Shallow Velocity Structure Beneath the Aira Caldera and Sakurajima Volcano as Inferred from Refraction Analysis of the Seismic Experiment in 2008(＜Special Section＞Sakurajima Special Issue)

We performed refraction analysis for the first P-wave arrival time data observed in the seismic experiment in 2008, and estimated a shallow velocity model up to 3 km depth beneath the Aira caldera and Sakurajima volcano. We found that a basement layer with a velocity of 4.6-5.0 km/s, which corresponds to geologically the Shimanto Group, inclines toward the central part of the Aira caldera. A low velocity zone with a velocity of 4.2-4.4 km/s is located in a depth range 1.5-3 km in the central part of the caldera. This low velocity zone suggests high activity of the magma plumbing system from the deep magma reservoir distributed beneath the caldera. It is found that the basement layer steeply falls down from 1 km to 2.5 km in depth along the northwestern boundary of the Kagoshima graben. The velocity structure in Sakurajima volcano is characterized by a zone with a velocity of 3.6-3.7 km/s. Moreover, we present a possibility that the underground structure strongly restricts an expanse of a focal region of each different type of the volcanic earthquakes.