Low‐velocity zones in the crust beneath Aso caldera, Kyushu, Japan, derived from receiver function analyses

Abstract

AbstractAso volcano, in central Kyushu Island in southwest Japan, has a large caldera (18 × 25 km) that formed by the ejection of more than 600 km3 of deposits 89 thousand years ago. We calculated receiver functions from teleseismic waveform data obtained from densely distributed stations in and around the caldera. We estimated the crustal S wave velocity structure from the receiver functions by using genetic algorithm inversion. We detected a low‐velocity zone (Vs > 2.2 km/s) at a depth of 8–15 km beneath the eastern flank of the central cones. A sill‐like deformation source has been detected at a depth of 15.5 km by analyses of GPS data, and a swarm of low‐frequency earthquakes exists at depths of 15–25 km just beneath this low‐velocity zone. Magma may be newly generated and accumulated in this low‐velocity zone as a result of hot intrusions coming from beneath it. Except for the region beneath the eastern flank of the central cones, a second low‐velocity zone (Vs > 1.9 km/s) extends in and around the caldera at a depth of 15–23 km, although phenomena representing intrusions have not been detected below it. From the estimated velocity structure, these low‐velocity zones are interpreted to contain a maximum of 15% melt or 30% water.