Repeating Long-Period earthquakes at Shishaldin volcano (Alaska) before a minor eruption: insights into the medium properties and source mechanism

We have looked for repeating earthquakes at Shishaldin volcano (Alaska) by a multi-stage clustering of the waveforms of a large catalog of Long-Period (LP) earthquakes (about 330,000 events) occurring between October 2003 and July 2004, that included a minor eruption. We found 551 repeaters mainly concentrated in two temporal clusters in December 2003 - January 2004 and in February 2004. By cross-correlation, we have estimated the time shifts Δt between pairs of repeaters and have found an increase of Δt moving along the signal. The increasing rate of Δt appears especially relevant (corresponding to a stretching factor in the order of 0.01) in December 2003 - January 2004, that we have defined as Transitory Phase (TP), during which most of the repeaters were detected and a sudden increase of the nucleation depth of the repeaters (inferred by the polarization dip angle) occurred. TP anticipated of about five months the climax of the eruption that culminated in May 2004 with steam and ash explosions and thermal anomalies. We have explored the possibilities that the signal stretching of the repeaters would be induced by a modification of the elastic properties of the medium and/or of the magmatic/hydrothermal source producing the LP events. We modelled the observations during TP as the effects of a sharp pressure drop inside the volcanic/hydrothermal feeding system possibly due to the opening of a path between magma chambers at different heights. Such a pressure drop could have been recovered after TP by an enhanced magma degassing that eventually led to the eruptive crisis.