At the Ibaraki-ken-oki region where the Paci.c plate subducts beneath the North American plate, repeating M7 class subduction-zone earthquakes have occurred with an interval of approximately 21 years. We estimated the source rupture processes of two M7 Ibaraki-ken-oki earthquakes from broadband strong motion records and compared them to investigate the source characteristics of repeating subduction-zone earthquakes. First, the source model of MJ 7.0 earthquake occurring on May 8, 2008 (2008MS) was estimated by the forward modeling through strong motion simulations using the empirical Green’s function method. Strong ground motions were simulated in the broad.band frequency range (0.3-10 Hz). A single rectangular Strong Motion Generation Area (SMGA) was assumed. The synthetic waveforms at stations within 150 km from the hypocenter reproduced the observed ones well. Then, we estimated the source model of another MJ7.0 earthquake occurring on July 23, 1982 (1982MS). We compared the observed waveforms of the 1982MS and the 2008MS at the same stations and identi.ed the initial rupture phase for the 1982MS. We assumed a single rectangular SMGA away from the hypocenter of the 1982MS. The SMGA was located about 33 km westward from the hypocenter and ruptured about 11s after the origin time; this location was just 7 km northward to the 2008MS hypocenter and so inside its SMGA. We analyzed the spectral ratio of the strong motion records of the 1982MS to the 2008MS and estimated the size, stress drop, and slip amount of the SMGA of the 1982MS. The estimated size was the same as that of the 2008MS, but the stress drop and slip amount were approximately 1.5 times larger. Synthetic waveforms reproduced the observed ones well when we assumed the rupture propagation characteristics were the same as that of the 2008MS. This simulation did not have the resolution to several kilometers’change of the location of the SMGA. Our results indicate that the two SMGAs may overlap. They were the same size, but had different stress drops and slip amounts. The hypocentral locations and initial rupture processes were also different. These results show the variety of the repeating earthquakes occurring in this region.
2011 The Seismological Society of Japan