エアガン人工地震データ解析による島原半島の地殻構造

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Unzen eruption starting from 1990 prompted us to put as much constraint as possible in the velocity structure of whole Shimabara peninsula, and we re-analyzed a controlled source seismic experiment data obtained on a seismic line with the strike of N-S in 1986. An airgun was shot repeatedly as the controlled source at Port Kuchinotsu, Bessho dam and Port Taira. These shots were recorded by 114 stations. A 2-D velocity model to a depth of 20 km was obtained by the 2-D ray tracing technique and reflectivity method using not only the traveltimes and the amplitudes, but also a relationship of the amplitudes versus the offsets from each shot position to each receiver station. We identified four layers beneath the Shimabara peninsula, (1) an uppermost layer (V_p=2.6-3.3 km/s), (2) a basement (V_p=3.6-3.9 km/s), (3) an upper crust (V_p=5.9-6.3 km/s), and (4) a lower crust (V_p=7.0-7.3 km/s). The upper crust consists of two parts, one is the upper part with large vertical velocity gradient (0.1s^<-1>) and the P-wave velocity of 5.9-6.3 km/s, and the other is the lower part with almost constant velocity of 6.3 km/s. The lower crust underlaying the upper crust also consists of two parts. The P-wave velocities of the upper and lower parts are inferred to be 7.0 km/s and 7.3 km/s from amplitude analyses of the reflected waves. Beneath the central part of the peninsula, the upper crust becomes thinner and the lower crust swells. The boundary between the upper and the lower crusts is located at a depth of 9-11 km. The bottom of the lower crust with high velocity contrast is located at a depth of 19-20 km.
特定非営利活動法人日本火山学会の論文
2002-11-29

エアガン人工地震データ解析による島原半島の地殻構造

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