Estimation of rupture propagation direction and strong motion generation area from azimuth and distance dependence of source amplitude spectra

Strong motion generation areas which reproduce ground motions in 0.2 to 10Hz were estimated using the empirical Green's function method. This strong motion generation area was somewhat smaller than the total rupture area, and coincident with the area of asperities derived from heterogeneous slip distributions estimated by waveform inversions using lower frequencies (<1Hz). We confirmed that the azimuth and distance dependence of observed source amplitude spectra in the near-source area, i.e. rupture directivity effects, were controlled by rupture propagation style and size of the strong motion generation area. We found that the source displacement spectra at stations in forward rupture propagation directions had higher corner frequencies and steeper high-frequency decays, compared with stations in sideways directions. Stations in backward directions had opposite tendencies. Different relationships between size and average corner frequencies of the strong motion generation area were proposed for unilateral and bilateral ruptures with radial propagation.