Velocity Structure Modeling of Beppu Basin, Japan

Abstract eng:
Beppu Bay sedimentary basin is about 50 km x 30 km bell shaped basin with maximum sediments thickness is 4 km. Velocity structure of Beppu Bay basin, Kyushu, Japan, has been investigated with microtremor array survey and ambient noise interferometry survey. Microtremor array survey with array radius about 20 to 1000 m has been conducted at 21 sites around Beppu Bay using T=10s velocity seismometers to estimate subsurface S-wave velocity structure. S-wave velocity structures down to the seismic basement have been estimated by converting phase velocities analyzed with SPAC method and V method using empirical relation. Overall performance of published velocity structure, J-SHIS model, is good, but remarkable discrepancy in phase velocities are found at Beppu fan sites. Ambient noise interferometry has been conducted using year-long data obtained with 12 broadband seismometers deployed around the Beppu Bay to investigate S-wave velocity structure beneath the sea covered area. Group velocities of the Rayleigh and Love waves between station pairs have been successfully retrieved from CCFs. Group velocities along the paths through Beppu Bay are found to be systematically smaller than those calculated from J-SHIS model, indicating the S-wave velocity beneath the sea area of the Beppu Bay is slower than modeled.

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