Shear wave splitting, v_P/v_S, and GPS during a time of enhanced activity at Aso caldera, Kyushu

摘要

We performed shear wave splitting analyses and calculated v_P/v_S‐ratios on local earthquakes recorded at seven stations around Aso volcano, Japan, between 2001 and 2008. The mean v_P/v_S of 1.61 ± 0.01 from all stations is among the lowest average values found anywhere in the world. A temporal change of shear wave splitting fast polarization is observed at one station. Additionally, spatial variations of shear wave splitting delay times and v_P/v_S‐ratios show a correlation at three stations. We suggest that stress‐controlled anisotropy dominates at these stations. GPS baselines and 2D strain reveal extension in the caldera in 2003, and compression before and after 2003. Strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003. Baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km. Compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low v_P/v_S‐ratios and the observed compression in the caldera. We propose that a small volume of magma intruded into the sill in 2003, caused extension at the surface, and triggered ash eruptions between 2003 and 2005. Associated localized migration of gases in overpressured cracks with aspect ratios potentially ～0.5 in the upper crust with porosities ≥0.1 could be the source of changes in seismic anisotropy, and explain the slight variations observed in v_P/v_S.