Long-period seismicity at Shishaldin volcano (Alaska) in 2003-2004: Indications of an upward migration of the source before a minor eruption

We have analyzed the long-period (LP) seismic activity at Shishaldin volcano (Aleutians Islands, Alaska) in the period October 2003-July 2004, during which a minor eruption took place in May 2004, with ash and steam emissions, thermal anomalies, volcanic tremor and small explosions. We have focused the attention on the time evolution of LP rate, size, spectra and polarization dip angle along the dataset.We find an evolution toward more shallow dip angles in the polarization of the waveforms during the sequence. The dip angle is a manifestation of the source location. Because the LP seismic sources are presumed to reflect the aggregation of gas slug or pockets within the melt, we use the polarization dip at the LP onset as a proxy for the nucleation depth of the seismic events within the conduit. We refer to this parameter as the nucleation dip and the position along the conduit of the gas aggregation as nucleation depth.The nucleation dip changes throughout the dataset. It shows a sharp decrease between the end of December 2003 and the end of January 2004, followed by a gradual increase until the onset of the eruption. At the same time, a general increase of the LP rate occurs. We have associated the dip evolution with a sinking and a subsequent decrease of the nucleation depth, which would quickly migrate up to about 8. km below the crater rim, followed by a slow depth decrease which culminates in the eruption.The change in the nucleation depth reflects either a pressure variation within the plumbing system, which would affect the confining pressure experienced by the gas aggregations. We have imputed such a pressure change to the intrusion of batches of magma from a deeper magma chamber (<10km) toward a shallower one (>5km). For a cylindric conduit with rigid walls, this leads to a volume of the injected new magma of 105-107m3, compatible with estimates in other areas, suggesting that the LP process can be considered a good proxy of the thermodynamical conditions of the shallow plumbing system.