RUPTURE DISTRIBUTION OF THE 1976 TANGSHAN EARTHQUAKE SEQUENCE INVERTED FROM GEODETIC DATA

Abstract

The Tangshan 1976 M7.8 earthquake was followed by two large aftershocks (the Luanxian M7.1 earthquake and the Ninghe M6.9 earthquake). The earthquake sequence occurred in an area with good geodetic observation, and the horizontal and vertical co-seismic displacements were obtained by triangulation net and leveling net. In this study, by using raw leveling data (not the subsidence image processed from raw leveling data) and triangulation observation data, we obtain the rupture distribution of strike slip and dip slip component on seismogenic faults. The geometry and size of the seismic faults of the Luanxian and Ninghe earthquake are considered in our model construction. The result shows that the seismic fault of the Tangshan mainshock manifested right-lateral strike slip with a maximum value greater than 6 m in the southern segment. The strike slip component on the northern segment is much less than that of the southern segment. The total seismic moment of the Tangshan mainshock is 2.58 × 10²⁰ N·m, which is consistent with that determined by seismic waves. The seismic fault of the Luanxian earthquake shows as a left-lateral normal fault with total moment of 4.95 × 10¹⁹ N·m. The seismic fault of the Ninghe earthquake shows as a left-lateral normal fault with total moment of 3.94 × 10¹⁹ N·m, which is an order of magnitude greater than the moment determined by seismic waveform. It is so inferred that the aseismic slip of the Tangshan earthquake occurred on the western part of the Ninghe earthquake fault, demonstrated nearly normal fault property, which is of great significances for geodynamic process and mechanism of aftershock occurrence after the Tangshan earthquake.