Imaging of crustal heterogeneous structures using a slowness-weighted back-projection with effects of scattering modes: 2. Application to the Nagamachi-Rifu fault, Japan, area

摘要

This is the second paper in a two-part series on a newly developed imaging approach for small-scale heterogeneities (<1 km) in the crust with effects of scattering modes. In the present paper, we estimated a detail three-dimensional spatial distribution of small-scale heterogeneities around the Nagamachi-Rifu fault, northeastern Japan, in a frequency range of 2–16 Hz, using the imaging approach presented in the first paper. We used seismograms recorded by dense three-component seismic arrays that were deployed in this region for 15 explosion sources. As one of our important results, there are concentrations for only P-S but not P-P scatterers near the surface trace of the fault. P-S scatterers in a frequency range of 8–16 Hz are localized near the surface trace of the fault, implying the possibility of strong heterogeneities with a size of 0.08 km there. In the Shirasawa caldera region, the characteristics of seismic scatterers seem to convert from large P-S to large P-P relative scattering coefficients with its transition depth range of 5–8 km. This feature implies that the materials composed of seismic scatterers may show a systematic variation with depth. Finally, the strength of scattering coefficients is rather weak in the coseismic area of the 15 September 1998 earthquake with a magnitude of 5.2, the largest recent event in this area. This result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system.