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

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 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.

Seismic activity related to the 2000 eruption of the Hekla volcano, Iceland

“…Although the degassing-related origin of the tremor is shallow, the resulting vibration can occur in a long channel and produce the characteristic low frequencies. Lack of evidence for a considerable Hekla magma chamber at the uppermost 14 km (Soosalu and Einarsson 2004) may give an estimate for the length of the eruptive conduit of Hekla.…”

“…No propagating front of seismicity from depth towards the surface could be detected. We have studied the location of the magma chamber of Hekla (Soosalu and Einarsson 2004) and concluded that no seismic indicators of a prominent magma chamber at Hekla can be found in the uppermost 14 km.…”

Degassing regime of Hekla volcano 2012–2013

“…Several early ground deformation studies inferred the magma reservoir depth at 5-10 km (Kjartansson and Gronvold, 1983;Sigmundsson et al, 1992), while a more recent and more encompassing work by Sturkell et al (2013) locate it closer to 10 km. However, other research has indicated that there is no significant molten material shallower than 14 km (Soosalu and Einarsson, 2004); and that the main storage chamber is located at 16 ± 2 km depth (Ofeigsson et al, 2011), or possibly as deep as 24 km (Geirsson et al, 2012). Petrological analysis provides evidence in support of magma storage at $9 km (Moune et al, 2007).…”

“…Hekla volcano (63.98°N 19.70°W) is one of the most frequently active, yet also one of the most unpredictable volcanoes in Europe, as a precursory seismic swarm has preceded its most recent eruptions by only a couple of hours or less (Soosalu and Einarsson, 2004 with the most recent one in 1104 AD (Thorarinsson, 1967;Larsen et al, 1999). Following this event and until 1947 CE Hekla continued to erupt 1-2 times per century with volumes up to 0.4 km 3 (Thorarinsson, 1950(Thorarinsson, , 1967Larsen et al, 1999).…”

Short-Term Seismic Precursors to Icelandic Eruptions 1973–2014

“…Short-term changes were recorded 23-79 min before the outbreak of the eruptions. This short precursor time appears to be inconsistent with the large depth of 15-20 km to the feeding magma reservoir (Soosalu and Einarsson, 2004;Ofeigsson et al, 2011;Geirsson et al, 2012). Sturkell et al (2013) discuss this in connection with their finding that the surface eruptive fissure only extends to shallow levels in the crust, below which a pipe-like conduit extends downwards to the reservoir.…”