DATアレイ地震観測による奥羽脊梁山地およびその周辺域における地震波散乱体分布の推定

Abstract

Seismic array observations were made for about 3 months from July 28 to November 4, 1998, to estimate inhomogeneous structure of the crust and uppermost mantle beneath the Ou backbone range, which is one of the most seismically active areas in northeastern Japan. We deployed L-shaped small aperture seismic arrays at 13 locations in and around the range. Each array is composed of 16 seismometer stations with a 50m separation and 3 sets of DAT recorders, which record 18-channel seismic signals continuously for about 4 weeks with a 100-Hz sampling. We recorded waveforms from 100kg explosions detonated at 11 locations for the seismic refraction/reflection experiment of the 1998 Joint Seismic Observation of Tohoku. We also observed many earthquakes, including aftershocks of M 6.1 earthquake that occurred on September 3, 1998, in the northern edge of our array observation network. Both explosion and natural earthquake data recorded by the arrays are used to estimate a spatial distribution of seismic scatterers in the crust and uppermost mantle. We evaluated semblance coefficients and powers of seismograms to detect coherent wave trains with large amplitudes. Then the azimuth and slowness of these wave trains are converted to their source locations by assuming a single scattering in the model velocity structure. Estimated distribution of scatterers shows that most of them are distributed within the crust beneath the Ou backbone range. Particularly, concentrations of scatterers are seen around the fault planes and their deeper extensions of active faults bounding both at eastern and western edges of the backbone range and the fault plane of the September 3, 1998, earthquake of M6.1. Some of the scatterers are also located around the S-wave reflectors (bright spots) detected by the previous study in the lower crust of this area.