Re-examination of the fault model for the Niigata earthquake of 1964.

The source mechanism of the Niigata earthquake of 1964 (Ms≈7.5) is re-studied in detail on the basis of the P wave first motion, S wave polarization angle, long-period surface wave, aftershock, precise levelling, tide gage, and tilt measurement data. The long-period surface wave data and geodetic data are interpreted consistently in terms of a thrust fault reaching the earth's surface and having a dip of 56° toward N81°W, a dimension of 80km (length)×30km (width), and an average dislocation of 3.3m. The seismic moment is 3.2×1027 dyne·cm. The stress drop is estimated to be 70bars. This value is not very different from the stress drops obtained for moderate to large shallow earthquakes which occurred in the Japanese islands. The fault plane geometry obtained here is slightly different from that determined from the P wave first motions. Combining this result with the weak beginning of the initial P waves, we may interpret the entire faulting process in terms of a multiple faulting which consists of the two events: the initial localized rupture is followed, after about 4sec, by the major faulting, which is responsible for the excitation of long-period waves.