Tomography of the 1995 Kobe earthquake area: Comparison of finite-frequency and ray approaches

We determined a detailed 3-D crustal model in the 1995 Kobe earthquake (M 7.2) area in southwest Japan using both finite-frequency and ray tomography methods. Our finite-frequency tomography technique is based on the single-scattering theory. The finite-frequency sensitivity kernel derived in this study reflects correctly the sensitivity of the heterogeneity off the geometrical ray path and the existence of Fresnel volume, and the kernel depends on the dominant frequency of the observed wave. The dominant frequency is estimated directly from the earthquake magnitude based on a relation that is obtained by regressively analyzing the displacement spectra of 20 earthquakes in the study area. We used 141118 P-wave and 133648 S-wave high-quality arrival-time data from 2813 Kobe aftershocks and 3140 other local earthquakes during 1995-2010. The tomographic images obtained with the finite-frequency and ray tomography methods show a high level of similarity, which is verified quantitatively by adopting the structural similarity index. Our results show that the Kobe main shock hypocentre is located in a distinctive zone characterized by a high Poisson's ratio and a low product V_P×V_S of P- and S-wave velocities, which is interpreted as a fluid-filled, fractured rock matrix that may have triggered the 1995 Kobe earthquake.