Source Processes of the Haicheng , China Earthquake from Observations of P and S Waves By

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 x 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. INTRODUCTION The Haicheng, Liaoning Province, China earthquake of February 4, 1975, was the first major earthquake to have been successfully predicted (Raleigh et al., 1977). Using a wide range of observations, principally tilt measurements and seismicity, Chinese seismologists continually improved their estimate of the time and location of the impending event. By 1970, long-term trends in seismicity suggested to Chinese seismologists that Liaoning Province, in northeastern China, would be the site of the next large event in a series of earthquakes which began in 1966. During this series, epicenters of large earthquakes {1966 Hsingtai earthquakes, 1967 Tientsin, and 1969 Pohai Gulf) migrated northeastward toward Liaoning Province. This long-term prediction led to increased geophysical studies in Liaoning Province such as new seismic stations and tilt observations. In late 1973, the number of small earthquakes showed a significant increase in frequency while previously slow ground tilting accelerated. These geophysical observations spurred provincial authorities to make preparations for a major disaster and to step up their program of alerting citizens. Foreshock activity, beginning on February 1 and continuing up until 6 hr before the main shock, appears to have been the deciding factor in predicting the earthquake and warning the populace. From the amount of damage to homes and other buildings the prediction of this earthquake undoubtedly saved many thousands of lives. Scholz (1977) has suggested that the series of large earthquakes including the Haicheng earthquake and its precursory phenomenon were triggered by a deformation front propagating through northeastern China. The main shock occurred at l l h 36m UTC on February 4, 1975 with a surface-wave magnitude of 7.4 (USGS)o Basic parameters of the main shock are listed in Table 1. This paper is concerned with the interpretation of the source processes of the Haicheng earthquake by analysis of teleseismic body waves. In particular, the agreement between P-wave and S-wave observations will be investigated. The 1903