制御震源地震探査データの共通散乱点重合法解析による北部フォッサマグナ地域の散乱体分布

Abstract

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 200m. The array consisted of 39 seismometers distributed two dimensionally with a spacing of 100m. 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 100m 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 cross-correlation 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 800m and 1600m. 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.