Jökull - 01.01.2005, Side 91

Seismic characteristics of the Hekla volcano
tal three-component stations in the south Iceland low-
land area, and more recently it has been expanded to
other parts of Iceland (Böðvarsson et al. 1999). Be-
cause the study area is situated at the edge of the orig-
inal SIL network, three permanent analogue vertical-
component stations in the vicinity of Hekla were used
in addition to get good station coverage for the whole
area (Figure 1). The events were relocated with the
location program HYPOINVERSE (Klein 1978), us-
ing a crustal model consisting of layers with constant
velocity gradients (Gebrande et al. 1980). The model
is an average model with time delay corrections used
for the seismograph stations to improve the location
accuracy. The location procedure is described in de-
tail in Soosalu and Einarsson (1997).
The dataset used spans the period from June 1990
to mid-August 2005 and covers the two latest Hekla
eruptions. Over one-thousand small earthquakes (ML
< 3) were detected at Hekla and its surroundings dur-
ing this period. Of these events, we plot in the sub-
sequent maps only the well-located ones, using the
location criteria: root mean square travel-time resid-
ual (rms) ≤ 0.2 s, horizontal error (erh) ≤ 1.0 km,
vertical error (erz)≤ 2.0 km, and largest gap between
observing stations ≤ 180◦.
SEISMICITY IN THE HEKLA AREA
BETWEEN ERUPTIONS
During non-eruptive times Hekla is characterised by
scarce seismicity (Einarsson 1991; Soosalu and Ein-
arsson 1997, 2002; Soosalu et al. 2005). The
earthquakes are sporadic and small in size, typically
ML ≤ 1. As an illustration of the seismicity at
Hekla and its vicinity, earthquakes in the area during
non-eruptive times, June 1990–January 1991, March
1991–February 2000 and March 2000–mid-August
2005 are plotted in Figure 2. Their hypocentral distri-
bution is shown in Figure 3. Inter-eruptionHekla seis-
micity is not volcano-related, but reflects the trans-
form tectonics of the South Iceland seismic zone to-
gether with the earthquake activity at the Vatnafjöll
volcano south of Hekla. The earthquakes at Hekla
and Vatnafjöll are loosely clustered in two N-S linea-
ments, similar to the faults of the South Iceland seis-
mic zone. One of the lineaments cuts the SW parts
of the volcanoes and the other the middle-NE parts
of them. The earthquakes occur typically at 8–13 km
depth, similar to earthquakes at the east end of the
South Iceland seismic zone. Three events in the NE
part of Hekla were located at considerable depths of
16, 18 and 26 km. For all these events the nearest
station was within the range of 17 to 18 km.
During non-eruptive periods, the surroundings of
Hekla are far more seismically active than the vol-
cano itself. Most intensive seismicity occurs at the
Torfajökull volcano; e.g. during the period 1990–1995
about 200 events, up to local magnitude 2.8 were
located in the Torfajökull area (Soosalu and Einars-
son 1997). Torfajökull is characterised by dual seis-
micity: high-frequency volcano-tectonic earthquakes
occur in the western part of its caldera and low-
frequency earthquakes in the south (Soosalu and Ein-
arsson 2003).
Seismicity in the easternmost section of the South
Iceland seismic zone, next to Hekla, is clustered in
two elongate N-S lineaments, roughly at longitudes
20◦W and 20◦10’W, both during eruptive and non-
eruptive periods for Hekla (Soosalu and Einarsson
1997, 2002). Small earthquakes, up to local mag-
nitude 2, are common in this area, and over 190
were observed in the period 1990–1995. They oc-
curred typically in the depth range of 6–12 km. The
events were located between latitudes 64◦08’N and
63◦49’N, but were most abundant around latitude
64◦N, where surface faults can be seen.
From the start of the study period, June 1990 to
half an hour before the start of the 1991 eruption, on
January 17, no earthquakes were located at Hekla or
its immediate vicinity (Soosalu and Einarsson 2002).
Thus, no long-term or intermediate term precursors
were observed prior to the eruption. Earthquakes in
this period were confined to the Torfajökull volcano
and South Iceland seismic zone.
Minor seismicity was observed within the Hekla-
Vatnafjöll area following the eruption in January 17–
March 11, 1991 until June 1, 1991 when an unusual
swarm of earthquakes occurred beneath the northern
flank of Hekla, most at less than 3 km depth. The
SIL network detected about thirty events with magni-
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