Dissimilar receiver functions observed at very close stations in the Kii Peninsula, central Japan: features and causes

Receiver function analysis is one of the most powerful methods for modeling the subsurface structure beneath a seismograph station. Based on the assumption that similar receiver functions should be observed at stations located close to each other, we can construct a spatial distribution of a seismic velocity interface, such as the Moho discontinuity, to trace the distribution of P-to-S (Ps) converted phases in receiver functions. After more than 15 years of observation by the seismograph networks, we confirmed that receiver functions observed at two stations very close to each other in the Kii Peninsula, central Japan, show significantly different characteristics from each other. The backazimuth dependence of later phase arrivals within 4 s after the direct P wave was strong, especially for transverse components. Because the Ps converted phases at the Moho discontinuity arrived around 4 s after the direct P wave, we assume that the dissimilarity is caused by a localized structural anomaly in the crust. This assumption is supported by the fact that only the cross-correlation coefficients for the short-period transverse component of teleseismic waveforms were small. According to the geological map, the two stations are located in the accretionary complex along the Pacific coast, and the strikes of the geological boundaries show an abrupt change around the stations. Based on forward modeling with the anisotropic subsurface models, we confirmed that the characteristics of the observed receiver functions can be explained roughly by considering a localized anomaly in the upper and middle crust.