Three-dimensional P- and S-wave velocity structure below Unzen volcano

Unzen volcano, located in southwestern Japan, erupted on November 17, 1990 after 198 years of dormancy and has been ejecting lava since May 20, 1991. In this paper, we present three-dimensional P- and S-wave velocity variations below Unzen volcano using 22,473 P- and 14,349 S-wave arrival times from 3457 local earthquakes recorded on a local network of twelve seismic stations. The model was parameterized by 24,000 approximately 2.0 km cubic blocks, targeting a volume of 120 × 100 × 20 km. A prominent low-velocity anomaly (greater than 4% slowness perturbation) starting from 2.5 to 5.0 km depth beneath the volcano is observed on both the P- and S-wave inversions. Below Unzen volcano slightly lower amplitude low-velocity anomalies are further observed to a depth of 12.5 km for P- and 7.5 km for S-waves. Shallow low-velocity anomalies are observed below the Chijiwa bay area (northwest of Unzen), and deeper anomalies are seen below Shimabara bay (southwest of Shimabara peninsula). This NW-SE-trending feature represents an elongated system of dykes which supply the shallow magma reservoirs.