Evaluating the Relevance of Moho Reflections in Accelerometric Data: Application to an Inland Japanese Earthquake

Abstract

The relevance of SmS seismic phase or multiple reflected SmS on the ground motion away from the earthquake source has been discussed in many studies during the past 30 years. The SmS and multiples are normally recognized by analyzing travel time arrivals on the basis of a priori well‐known crustal models. Moho reflections in borehole and surface accelerometric data of the KiK‐net network for an inland Japanese earthquake are investigated. When reflected phases are transmitted efficiently, both amplitude and frequency content can be retrieved by using a simple time‐frequency analysis of accelerometric records. For the 15 April 2007 M 5.4 Kinki crustal earthquake, Moho reflections are found at epicentral distances from 70 to 180 km. Borehole data show that SmS and multiples can be easily found in the frequency range from 0.25 to 2 s and in the group velocity window from 2.8 to 3.5 km/s. The coupling of borehole and surface stations of KiK‐net data allows for the separate evaluation of the propagation effect along the source‐receiver path and of the transfer function due to soft soils for these specific phases. An amplitude–period scaling relationship is found between the National Earthquake Hazards Reduction Program site classification and the observed transfer function for SmS phase and multiples. Online Material: Figures showing the effect of the source‐time function duration on the multiple filter technique (MFT) diagrams, an MFT analysis of surface and borehole accelerograms, period–distance domain spectral amplitudes, and the average period–velocity transfer function for different NEHRP soil classes.