Abstract
Networks of seismographs of high sensitivity have been in use in the vicinity of active volcanoes in Iceland since 1973. During this time 21 confirmed eruptions have occurred and several intrusions where magma did not reach the surface. All these events have been accompanied by characteristic seismic activity. Long-term precursory activity is characterised by low-level, persistent seismicity (months-years), clustered around an inflating magma body. Whether or not a magma accumulation is accompanied by seismicity depends on the tectonic setting, interplate or intraplate, the depth of magma accumulation, the previous history and the state of stress. All eruptions during the time of observation had a detectable short-term seismic precursor marking the time of dike propagation towards the surface. The precursor times varied between 15 minutes and 13 days. In half of the cases the precursor time was less than 2 hours. Three eruptions stand out for their long duration of the immediate precursory activity, Heimaey 1973 with 30 hours, Gjalp 1996 with 34 hours, and Barðarbunga 2014 with 13 days. In the case of Heimaey the long time is most likely the consequence of the great depth of the magma source, 15-25 km. The Gjalp eruption had a prelude that was unusual in many respects. The long propagation time may have resulted from a complicated triggering scenario involving more than one magma chamber. The Barðarbunga eruption at Holuhraun issued from the distal end of a dike that took 13 days to propagate laterally for 48 km before it opened to the surface. Out of the 21 detected precursors 14 were noticed soon enough to lead to a public warning of the coming eruption. In 4 additional cases the precursory signal was noticed before the eruption was seen. In only 3 cases was the eruption seen or detected before the seismic precursor was verified.
Highlights
The interaction of the Iceland hotspot with the mid-Atlantic plate boundary leads to volcanism of unusually wide variety
The slow intrusion lasted about 1 year and ended without an extrusion to the surface. Even though it has been general knowledge in Iceland for centuries that the outbreak of volcanic eruptions is commonly associated with earthquakes, it wasn’t until sensitive seismographs were installed in Iceland and had been in operation for a few decades that it became clear that all eruptions are preceded by characteristic seismicity, eathquakes and continuous tremor
We find that the precursor time of the 21 eruptions is highly variable, between 15 min and 13 days, see Table 1
Summary
The interaction of the Iceland hotspot with the mid-Atlantic plate boundary leads to volcanism of unusually wide variety. Following a small eruption in 1934, it went into a quiet state for almost half a century At least three events have taken place, during which cauldrons have been formed in the ice cover of the volcano and floods have issued from the glacier edge These occurred in 1955 (June 25), 1999 (July 18), and 2011 (July 8–9), and were accompanied by earthquakes and seismic signals that resemble volcanic tremor (Tryggvason, 1960; Einarsson, 2000; Gudmundsson et al, 2000; Sgattoni et al, 2017). The slow intrusion lasted about 1 year and ended without an extrusion to the surface
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