Determination of rupture propagation by measurements of durations of high frequency energy radiation

Hara (2011, Earth Planets Space, 63, 525-528) measured durations of high frequency energy radiation (HFER) for the 2011 off the Pacific coast of Tohoku Earthquake using tele-seismic broadband waveform data following the procedure of Hara (2007, EPS, 59, 227-231). He showed that the measured HFER durations had azimuthal dependence, and suggested that the rupture that radiated high frequency energies propagated in the southwest direction. In this study, in order to quantitatively estimate rupture propagation using measured HFER durations, we performed a grid search to determine the end point of rupture. First, we set spatial grids on an assumed fault plane. We calculated theoretical arrival times of P waves radiated at each spatial grid by adding theoretical travel times for each grid and rupture duration inferred from the mean of the measured HFER durations. Then, we calculated theoretical durations by subtracting the theoretical travel times computed for the hypocenter (i.e., the starting point of rupture) from the theoretical arrival times of P waves computed by the above mentioned procedure. We performed a grid search in which these theoretical durations were compared to the observed durations to find the optimal end point of rupture. We applied this method to the 2011 off the Pacific coast of Tohoku Earthquake. The optimal end point of rupture is obtained at the deep part of the assumed fault in the southwest direction. This result is consistent with the locations of strong motion generation areas obtained by previous studies (e.g., Kurahashi and Irikura, Earth Planets Space, 63, 571-576). Then, we applied this method to the 1994 far east off Sanriku earthquake. The optimal end point of rupture is obtained at the deep part of the assumed fault, which is consistent with the result of Nakahara et al. (1998, JGR, 103, 855-867, doi:10.1029/97JB02676), who performed seismogram envelope inversion to determine spatial distributions of high frequency energy radiation. We also applied this method to the 2003 Tokachi-oki earthquake. The optimal end point of rupture is again obtained at the deep part of the assumed fault. This is consist with the HFER centroid obtained by Wang (2010, Bulletin of IISEE, 44, 7-12), who conducted analyses using empirical Green's functions. The results for these three events seem consistent with the variation of rupture properties with depth suggested by Lay et al. (2012, JGR, 117, B04311, doi:10.1029/2011JB009133). The result of this study suggests the possibility of rapid determination of rupture propagation radiating high frequency energies by the grid search using HFER durations from tele-seismic data.