Characteristics of Small-Scale Heterogeneities in the Hidaka, Japan, Region Estimated by Coda Envelope Level

We estimated the spatial distribution of small-scale heterogeneities as anomalous amplification of coda level in the Hidaka, Japan, region. We analyzed 2768 seismograms in a frequency range of 1-32 Hz for 24 earthquakes recorded at 62 stations. First, we estimated the site effect of each station, using the coda nor- malization method with regional earthquake data of large epicentral distances. All the data processing was done after the correction of this site effect. Next, we deter- mined coda amplitude factor (CAF), that is, the amplitude ratio of coda waves on each source-station pair relative to the averaged coda amplitude over stations, for earthquakes inside the region. We found systematic spatial variations of CAF, im- plying the existence of localized heterogeneities. At frequencies lower than 2 Hz, the CAF is relatively large in the west of the Hidaka Mountains, implying the possibility of strong heterogeneities with a scale of 0.6-1.3 km there. In a high-frequency range ( 16 Hz), CAF values are large on paths crossing the Hidaka Mountains. From the corresponding lapse time of coda (65 sec), there may be a zone of concentrated heterogeneities beneath the Hidaka Mountains at the depth of 100-120 km.