Seismogenesis and occurrence of earthquakes as observed by temporally continuous 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) 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 earth's surface, the predominant 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 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 local stress concentration, and/or pore over-pressure.