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Seismic constraints on magma chambers at Hekla and Torfajökull volcanoes, Iceland

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Abstract

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 S-wave 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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Acknowledgements

H. Soosalu was supported by Finnish Cultural Foundation and the Vilho, Yrjö and Kalle Väisälä Foundation of the Finnish Academy of Science and Letters. The Icelandic Meteorological Office provided the digital SIL data. The National Power Company of Iceland funds the analogue seismograph network. Comments and suggestions of Dave Hill, Rune Selbekk and an anonymous reviewer improved the manuscript.

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  1. Heidi Soosalu

    Present address: Nordic Volcanological Institute, Grensásvegur 50, Reykjavík, Iceland

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  1. Institute of Seismology, University of Helsinki, P.O. Box 26 (Teollisuuskatu 23), Finland

    Heidi Soosalu

  2. Science Institute, University of Iceland, Hofsvallagata 53, Reykjavík, Iceland

    Páll Einarsson

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  1. Heidi Soosalu
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Correspondence to Heidi Soosalu.

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Soosalu, H., Einarsson, P. Seismic constraints on magma chambers at Hekla and Torfajökull volcanoes, Iceland. Bull Volcanol 66, 276–286 (2004). https://doi.org/10.1007/s00445-003-0310-1

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