Páll Einarsson (Emeritus, IES)

"Three types of seismic tremor associated with activity of the subglacial Grímsvötn volcano"

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Abstract:

The Grímsvötn volcano, one of the most active volcanoes in Iceland, is covered to a large extent by the Vatnajökull glacier. High geothermal activity within its caldera maintains a caldera lake with variable water level. Large floods from the lake (jökulhlaups) are initiated when the water level exceeds a critical level, that varies with time. In several cases the falling lake level is known to have triggered eruptions of the volcano. The eruptions of 1983, 1998, and 2011 were not triggered by jökulhlaups, on the other hand, and several jökulhlaups have not triggered eruptions, such as those of 2008 and 2010. All these processes, i.e. volcanic activity, water floods, and geothermal activity, are accompanied by seismic tremor that is detectable by the surrounding network of seismic stations. For monitoring purposes it is of great importance to be able to separate the different types of tremor and identify their sources. The tremorplot application accessible through the webpage of the Icelandic Meteorolgical Office is used to characterize the types of tremor recorded during several recent events at Grímsvötn. By comparing tremorplots of the jökulhlaups of 2008 and 2010, and the eruptions of 2004 and 2011, we can identify three types of tremor:

1. Water flood tremor. Jökulhlaups from the caldera are always accompanied by high-frequency tremor, recorded on the seismic stations near the caldera.

2. The second type of tremor appears to be switched on when the drop in water level reaches 10-30 m. It remains after all water has been drained from the lake and is therefore not associated with flowing water. The range of this tremor is short, it is seldom recorded beyond the edge of the glacier. It is suggested that it is generated by flash-boiling of the geothermal system within the caldera, triggered by the pressure drop of the lake level.

3. Eruption tremor. Eruptions of Grímsvötn are accompanied by tremor that begins simultaneously with the eruption and is distinctly different from the other two types of tremor. It contains lower frequencies and has a wider range.