ON THE CAUSE OF GROUND STRESS TENSILE PULSES OBSERVED BEFORE THE 1976TANGSHAN EARTHQUAKE

It has been asserted that earthquakes cannot be predicted (Geller et a l., 1997). This may be true to some extent for the present, but it sho uld not stop further research. The discussion of stress changes in thi s article gives an example to show that what people observe near the s urface may not be the situation at depth. Nonetheless, such surficial observations reflect the movements of the crust associated with earthq uakes. Before the great M 7.8 Tangshan, China, earthquake in 1976, the Douhe and Zhaogezhuang stress-monitoring stations on the supposed fau lt zone observed significant tensile pulses of ground stress normal to the fault zone. The data are both reliable and reasonable. Analysis o f the stress field adjacent to a fracture front shows that, when the f racture front passes by the measuring point, the stress normal to the fracture will first rise and then drop, while the stress at an angle o f 30 degrees to the fracture has much less significant variation in th e observations. The pulses are attributed to the near-surface tensile fractures generated along the fault zone.