Generating Mechanism of Large Earthquakes Associated with the Caldera Collapse of Bárðarbunga Volcano during the 2014-2015 Holuhraun Eruption, as seen by Seismic Waveform Analysis

Abstract:

The volcanic unrest in and around the Bárðarbunga volcano, between Aug 16th 2014 and Feb 27th 2015, was accompanied by a sequence of large events occurring close to the caldera rim. More than 70 events with M >= 5 were reported by the Icelandic Meteorological Office (IMO). The events are in many aspects unusual; 1) Moment tensors have a large negative CLVD component (www.globalcmt.org), 2) Many of the largest events are accompanied by a sudden subsidence at the center of the caldera as measured by GPS (E. Magnússon/B. Ofeigsson) and InSAR (Riel et al 2015), with decreasing steps in time, even for events of the same magnitude, 3) The events have a very long duration for their size (this study), 4) event hypocenters are very shallow (this study), 5) Relatively relocated M < 3 events show subvertical caldera walls (Vogfjord et al, this conference).

Two mechanisms for generating earthquakes with large vertical CLVD components in volcanoes have been suggested; 1) slip on a ring fault (Ekström 1994), 2) collapse of an underlying horizontal sill (Riel et al 2015). However, neither model is consistent with all observations. In this presentation we analyze both regional and teleseismic waveforms from the largest of the events in detail, with the objective of deciphering their generating mechanism. Preliminary results indicate that the complex waveforms cannot be replicated by models with slip on differently oriented simple fault planes or vertical ring fault structures.