates resulting from fractionation of
mafic intrusions and partial melting of subsiding hy-
drothermally altered rocks.
GPS measurements were conducted on nunataks
on the caldera rim between 1999 and 2004 and the
observed horizontal velocities were interpreted as due
to inflation and possible magma accumulation at ap-
proximately 5 km depth beneath the caldera (Sturkell
JÖKULL No. 67, 2017 3