abstract

The static and dynamic parameters of the Ms = 6.8 Izu-Oshima earthquake were determined from far-field SH wave forms, near-field displacement-type strong-motion seismograms, surface-wave spectral amplitudes, and static displacements in the vicinity of the fault. The main shock involved right-lateral strike-slip motion together with a small normal dip-slip component on an EW trending, steeply northward dipping fault plane which lies under the sea between the Izu Peninsula and Izu-Oshima. The mean seismic moment estimated from surface-wave and SH-wave amplitudes is close to the value estimated independently from geodetic data, implying that the rupture process inferred from seismic waves represents the main faulting process. The total length of this fault and a buried subsidiary (and possibly aseismic) inland fault is less than half the overall extent of the aftershock zone which extends from Izu-Oshima right across the Izu Peninsula. Anomalous uplift near Ito on the northeast corner of the Izu Peninsula which occurred in 1975 appears to be mechanically related to the Izu-Oshima earthquake, because the M = 7.0 North Izu earthquake of 1930, which involved left-lateral slip on the N-S striking Tanna fault, was also preceded by anomalous uplift near Ito. This conjugate fault pair reflects NW-SE compressional stress and may be regarded as accommodating NW-SE shortening in the vicinity of the collison zone between the Philippine Sea plate and the Eurasian plate. Parameter values are: origin time = 03:24:44.8 (GMT), latitude = 34°46′08″N, longitude = 139°12′24″E, depth = 4 km, moment = 1.1 × 1026 dyne-cm, strike = N270°E, dip angle = 85°N, slip angle = 188°, fault length = 17 km, fault width = 10 km (surface faulting), dislocation = 183 cm (right-lateral strike-slip), 26 cm (normal dip-slip), rupture velocity = 2.8 km/sec westward, rise time = 2 sec.

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