Abstract

The rupture process of the 2008 northern Iwate, Japan, earthquake (M 6.8) has been derived from strong-motion records obtained at 22 regional stations by the multi-time-window linear waveform inversion method. This event occurred in the lower plane of the double seismic zone in the subducting Pacific slab at a depth of 115 km. A preliminary inversion analysis revealed that a single planar fault plane model was insufficient to satisfactorily explain waveforms at stations in a specific azimuth. A two-segment fault plane model was then assumed in which the fault model consisted of the northern and southern segments with a 44° difference in strike direction, which respectively followed the source mechanisms determined by moment tensor inversion and P-wave polarity analysis. The source model derived from the two-segment fault plane model greatly improved the waveform fitting at stations for which there was a large discrepancy in the preliminary analysis with the single planar fault plane model. The largest asperity was located on the northern segment, which generated the main phase observed at most stations. The amount of slip on the southern segment was smaller but contributed significantly to the first of the two pulses observed at the stations to the south of the source area. The stress drop of the largest asperity for this intraslab earthquake was large in comparison with that predicted from empirical relations obtained for crustal earthquakes, which may be one of the causes of the relatively large accelerations recorded during this earthquake.

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