2001 Volume 54 Issue 1 Pages 233-249
We carried out a seismic survey in the northwestern Kii Peninsula, Japan. A major active fault system, called the Median Tectonic Line (MTL), passes through the study area. The detailed subsurface structure of the MTL in the study area has been investigated by seismic reflection surveys and borehole data of the Geological Survey of Japan. The MTL is composed of high-angle right lateral faults and low-angle reverse-slip faults. Since such a complicated fault system is expected to produce strong scattered waves, the MTL fault system in the study area is an ideal location to study the relationship between distribution of seismic scatterers and the subsurface structure. The survey was designed to record scattered waves as effectively as possible. First, we obtained a seismic velocity structure with spatially high resolution by combining both reflection and refraction methods. Next, we applied a semblance analysis to estimate the distribution of seismic scatterers using the obtained velocity structure. The semblance coefficient at a particular point in the profile is related to the magnitude of scattering there. A high semblance coefficient implies effective excitation of scattered waves. A test with synthetic data, it shows that the spatial resolution for scatterer location is less than 20m. Three regions are detected with high semblance coefficients, and corresponding to the fractured Izumi group (Cretaceous), conglomerate-predominant layers in the Shobudani Formation (Plio-Pleistocene) and rugged unconformity surface at the top of Sambagawa metamorphic rocks. One of the two detected low semblance regions coincides with mud-predominant layers in the Shobudani Formation, and the other with the low-angle MTL fault zone. The rocks in the fault zone have been crushed so homogeneously by repeated faultings that the zone must be recognized as a low semblance region.
