Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland

Soosalu, Heidi; Einarsson, Páll

doi:10.1007/s00445-001-0177-y

Cited by 20 publications

(21 citation statements)

References 14 publications

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“…Precursory seismicity develops only 25-80 min before an eruption (Einarsson and Björnsson 1976;Grönvold et al 1983;Gumundsson et al 1992;Einarsson 2000;Soosalu and Einarsson 2002). Paradoxical as it may sound, the lack of seismicity preceding eruptions can be taken as a sign of a deeper magma source.…”

Section: Discussionmentioning

confidence: 96%

Seismic constraints on magma chambers at Hekla and Torfaj�kull volcanoes, Iceland

Soosalu

Einarsson

2004

Bulletin of Volcanology

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Hekla and Torfajökull are active volcanoes at a rift-transform junction in south Iceland. Despite their location next to each other they are physically and geologically very different. Hekla is an elongate stratovolcano, built mainly of basaltic andesite. Torfajökull is a prominent rhyolitic centre with a 12-km-diameter caldera and extensive geothermal activity. The scope of this study is to examine the propagation of body waves of local earthquakes across the Hekla-Torfajökull area and look for volumes of anomalous S-wave attenuation, which can be evidence of magma chambers. So far the magma chamber under Hekla has been modelled with various geophysical means, and its depth has been estimated to be 5-9 km. A data set of 118 local earthquakes, providing 663 seismic rays scanning Hekla and Torfajökull, was used in this study. The major part, 650 seismograms, did not show evidence for S-wave attenuation under these volcanoes. Only six seismograms had clear signs of Swave attenuation and seven seismograms were uncertain cases. The data set samples Hekla well at depths of 8-14 km, and south part of it also at 4-8 km and 14-16 km. Western Torfajökull is sampled well at depths of 4-14 km, eastern and southern Torfajökull at 6-12 km. Conclusions cannot be drawn regarding the existence of magma beyond these depth ranges. Also, magma volumes of smaller dimensions than about 800 m cannot be detected with this method. If a considerable molten volume exists under Hekla, it must be located either above 4 km or below 14 km. The former possibility seems unlikely, because Hekla lacks geothermal activity and persistent seismicity, usually taken as expressions of a shallow magma chamber. An aseismic volume with a diameter of 4 km at the depth of 8 km in the west part of Torfajökull has been inferred in earlier studies and interpreted as evidence for a cooling magma chamber.Our results indicate that this volume cannot be molten to a great extent because S-waves travelling through it are not attenuated. Intense geothermal activity and low-frequency earthquakes are possibly signs of magma in the south part of Torfajökull, but a magma chamber was not detected there in the areas sampled by this study.

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Section: Discussionmentioning

confidence: 96%

Seismic constraints on magma chambers at Hekla and Torfaj�kull volcanoes, Iceland

Soosalu

Einarsson

2004

Bulletin of Volcanology

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“…The SIL stations recorded premonitory earthquakes (Stefµnsson et al 1993;Soosalu and Einarsson 2002), and subsequently volcanic tremor associated with the beginning of the eruption. The initial plinian phase of the eruption was short-lived.…”

Section: The Seismograph Stations Observing the Volcanic Tremor Of Thmentioning

confidence: 99%

“…In accordance with this, no earthquakes were observed at Hekla before 17 January, and thus there are no signs of precursory seismicity. Numerous small earthquakes were related to the onset of the eruption (Soosalu and Einarsson 2002). Sixty earthquakes were observed by the SIL system before the onset of the eruption (16:30-17:01 GMT), and 320 events between 17:02 and midnight (see below).…”

Section: Time History Of the Eruption In The Light Of Tremor Observatmentioning

confidence: 99%

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Volcanic tremor related to the 1991 eruption of the Hekla volcano, Iceland

Soosalu

Einarsson

Jakobsdóttir

2003

Bulletin of Volcanology

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Volcanic tremor at the Hekla volcano is directly related to eruptive activity. It starts simultaneously with the eruptions and dies down at the end of them. No tremor at Hekla has been observed during noneruptive times. The 1991 Hekla eruption began on 17 January, after a short warning time. Local seismograph stations recorded small premonitory earthquakes from 16:30 GMT on. At 17:02 GMT, low-frequency volcanic tremor became visible on the seismograph records, marking the onset of the eruption. The initial plinian phase of the eruption was short-lived. During the first day several fissures were active but, by the second day, the activity was already limited to a segment of one principal fissure. The eruption lasted almost 53 days. At the end of it, during the early hours of 11 March, volcanic tremor disappeared under the detection threshold and was followed by a swarm of small earthquakes. At the start of the eruption, the tremor amplitude rose rapidly and reached a maximum in only 10 min. The tremor was most vigorous during the first hour and started to decline sharply during the next hour, and later on more gently. During the eruption as a whole, the tremor had a continuous declining trend, with occasional increases lasting up to about 2 days. Spectral analysis of the tremor during the first 7 h of the eruption shows that it settled quickly, within a couple of minutes, to its characteristic frequency band, 0.5-1.5 Hz. The spectrum had typically one dominant peak at 0.7-0.9 Hz, and a few subdominant peaks. Hekla tremor likely has a shallow source. Particle motion plots suggest that it contains a significant component of surface waves. The tremor started first when the connection of the magma conduit with the atmosphere was reached, suggesting that degassing may contribute to its generation.

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“…The earthquake swarm began at 16:30 (Figure 6) with small events, which quickly became larger until the eruption broke out between 17:00 and 17:02 (Gudmundsson et al, 1992;Linde et al, 1993;Soosalu and Einarsson, 2002;Soosalu et al, 2003). The recorded seismic activity was detected by people before the eruption was seen, but only after the eruption had already started.…”

Section: Heklamentioning

confidence: 99%