SEISMOGENESIS AND OCCURRENCE OF EARTHQUAKES AS OBSERVED BY TEMPORALLYCONTINUOUS GRAVITY VARIATIONS IN CHINA

In the last decade and a half, a number of earthquakes of magnitude 4- 5 have occurred in the Beijing-Tianjin-Tangshan-Zhangjiakou (BTTZ) reg ion. On the basis of the analysis of the temporally continuous gravity variation data principally from the Baijiatuan (BJTN) semi-permanent gravity base station, a general picture of gravity variation related t o the seismogenesis and occurrence of earthquakes has emerged. As grav ity variation is generally observed on the earth's surface, the predom inant influence is that of the near-surface groundwater. The substance fluids are distributed throughout all depths in the crust and respond to the seismogenic process of earthquakes as well. The influence of t he subsurface fluid on gravity variation is, therefore, of equal impor tance. The fluids, which include the near-surface groundwater and the subsurface fluids distributed throughout all depths in the crust, play a more important role in the gravity variations in terms of the seism ogenesis and occurrence of earthquakes than previously realized. The a bundance of accumulated data shows that the dilatancy instability (IPE ) model seems not applicable at least to the seismogenesis and occurre nce of earthquakes in the BTTZ region. In order to reflect the physica l reality, the earlier proposed combined dilatancy model requires modi fication. The seismogenic area in the BTTZ region may be modelled as a large pre-stressed volume of a fluid-filled poroelastic medium, inclu ding not only the pre-stressed volume surrounding the impending ruptur e zone but also the volume containing the rupture of the fault zone it self. The pre-stressed volume outside the impending rupture zone is un der a state of relatively small change of the pre-existing regional te ctonic stress, while the volume containing the impending rupture zone is an induced region of very local stress concentration, and/or pore o ver-pressure.The calculated gravity variations based on the modified c ombined dilatancy model (MCDM) with the known physical parameters of t he region resemble the observed residual gravity variations. Apparentl y the residual gravity variations, in addition to responding to the de ep-seated seismogenesis and occurrence of earthquakes, predominantly r espond to the near-surface groundwater, and the subsurface fluids, whi ch themselves also respond to the seismogenesis and occurrence of eart hquakes. On the basis of comparison between the calculated MCDM gravit y variations and the observed residual gravity variations, the change of the regional tectonic stress field for the earthquakes of magnitude 4-5 in the BTTZ region could be approximately estimated to be in the neighbourhood of 5% - 7%. It is apparent that simultaneously monitorin g the temporally continuous variations of the near-surface groundwater , subsurface fluids, and gravity coupled with modelling would provide vital information on the history and evolution of the seismogenic proc esses about 10 months to 1 year prior to the occurrence of an earthqua ke of magnitude 4-5 and tens of years prior to that of an earthquake o f magnitude 7-8 such as the Haicheng earthquake in 1975 and the Tangsh an earthquake in 1976 in the BTTZ region. These earthquakes of magnitu de 4-5, which so far have occurred in the BTTZ region, may well be the precursory events to a larger earthquake.