Detection of Localized Small-Scale Heterogeneities in the Hanshin-Awaji Region, Japan, by Anomalous Amplification of Coda Level

We estimated the spatial distribution of small-scale heterogeneities in and around the Hanshin-Awaji region including the coseismic fault area of the 1995 Hyogo-ken Nanbu, Japan, earthquake, by using the amplification factor of the coda level as a measure of scattering strength. We analyzed 137 earthquakes recorded at 64 stations. Because the coda level is strongly affected by site amplification, the determination of the amplification factor of the coda level requires the following three major steps: (1) checking the stability of coda waves among different stations such as coda Q and lapse time, (2) estimating the site amplification of coda waves for events outside the fault region, and (3) taking the spectral ratio of coda waves for events inside the fault zone after correcting the site characteristics obtained in step 2. The coda decay rate, Qc^-1 , is common among stations for each event. The site amplification factor at each site estimated from 25 events correlates with the surface geological setting. The most important result is that coda amplification anomalies exist as clusters along the coseismic fault area, depending on frequency: beneath Awaji Island in the range of 1-4 Hz and beneath the area southwest of Kobe, 8-16 Hz. These high coda levels imply that strong heterogeneities exist there. The scales of these heterogeneities are estimated to be about 0.2-1.4 and 0.06-0.18 km, corresponding to the observed dominant frequencies (1-4 and 8-16 Hz, respectively).