Source Models of Two Large Intraslab Earthquakes from Broadband Strong Ground Motions

We construct source models of two large intraslab earthquakes (the 1993 Kushiro-oki and 1994 Hokkaido Toho-oki earthquakes) along the southern Kurile-Hokkaido arc based on strong motion records; the source models well explain observed broadband strong ground motions. We assume a distribution of rectangular-shaped asperities on a fault plane and apply the empirical Greenʹs function method to the synthesis of strong ground motions from the asperities. The asperity parameters (area and stress drop) are estimated by matching the synthetic waveforms (displacement, velocity, and acceleration time histories) and spectra to the observed ones. Our source models have the following characteristics compared with those of inland and plate-boundary earthquakes having a comparable seismic moment: (1) the total asperity area is much smaller, and (2) the stress drop is extremely high (about 200-400 MPa). These indicate that the large intraslab earthquakes radiate seismic energies from a small area in a short time. This rupture process results in extremely strong radiation of short-period seismic waves. Finally, we confirm a correlation between outer (entire fault size, seismic moment, and short-period level of the acceleration source spectrum) and inner fault parameters (areas and stress drops of asperities) for the two intraslab earthquakes.