Heterogeneous structure in and around the source region of hazardous inland earthquake

Abstract:

A detailed crustal structure and deep geometry of the active fault provide important information to constrain the process of earthquake occurrence. Dense seismic array observation is one of the most effective techniques to reveal inhomogeneous structure. Recently, several dense seismic array observations have been conducted across the active fault in Japan: for example, Tachikawa fault located near the Tokyo metropolitan area, and the Neodani fault located in the central Japan. Previous studies indicated that the heterogeneous lower crust should be the key to understand the mechanism of earthquake occurrence (e.g., Iio et al., 2002). In November of 2012, a highly dense seismic experiment was conducted in and around the source region of the 1891 Nobi-earthquake (M 8.0), central Japan. The transect line ran from Fukuchiyama-city to Ina-city over a 260 km profile, on which 8 explosives were fired. We deployed 1,793 off-line recorders to record the explosive seismic signal. The collected data have high signal-to-noise ratios, from which we can easily recognize not only the first arrival phases but also latter phages. Two-dimensional (2-D) P-wave velocity structure beneath the survey line was derived by ray tracing method. Remarkable characteristics of the 2-D velocity structure are relatively low-velocity middle and lower crusts (Vp=5.9-6.3 km/s) and change in crustal thickness beneath the central part of the profile (Shiga and Gifu Prefecture). This low velocity region involves the deeper extension of the Neodani fault. The 1891 Nobi-earthquake was associated with rupture of the Neodani fault. The subducting Philippine Sea plate (PHS) is in contact with the low-velocity lower island-arc crust. The contact zone between the base of the low-velocity lower crust and the top of the PHS is located at a depth of about 28km. The island arc Moho is about 33 km deep beneath the eastern part of the profile (Gifu and Nagano Prefecture). Several reflectors can be recognized within low-velocity lower island-arc crust and the PHS below the contact zone. These results suggest that the inhomogenity around the source region of the 1891 Nobi-earthquake was caused by fluids dehydrated from the subducted PHS. Such inhomogenity is a possible cause of stress concentration, and consequently it may be the cause of earthquake occurrence.