Distribution of scatterers in the lower crust beneath the focal region of the 1995 Hyogo-ken Nanbu (Kobe) earthquake from explosion seismic observations

Recent studies indicate that the structure of the lower crust affects the occurrence of large intraplate earthquakes. Therefore, for understanding the generating processes of intraplate earthquakes, it is important to reveal the detailed structure of the lower crust. After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M 7.3), the Research Group for Explosion Seismology (RGES) conducted a seismic refraction experiment in the northern part of Awaji Island in order to investigate the detailed structure around the focal region. In the surveyed area of 5 km × 10 km, 128 temporal observation stations were deployed two-dimensionally, and seven explosive shots with a 150-kg charge were detonated. The recorded waveforms contained many later phases with lapse times of 6-11 s from the shots. These later phases imply that strong inhomogeneity exists in the lower crust. Waveforms of the later phases were very complicated and appeared to be coherent only among 8-11 adjacent stations. In this study, we considered the later phases as P-to-P scattered waves and estimated the spatial distribution of scatterers by using the following procedure. We divided the stations into 18 groups, each consisting of 8-11 stations. For each group and each shot, the semblance coefficient is calculated using the seismograms in a frequency range of 4-8 Hz to pick up coherent later phases. The location of a scatterer was estimated from travel time, incoming azimuth, and incident angle of the phase. By applying this procedure, we obtained an image of the scatterer distribution. The scatterers were distributed in regions around 25 km depth in the central part and 30 km depth to the northwest of Awaji Island with a horizontal extent of about 10 km. These two regions were located on the edge of the low-Vp, low-Vs, and high-Vp/Vs zones. The spatial relationship between the source fault, scatterer distribution, and velocity anomalies found in this study was similar to that obtained in other focal regions of intraplate earthquakes, the 2002 western Tottori earthquake (M 7.3) and the 1962 northern Miyagi earthquake (M 6.5).