Jökull - 01.01.2005, Síða 87

Reviewed research article
Seismic characteristics of the Hekla volcano and its
surroundings, Iceland
Heidi Soosalu1 and Páll Einarsson2
1Nordic Volcanological Center, Institute of Earth Sciences University of Iceland, Sturlugata 7, IS-101 Reykjavík, Iceland;
now at: Department of Earth Sciences, Bullard Laboratories, Cambridge University, Madingley Road,
Cambridge CB3 0EZ, United Kingdom; heidi@hi.is
2Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík; palli@raunvis.hi.is
Abstract— The volcano Hekla is located in south Iceland at the junction of a transform segment, the South
Iceland seismic zone, and a ridge segment, the Eastern volcanic zone, of the mid-Atlantic plate boundary.
Hekla is one of the most active volcanoes in Iceland, with at least 18 eruptions during the last 1100 years. In
recent decades it has had relatively small eruptions, approximately once in a decade, most recently in 1991 and
2000. During non-eruptive periods Hekla is virtually aseismic, and does not give long-term or intermediate-
term precursory warnings before its eruptions. Eruption-related seismicity starts 25–80 minutes before its
onset. Hundreds of small volcano-tectonic earthquakes (magnitude < 3), related to magma intrusion, occur
during the first hours when the eruption is violent and explosive. This seismicity soon diminishes, along with
the eruptive activity. Subsequent eruptive activity consists mainly of lava effusion and occasional gas bursts
associated with very few earthquakes. Volcanic tremor, continuous low-frequency vibration of the ground,
starts simultaneously with the eruption, and continues throughout it. It is most vigorous during the explosive
onset, and decreases along with the eruptive activity. The few earthquakes at Hekla and its immediate vicinity
during non-eruptive times are small (magnitude < 2), and apparently not related to the Hekla volcano itself.
They follow a distribution similar to the events of the South Iceland seismic zone. They form two north-south
lineaments analogous to the seismic zone faults, and occur mainly at depths of 8–12 km. Thus seismically the
Hekla area has a dual nature: on one hand the seismicity is ruled by the tectonics of the South Iceland seismic
zone, and on the other hand by the internal processes of the volcano. Transform zone tectonics dominate
during the non-eruptive periods. The volcano-related seismicity of Hekla is almost exclusively associated with
eruptions. Volcanic tremor has never been recorded during non-eruptive periods.
INTRODUCTION
The two most recent eruptions of Hekla occurred in
January 17–March 11, 1991 and February 26–March
8, 2000. These events produced a rather similar
amount of eruptives of basaltic-andesitic composition,
0.15 km3 (Guðmundsson et al. 1992) and ∼0.2 km 3
(Höskuldsson et al. submitted), respectively. Both
eruptions had a short-lived initial Plinian phase and
were most vigorous in the first hours. Effusion of lava
began at the same time as the explosive activity, or
shortly after. Initially, large segments of fissures were
active, but the eruptions became more localised dur-
ing the later phases. In both cases, lava production
was largest during the first days of activity. Seismi-
cally, the eruptions were very similar. No long-term
precursory seismicity was detected. The onset of each
eruption was accompanied by an initial swarm of hun-
dreds of small volcano-tectonic earthquakes (ML <
3) which increased in magnitude towards the onset of
the eruption, few earthquakes in the later phases and
continuous low-frequency volcanic tremor with dom-
inant frequencies at 0.7–0.9 Hz (Soosalu and Einars-
son 2002; Soosalu et al. 2003, 2005).
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