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Seismic waves are refracted, reflected, or scattered by heterogeneities in the Earth's crust. Small-scale structures less than wave-lengths are difficult to be observed by conventional refraction and/or reflection methods although scattered wave energy is observed as coda waves. In March 1997, a seismic reflection survey was conducted to reveal the active tectonics in the Japan arc across the Northern Fossa Magna in Japan. The profiling revealed a strongly deformed thick sedimentary layer. At the same time, we carried out a seismic array observation using off-line seismic recorders in the same area to image crustal structure. We used four large vibrators as seismic sources with a spacing of 200 m. The array consisted of 39 seismometers distributed two dimensionally with a spacing of 100 m. We applied the CSP (common scatterer point) stacking method for all the combination of sources and receivers. We assumed 3-D grid points with a spacing of 100 m in a studied area. We calculated a travel time field for the 3-D structure by a finite difference method. To investigate whether a scattered energy was really originated from a particular grid point or not, we calculated crosscorrelation coefficients between the CSP stacked waveforms and a source waveform. We interpret that the correlation coefficients represent a distribution of point scatterers. The scatterers are distributed near the Earth's surface and at depths of 800 m and 1600 m. Compared with the reflection profile, scatterers are distributed in severely deformed strata. Since our method strongly depends on the background velocity structure, it is important to use a realistic background velocity structure to estimate scatterer distributions accurately.

Scatterer Distribution in the Northern Fossa Magna, Japan, by the Common Scatterer Point Stacking Method Applied to Controlled Source Experiment Data