Jökull - 01.01.2005, Síða 94

Heidi Soosalu and Páll Einarsson
(Soosalu and Einarsson 2002) and the 2000 eruptions
(Soosalu et al. 2005). After the 1991 eruption, the
last low-frequency event was detected in March 1999.
High-frequency earthquakes started to occur at Hekla
proper at the end of the decade: three of them were
observed in February 1998 and one in July 1999. Af-
ter then, no earthquakes of any sort were detected at
Hekla itself before the 2000 eruption, which started
on February 26.
Since the 2000 eruption, Hekla seismicity has
been modest. Until mid-August 2005 nineteen low-
frequency Hekla events with ML magnitudes between
0.3 and 1.6 have been detected. A clearly high-
frequency Hekla earthquake (ML 0.9) was observed
at 5 km depth in September 2004, and another one
(ML 0.4) at 6 km depth in March 2005.
SEISMIC BEHAVIOUR OF HEKLA
DURING ITS ERUPTIONS
Hekla is a notorious volcano because of the short
warning time before its eruptions. Seismicity related
to eruptions starts gradually and reaches the detection
threshold very shortly before the onset; approximately
25 minutes in the eruptions in 1970 (Einarsson and
Björnsson 1976) and in 1980 (Grönvold et al. 1983),
half an hour in 1991 (Guðmundsson et al. 1992; Soos-
alu and Einarsson 2002) and 79 minutes in 2000 (Ein-
arsson 2000; Soosalu et al. 2005).
Based on knowledge gained from seismicity and
strain observations in 1991 (Linde et al. 1993), the
2000 Hekla eruption was successfully predicted. In
1991 the initial swarm of small earthquakes was ac-
companied by a compressive strain signal observed at
a strain station 15 km from Hekla (BUR in Figure 1),
interpreted as a result of a feeder dyke propagating at
depth towards the surface. The same pattern was ob-
served in 2000, which led to a successful short-term
prediction for the eruption, some 50 minutes before its
onset (Ágústsson et al. 2000; Stefánsson et al. 2000).
Two expressions of seismicity are observed dur-
ing the Hekla eruptions: volcano-tectonic earthquakes
(Figure 5a) and low-frequency volcanic tremor (Fig-
ure 5b). The majority of the earthquakes occur around
the onset of the eruption. During later phases of the
eruption, only occasional earthquakes are observed.
The tremor starts simultaneously with the eruptive ac-
tivity, continues throughout it and fades away together
with it. No tremor has ever been observed at Hekla
during non-eruptive periods. The tremor is most vig-
orous during the first hours of eruptions which are
characterized by explosive activity, and declines later.
The initial earthquake swarm
The initial earthquake swarms in 1991 and 2000 were
very similar (Figure 6). The very first observed events
were tiny, of about magnitude 0. The size of the events
grew towards the onset of the eruption and culminated
around it. In 1991 the detection threshold was higher
than in 2000, as the analogue station, HE, on the flank
of the volcano was broken and the closest digital sta-
tion, HAU, was down. Both of these stations oper-
ated well in 2000. The seismicity in 1991 quite likely
started in a similar manner as in 2000 but reached
the detection threshold later, first 30 minutes before
the onset of the eruption, compared to the 79 min-
utes of 2000. Both in 1991 and 2000 a strain signal
of contraction, indicating propagation of an intrusion
from depth, was observed at the close strain station
about half an hour before the eruption reached the sur-
face (Linde et al. 1993; Ágústsson et al. 2000). The
earthquake swarm started some 50 minutes before the
strain station observed any deformation. It is possi-
ble that small strain changes occurred but went un-
detected because of the large distance (15 km) to the
nearest strain station (Alan Linde, pers. comm. 2004).
The initial earthquake swarm soon became very
intense, and events were observed with intervals of
less than oneminute. The events were relatively small;
in 1991 the maximum magnitude was ML 2.5 and in
2000 ML 2.1. In 1991 the total seismic energy of the
initial earthquakes corresponded to a single event of
ML 3.4 and in 2000 to an event of ML 3.2.
In total, 380 earthquakes were detected during the
initial swarm in 1991, some 60 of which occurred
before the onset of eruptive activity. In total, 340
events were observed in 2000, 208 of which preceded
the start of the eruption. Figure 7 shows the epicen-
tral map of well-located earthquakes on February 26,
2000. In the first few hours the earthquakes were
rather even in magnitude, ≥ 2 in 1991 and ≤ 2 in
94 JÖKULL No. 55