[S03-1] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise I

[S03-1-04] Role of localized heterogeneities on distortion of the apparent radiation patters: aftershock sequence of the 2016 Kumamoto earthquake

^○Shunsuke Takemura, Tatsuhiko Saito, Hisahiko Kubo, Katsuhiko Shiomi (National Research Institute for Earth Science and Disaster Resilience, Tsukuba, Japan)

The azimuthal variations of observed maximum amplitudes during local earthquakes (called the “apparent radiation patterns") are gradually distorted from the expected four-lobe patterns of a double-couple point source with increasing frequency and distance (e.g., Liu and Helmberger, 1985; Takemura et al., 2009, 2016). In this study, we examine the frequency- and distance-dependent characteristics of the apparent radiation patterns for P and S waves using NIED Hi-net waveforms of 30 aftershocks of the 2016 Mw 7.1 Kumamoto earthquake. Analyzed aftershocks are concentrated around two regions: the fault zone of the 2016 Kumamoto earthquake and Aso volcano area. Thus, we separately analyze the aftershocks in the fault zone (region A) and the volcanic area (region B). To quantify distortion from the four-lobe pattern, we calculate cross-correlation coefficients (CCCs) between observed and theoretical apparent radiation patterns as a function of normalized hypocentral distance kL, where k is wavenumber and L is hypocentral distance. Focal mechanisms are referred from the NIED F-net moment tensor catalog.
In the Chugoku region, CCCs show linear decay from 0.75 to 0.25 with increasing log(kL) from 1.64 to 2.85 and no significant difference of decay rate between P and S waves (Takemura et al., 2016). However, CCCs in this study show different characteristics from ones in Chugoku region. In region A, CCCs for P and S waves linearly decay in the same log(kL) range, but values are smaller (0.45-0.2) than Chugoku region, indicating that crustal heterogeneity in region A is stronger than Chugoku region. On the other hand, in region B, CCCs for S wave are very small (=0.15-0.2) even for small log(kL), but ones for P wave well agrees with Chugoku region, suggesting that around the volcanic area S-wave velocity structure has relatively strong heterogeneous nature compared to P-wave one.