Abstract

We investigated foreshock and aftershock sequences of a M 5.0 earthquake that occurred on 15 September 1998 near the Nagamachi-Rifu fault, an active fault crossing the urban area of Sendai city, northeastern Japan. The focal mechanism of this event was a reverse-fault type with a nearly horizontal P axis parallel to the plate conversion direction. Aftershock distribution located by the homogeneousstation method delineates the fault plane of this event, dipping to the northwest at an angle of ∼30° in the depth range of 10–12 km. The location of the fault plane and the slip direction thus estimated suggest that this event was a slip at a deep portion of the Nagamachi-Rifu fault. Three days prior to the mainshock, many foreshocks with similar waveforms started to occur very close to the mainshock hypocenter. We estimated relative locations of these similar foreshocks based on crossspectrum analyses of waveforms recorded at nearby stations. They are located at the deepest portion of the mainshock fault plane and just above the mainshock hypocenter. Migration of the foreshocks is observed from deeper to shallower depths. Cumulative Coulomb stress changes caused by preceding events show negative values at hypocenters of similar waveform foreshocks. Our results do not support the cascade model for the process of foreshock generation. Nucleation phase prior to the high-speed rupture of the mainshock is observed in seismograms at nearby stations. The initial point of the nucleation phase is located on a deeper extension of the foreshock area. The high-speed breakout point is located in the area where a large amount of seismic moment release was estimated from waveform inversions. These observations probably reflect an underlying aseismic nucleation process preceding the main dynamic rupture of the M 5.0 event.

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