A DISLOCATION MODEL OF THE TANGSHAN(唐山) EARTHQUAKE OF 1976 FROM THE INVERSION OF GEODETIC DATA
CHEN YUN-TAI HUANG LI-RENLIN BANG-HUI LIU MIAO-LONGWANG XIN-HUA(Institute of Geophysics, (The Geodetic Survey Brigade for Earthquake State Seismological Bureau) Research, State Seismological Bureau)
The fundamental principles and method of deducing a source model of the earthquake from "zero frequency" data are developed, and a dislocation model of the Tangshan earthquake (M8 = 7.8) of 1976 is deduced from the inversion of geodetic data. The results obtained indicate that the source of the Tangshan earthquake is a right-lateral normal fault striking generally N 49°E, and diping 76°SW. The fault is of length, 84 km, width 34 km, average strike-slip dislocation 459 cm, average dip-slip dislocation 50 cm, seismic moment 4.3 X 1027 dyne-cm, stress-drop 29 bar, strain-drop 4.3 X 10-5, and strain-energy release 3.7 X 1023 ergs. The fact that the average disloca-tion deduced from the geodetic data is much larger than that from seismic data, implies that before the earthquake, an aseismatic slip i.e. a precursory fault creep, had occurred within the crust beneath the epicentral area. The moment of the aseismic slip is estimated to be 2.5 X 1027 dyne-cm. It is concluded that although before Tangshan earthquake, no foreshock was observed, a large-scale pre-creep had occurred instead of the ordinary foreshock. This may be an essential characteristic of the Tangshan earthquake, and many peculiar precursory phenomena before the shock may be related to this pre-creep.