PS Converted-Wave Reflection Survey in Southern Osaka Plain, Japan

Osaka plain is one of the most populated areas in Japan. The velocity structure models have been created for the simulation of strong ground motion generated by expected earthquakes. But in southern Osaka plain, the information for the basement depth by wells and reflection surveys is few, and the models are created using the surface geology and the gravity information. We conducted a 3-component survey in southern Osaka plain, Japan. The survey line is 3.6km long. The Ryoke granite (Cretaceous) crops out in the south end of the line. It deepens to the north and covered by Osaka group (late Pliocene to early Pleistocene). The purposes of this survey are the delineation of Ryoke granite top and the estimation of P and S-wave velocity structure of Osaka group in this area. The source was a vibrator Y2400. The attenuation was large in southern part of the line and refractions are visible only to the distance about 300m. In one quarter of the line, coherent noises probably caused by the water line were very strong and made refractions and reflections hard to be recognized. From the P-wave section, we can see the Osaka group strata in the northern part of the section. And the basement structure with steps can be seen clearly. The depth is about 700m at the north end of the line. But because of the strong coherent noises, the Osaka group strata are not imaged in the middle and southern part of the section. That makes the interpretation of the steps difficult. They can be faults related to Uemachi Fault and/or Izumi mountains, or they can be just eroded unconformities. The radial component data synthesized from H1 and H2 components were processed for the PS converted-wave section. The same structure and the problem can be observed in the PS converted-wave section. The comparison of the two sections suggests the Vp/Vs is almost 3 for the Osaka group strata in the northern part of the section.