Source processes of the Haicheng, China earthquake from observations of P and S waves

The Haicheng, China earthquake of February 4, 1975, was the first major seismic event to be predicted. In this paper, long-period teleseismic P waves and S waves from this event are compared directly to time-domain synthetic seismograms to infer the source parameters. Results indicate the focal mechanism of the earthquake is nearly left-lateral strike slip along a northwest striking nodal plane (strike = 288°, dip = 78° N, rake = 342°). The strike of this nodal plane agrees with the trend of the aftershock distribution. Seismic moment is 3 × 10²⁶ dyne-cm and source duration is 7 sec. Azimuthal variation of P-wave duration is attributed to fault propagation in a northwesterly direction along the strike of the aftershock zone. A model with a fault length of 22 km and rupture velocity of 3.2 km/sec can explain the observed P waves quite well. There is considerable discrepancy between observed SH waves and synthetics computed using this model. These discrepancies are due to source structure complexities and/or changes of fault mechanism as the rupture propagated along strike. The average dislocation is computed to be 2.8 m and the stress drop is 53 bars.