The new discovery of a major coseismic fault of the great 1976 Tangshan earthquake has been reported. Unlike the inferred strike slip fault (apparently about 10km long) passing through the city of Tangshan, whose maximum right-lateral offset is about 1.5m at the surface, the newly discovered fault takes remarkable appearance of a normal fault dipping to northwest with a scarp 3m deep. The newly discovered coseismic fault of Tangshan earthquake is much longer (more than 40km) and can be obviously recognized in aerial photos taken shortly after the main shock. Epicenters of aftershocks are located mostly on northwest to the fault. Whereas the inferred strike slip fault has neither been found in the aerial photos nor in seismic reflection profiles, the normal fault is already proved with fine geophysical explorations carried out by Jidong Oilfield Company, PetroChina.
The fault is in listric shape but can not be explained with the prevailing model of listric fault. Another model, the lateral tensile fracturing model, to explain how a listric fault occurs is put forward here.
Background tectonics of Tangshan and adjacent areas is considered undergoing vertical differential movement, relatively uplift on the northwest and subsidence on the southeast. The seismic region is located on an old Kaiping syncline striking northeast with northwest wing nearly vertical and southeast wing rather flat. Once a dip slip blind fault occurs at depth in such tectonic setting, an extreme stress concentration will occur at the tip and new fracture branch, which is termed lateral tensile fracture in this case, will propagate along a path normal to the greatest tensile stress component.
Based on fracture mechanics, laboratory experiments as well as numerical simulations, a double-couple of forces without moment is demonstrated to be applicable to simulate the source mechanism related to the lateral tensile fracture. The model is against the assumption of stick-slip on existing fault as the cause of the earthquake but not in conflict with seismological observations. As a matter of fact, global statistics of CMT solutions of great earthquakes raises significant support to the idea that lateral tensile fracturing might account for not only the Tangshan earthquake but also others.
The lateral tensile fracture model does not only account for the generation of normal fault. Reverse fault can take place near the surface as well if horizontal compression normal to the strike of lateral tensile fracture is strong enough.
