Seismic image and its implications for an earthquake swarm at an active volcanic region off the Miyake-jima-Kozu-shima, Japan

Abstract

An intensive earthquake swarm, which is proposed to be attributed to the magmatic activity, has continued in more than six months around the Miyake-jima-Kozu-shima. A wide-angle seismic survey was conducted immediately above the swarm. The purpose of this study is to map the seismic velocity structure of the swarm area, and to investigate structural evidence indicating magmatic activity. From the result of first arrival refraction tomography, we found a 5-8% decrease in P-wave velocity surrounding the swarm. A depth migration of the intra-crustal reflection phase also shows an existence of a reflector immediately below the swarm (10-13 km depth). Compared with the seismic imaging, geodetic data and earthquake focal mechanisms, the intra-crustal reflector we found might be the top of magmatic body which has caused the earthquake swarm between Miyake-jima and Kozushima.