关键词: 地电阻率, 自然电位, 地震前兆, 高阻层

Abstract: Resistivity measurement with an active source has been practised for thirty years at an enormous number of station in China. Resistivity decreases of several percent, which began 2～3 years prior to the Tangshan earthquake and other events and gradual increases afterward were observed. These anomalous variations were larger than their background fluctuations and hence statistically significant. Resistivity decreases were generally seen first and larger at stations close to epicenters and later and smaller at more distant stations. This implies that in the preseismic intermediate term stage, the fault was closed and the accumulated strain started first in the epicentral area and then migrated outwards from the epicentre. Anomalies in self-potential of the specific pattern of rapid onset and slow decay recorded at the station XZ are almost simultaneous with oil blowouts from the borehole W11. These short term or impending earthquake precursors migrated towards the epicentre, i.e., strain energy releasing occurred first at stations more distant to the epicentre, later at closer stations and at last the main shock took place. For short periods the industrial interference magnetotellurics (IIMT) determined that the Tangshan earthquake took place in the resistive crust. TM mode (H polarization) data at long periods of the MT measurements determined that electrical properties of resistive zone and the existence of a deep fault zone (fluid leakage paths) was found in resistive crust of the Tangshan earthquake area. Electrical measurements are the geophysical measurements most closely related to fluid volumes, connectivities and pressures, and applicable to earthquake prediction.

Key words: Georesistivity, Self-potential, Earthquake precursors, Resistive zone