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

Aso volcano, in central Kyushu Island in southwest Japan, has a large caldera (18 × 25 km) that formed by the ejection of more than 600 km 3 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.