Seismogenesis and occurrence of earthquakes as observed by temporally continuous gravity variations in China

Abstract

In the last decade and a half, a number of earthquakes of magnitude 4-5 have occurred in the Beijing_Tianjin_Tangshan_Zhangjiakou (BTTZ) region. 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 to the seismogenesis and occurrence of earthquakes has emerged.\;As gravity variation is generally observed on the earths surface, the predominant influence is that of the near_surface groundwater. The subsurface fluids are distributed throughout all depths in the crust and respond to the seismogenic processes of earthquakes as well. The influence of the subsurface fluid on gravity variation is, therefore, of equal importance. 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 seismogenesis and occurrence of earthquakes than previously realized.\;The abundance of accumulated data shows that the dilatancy instability (IPE) model seems not applicable at least to the seismogenesis and occurrence of earthquakes in the BTTZ region. In order to reflect the physical reality, the earlier proposed combined dilatancy model requires modification. The seismogenic area in the BTTZ region may be modelled as a large pre_stressed volume of a fluid_filled poroelastic medium, including not only the pre_stressed volume surrounding the impending rupture zone but also the volume containing the rupture of the fault zone itself. The pre_stressed volume outside the impending rupture zone is under a state of relatively small change of the pre_existing regional tectonic stress, while the volume containing the impending rupture zone is an induced region of very high local stress concentration, and/or pore over_pressure.\;The calculated gravity variations based on the modified combined dilatancy model (MCDM) with the known physical parameters of the region resemble the observed residual gravity variations. Apparently the residual gravity variations, in addition to responding to the deep_seated seismogenesis and occurrence of earthquakes, predominantly respond to the near_surface groundwater, and the subsurface fluids, which themselves also respond to the seismogenesis and occurrence of earthquakes. On the basis of comparison between the calculated MCDM gravity 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 monitoring 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 processes about 10 months to 1 year prior to the occurrence of an earthquake of magnitude 4-5 and tens of years prior to that of an earthquake of magnitude 7-8 such as the Haicheng earthquake in 1975 and the Tangshan earthquake in 1976 in the BTTZ region. These earthquakes of magnitude 4-5, which so far have occurred in the BTTZ region, may well be the precursory events to a larger earthquake.