Determination of subsurface structure of Tottori plain using microtremors and gravity anomaly

Microtremor observations and a gravity survey were conducted to determine the subsurface structures of the Tottori plain. This area was severely damaged during the 1943 Tottori earthquake (M 7.2), and the damage was concentrated in the plain. Microtremors were recorded by seismic arrays (8 sites) and 3-component single-site observation (417 points). Gravity data were obtained at about 400 points. The microtremor data were analyzed by the spatial auto correlation method and the horizontal-to-vertical spectral ratio (H /V). The subsurface structures were determined by 1) S-wave velocity structure models obtained at the array observation sites, 2) a 3D bedrock configuration based on H/V and the residual gravity anomaly, and 3) 2D density structures for three cross sections obtained using the gravity anomaly based on depths of S-wave velocity structures. A shallow bedrock area extends over a belt-like zone along the coast. This bedrock suddenly deepens from the eastern mountainous area to the plain. The depth to the V subscript S = 3500 m/s layer ranges from a minimum of 300 m to a maximum of 800 m.