A STUDY OF THE FOCAL PROCESS OF THE HEZE EARTH-QUAKE(MS=5.9), SHANDONG PROVINCE OF NOVEMBER 7, 1983

Based on the data of the first P-motion recorded at 178 seismographic stations located in China and other parts of the world,the fault plane solution of the Heze earthquake of November 7,1983,Shandong province was determined. Using the directivity function and its generalized form of surface waves,the modal plane striking NE was preferred as the fault plane of this earthquake. The direction of propagation of the fracture was northeast with a velocity of 0.6 km/s and the total length of the fracture 3 km. The estimation of the seismic moment of this earthquake by spectral density analysis gave a value of 1.71020N cm.The results of levelling survey from the geodetic line near the northeast and of the main shock area (Ms = 5.9) showed that a precursory change occurred about a half year before the main shock. This precursory change was a conspicuous uplift in an area over 20 km,the average uplift is about 2 mm,the maximum value reached 2.8 mm. This area was experiencing -strain softening about a half year before the main shock. During this time,the seismicity was distinctly lower in this anomalous area where small shocks had usually occurred since 1937. It is called the Heze swarm. We interpret these observation results as indicating that strain softening by fault creep in this anomalous area,transferring stress into the Ms=5.9 source area.