ON THE ANOMALOUS CRUSTAL BULGE AND ASEISMIC CREEP PRIOR TO THE 1976 TANGSHAN EARTHQUAKE

Based on the observed data of 11 repeated surveys in the Tangshan area prior to the 1976 earthquake, an analysis has been made for the temporal and spatial variations of erustal deformation within this region. The result indicates that before the earth-quake, from 1968 to 1969 a surface bulge of amplitude 50mm, lasting for two years, took place in the epicentral region. Near Tangshan and Baodi, on the Tangshan fault and the JiYunhe fault, superposition of accumulated strains and aseismic creeping are shown.By adopting the theories of uniform strain accumulation and ,elastic dislocation, theoretical values of vertical displacements caused by creeping at the various parts on the faults respectively, for the observation points near Tangshan and Baodi have been calculated and by the principle of least residual root mean squares to determine the optimal values of the parameters of strain accumulating and creeping fault.From 1969 to 1975, the creeping fault underneath Tangshan strikes 47 NE, dipping 87toward SE, right-lateral normal fault in character. Its length is 8 km, width 6 km, depth of burial 2 km, strike and dip slips amount, to 104 cm and 8 cm respectively, average rate of strain accumulation; 0.910-7/year. At the same time, the creep rate of the fault in the strike and dip directions are found to be 18.6 cm/year and 1.4 cm/year respectively. Besides, from the result on the JiYunhe fault, the creeping fault near Baodi is smaller in dimension than near Tangshan, but its creeping rate is greater than near Tangshan.In this paper, it is also discussed in connection with the relation of the above-mentioned anomalous erustal bulge with the occurrence of the Tangshan earthquake. Anomalous erustal deformation is considered as the early indication of the development of the earthquake, while aseismie creeping probably may be another precursory feature of a shallow earthquake. The occurrence of the Tangshan earthquake had come through a long process of development of strain accumulation-volumetric dilatancy-inverse erustal deformation (or decrease of creeping rate).