Bulletin of Volcanology

, Volume 66, Issue 3, pp 276–286 | Cite as

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

  • Heidi SoosaluEmail author
  • Páll Einarsson
Research Article


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.


Hekla Iceland Magma chamber S-wave attenuation Torfajökull 



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.


  1. Ágústsson K, Linde AT, Stefánsson R, Sacks SI (2000) Borehole strain observations for the February 2000 eruption of Hekla, south Iceland (abstract). AGU 2000 Spring meetingGoogle Scholar
  2. Alfaro R, White RS, Brandsdóttir B, Einarsson P, Guðmundsson MT (2003) Structure of the Grímsvötn volcano under the Vatnajökull ice cap. (in press)Google Scholar
  3. Björnsson A, Saemundsson K, Einarsson P, Tryggvason E, Grönvold K (1977) Current rifting episode in north Iceland. Nature 266:318–323Google Scholar
  4. Björnsson A, Johnsen G, Sigurðsson S, Þorbergsson G, Tryggvason E (1979) Rifting of the plate boundary in north Iceland 1975–1978. J Geophys Res 84:3029–3038Google Scholar
  5. Björnsson H, Einarsson P (1990) Volcanoes beneath Vatnajökull, Iceland: evidence from radio echo-sounding, earthquakes and jökulhlaups. Jökull 40:147–168Google Scholar
  6. Björnsson H, Björnsson S, Sigurgeirsson Þ (1982) Penetration of water into hot rock boundaries of magma at Grímsvötn. Nature 295:580–581Google Scholar
  7. Bower S, Woods AW (1998) On the influence of magma chambers in controlling the evolution of explosive volcanic eruptions. J Volcanol Geotherm Res 86:67–78Google Scholar
  8. Brandsdóttir B, Einarsson P (1992) Volcanic tremor and low-frequency earthquakes in Iceland. In: Gasparini P, Scarpa R, Aki K (eds) Volcanic seismology. Springer, Berlin Heidelberg New York, pp 212–222Google Scholar
  9. Brandsdóttir B, Menke W, Einarsson P, White RS, Staples RK (1997) Faroes–Iceland Ridge experiment, 2. crustal structure of the Krafla central volcano. J Geophys Res 102:7867–7886Google Scholar
  10. Cervený V, Pšencik I (1983) SEIS83-numerical modelling of seismic wave fields in 2-D laterally varying layered structure by the ray method. In: Engdahl (ed) Documentation of earthquake algorithms. WDC(A) for Solid Earth Geophysics, Boulder, Rep SE-35, pp 36–54Google Scholar
  11. Einarsson P (1978) S-wave shadows in the Krafla caldera in NE-Iceland, evidence for a magma chamber in the crust. Bull Volcanol 41:187–195Google Scholar
  12. Einarsson P (1991) The volcano-tectonic activity at Krafla 1975–89 (in Icelandic). In: Garðarsson A, Einarsson Á (eds) Náttúra Mývatns. Hið íslenska náttúrufraeðifélag, Reykjavík, pp 97–139Google Scholar
  13. Einarsson P (2000) The seismograph station on Litla-Hekla and the 2000 eruption (abstract, in Icelandic). Geoscience Society of Iceland, Spring meeting 2000Google Scholar
  14. Einarsson P, Björnsson S (1976) Seismic activity associated with the 1970 erruption of volcano Hekla in Iceland. Jökull 26:8–19Google Scholar
  15. Einarsson P, Björnsson S (1979) Earthquakes in Iceland. Jökull 29:37–43Google Scholar
  16. Einarsson P, Brandsdóttir B (1984) Seismic activity preceding and during the 1983 volcanic eruption in Grímsvötn, Iceland. Jökull 34:13–23Google Scholar
  17. Eysteinsson H, Hermance JF (1985) Magnetotelluric measurements across the eastern neovolcanic zone in south Iceland. J Geophys Res 90:10093–10103Google Scholar
  18. Einarsson P, Saemundsson K (1987) Earthquake epicenters 1982–1985 and volcanic systems in Iceland. In: Sigfússon ÞI (ed) Í hlutarins eðli, Festschrift for Þorbjörn Sigurgeirsson. Menningarsjóður, Reykjavík (map)Google Scholar
  19. Foulger GR, Toomey DR (1989) Structure and evolution of the Hengill–Grensdalur volcanic complex, Iceland: geology, geophysics, and seismic tomography. J Geophys Res 94:17511–17522Google Scholar
  20. Grönvold K, Larsen G, Einarsson P, Þórarinsson S, Saemundsson K (1983) The Hekla eruption 1980–1981. Bull Volcanol 46:349–363Google Scholar
  21. Guðmundsson Á (1988) Formation of collapse calderas. Geology 16:808–810CrossRefGoogle Scholar
  22. Guðmundsson MT, Milsom J (1997) Gravity and magnetic studies of the subglacial Grímsvötn volcano, Iceland: implications for crustal and thermal structure. J Geophys Res 102:7691–7704Google Scholar
  23. Guðmundsson Á, Óskarsson N, Grönvold K, Saemundsson K, Sigurðsson O, Stefánsson R, Gíslason SR, Einarsson P, Brandsdóttir B, Larsen G, Jóhannesson H, Þórðarson Þ (1992) The 1991 eruption of Hekla, Iceland. Bull Volcanol 54:238–246Google Scholar
  24. Guðmundsson Ó, Brandsdóttir B, Menke W, Sigvaldason GE (1994) The crustal magma chamber of the Katla volcano in south Iceland revealed by 2-D seismic undershooting. Geophys J Int 119:277–296Google Scholar
  25. Jakobsson SP (1979) Petrology of Recent basalts of the Eastern Volcanic Zone, Iceland. Acta Naturalia Islandica 26:1–103Google Scholar
  26. Jóhannesson H, Jakobsson SP, Saemundsson K (1990) Geological map of Iceland, sheet 6, South-Iceland, 3rd edn. Icelandic Museum of Natural History and Iceland Geodetic Survey, ReykjavíkGoogle Scholar
  27. Kjartansson E, Grönvold K (1983) Location of a magma reservoir beneath Hekla volcano, Iceland. Nature 301:139–141Google Scholar
  28. Klein FW (1978) Hypocenter location program HYPOINVERSE. US Geol Surv Open-file Rep 78-694Google Scholar
  29. Larsen G (1984) Recent volcanic history of the Veiðivötn fissure swarm, southern Iceland—an approach to volcanic risk assessment. J Volcanol Geotherm Res 22:33–58CrossRefGoogle Scholar
  30. Linde AT, Ágústsson K, Sacks IS, Stefánsson R (1993) Mechanism of the 1991 eruption of Hekla from continuous borehole strain monitoring. Nature 365:737–740CrossRefGoogle Scholar
  31. McGarvie DW (1984) Torfajökull: a volcano dominated by magma mixing. Geology 12:685–688Google Scholar
  32. Miller AD, Julian BR, Foulger GR (1998) Three-dimensional seismic structure and moment tensors of non-double-couple earthquakes at the Hengill–Grensdalur volcanic complex, Iceland. Geophys J I 133:309–325CrossRefGoogle Scholar
  33. Ólafsdóttir R, Höskuldsson Á, Grönvold K (2002) The evolution of the lava flow from Hekla eruption 2000. The 25th Nordic Geological Winter Meeting, Reykjavík, 6–9 January 2002Google Scholar
  34. Óskarsson N, Sigvaldason GE, Steinþórsson S (1982) A dynamic model of rift zone petrogenesis and the regional petrology of Iceland. J Petrol 23:28–74Google Scholar
  35. Rubin AM (1995) Propagation of magma-filled cracks. Annu Rev Earth Planet Sci 23:287–336Google Scholar
  36. Sacks S, Linde AT (2001) Magma chamber, pressure and eruptions (abstract). Symposium on Icelandic plume and crust, Svartsengi, Iceland, 8–10 September 2001Google Scholar
  37. Saemundsson K (1972) Geological notes on the Torfajökull area (in Icelandic). Náttúrufraeðingurinn 42:81–99Google Scholar
  38. Saemundsson K (1982) Calderas in the neovolcanic zones of Iceland (in Icelandic). In: Eldur er í norðri, Festschrift for Sigurður Þórarinsson. Sögufélag, Reykjavík, pp 221–239Google Scholar
  39. Saemundsson K (1991) The geology of the Krafla volcanic system (in Icelandic). In: Garðarsson A, Einarsson Á (eds) Náttúra Mývatns. Hið íslenska náttúrufraeðifélag, Reykjavík, pp 25–95Google Scholar
  40. Sigmundsson F, Einarsson P, Bilham R (1992) Magma chamber deflation recorded by the Global Positioning System: the Hekla 1991 eruption. Geophys Res Lett 19:1483–1486Google Scholar
  41. Sigvaldason GE (1974) The eruption of Hekla 1947–1948: vol 1. The petrology of Hekla and origin of silicic rocks in Iceland. Soc Scientiarum Islandica, ReykjavíkGoogle Scholar
  42. Soosalu H, Einarsson P (1997) Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991–1995. Bull Volcanol 59:36–48CrossRefGoogle Scholar
  43. Soosalu H, Einarsson P (2002) Earthquake activity related to the 1991 eruption of the Hekla volcano, Iceland. Bull Volcanol 63:536–544CrossRefGoogle Scholar
  44. Stefánsson R, Böðvarsson R, Slunga R, Einarsson P, Jakobsdóttir S, Bungum H, Gregersen S, Havskov J, Hjelme J, Korhonen H (1993) Earthquake prediction research in the South Iceland seismic zone and the SIL project. Bull Seism Soc Am 83:696–716Google Scholar
  45. Stefánsson R, Ágústsson K, Guðmundsson GB, Þorbjarnardóttir BS, Einarsson P (2000) A successful prediction and warning of an eruption in the Hekla volcano, Iceland (abstract). Am Geophys Union 2000 Spring meetingGoogle Scholar
  46. Sturkell E, Sigmundsson F (2000) Continuous deflation of the Askja caldera, Iceland, during the 1983–1998 noneruptive period. J Geophys Res 105:25671–25684Google Scholar
  47. Sturkell E, Einarsson P, Sigmundsson F, Hreinsdóttir S, Geirsson H (2003a) Deformation of Grímsvötn volcano, Iceland: 1998 eruption and subsequent inflation. Geophys Res Lett 30, 10.1029/2002GL016460Google Scholar
  48. Sturkell E, Sigmundsson F, Einarsson P (2003b) Recent unrest and magma movements at Eyjafjallajökull and Katla volcanoes, Iceland. J Geophys Res 108:2369, doi:10.1029/2001JB000917Google Scholar
  49. Tryggvason E (1980) Subsidence events in the Krafla area, North Iceland, 1975–1979. J Geophys 47:141–153Google Scholar
  50. Tryggvason E (1986) Multiple magma reservoirs in a rift zone volcano: ground deformation and magma transport during the September 1984 eruption of Krafla, Iceland. J Volcanol Geotherm Res 28:1–44CrossRefGoogle Scholar
  51. Tryggvason E (1989) Ground deformation in Askja, Iceland: its source and possible relation to flow of the mantle plume. J Volcanol Geotherm Res 39:61–71CrossRefGoogle Scholar
  52. Tryggvason E (1994) Observed ground deformation at Hekla, Iceland prior to and during the eruptions of 1970, 1980–1981 and 1991. J Volcanol Geotherm Res 61:281–291CrossRefGoogle Scholar
  53. Wessel P, Smith WHF (1998) New, improved version of Generic Mapping Tools released. Eos 79:579Google Scholar

Copyright information

© Springer-Verlag 2004

Authors and Affiliations

  1. 1.Institute of SeismologyUniversity of HelsinkiFinland
  2. 2.Science InstituteUniversity of IcelandReykjavíkIceland
  3. 3.Nordic Volcanological InstituteReykjavíkIceland

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