One postulated earthquake mechanism is the dilatancy model. The dilatations due to deformation, microfracture (fracture), and stress relaxation, either accompanied by fluid invasion or not, change the density, the geometrical configuration in space, and the porosity and the contents of the stressed volume of a tectonically active region. Observed gravity therefore is expected to respond to the sequence of events in the deformation cycle of dilatation. In this paper, a formulation of the gravity variations associated with the dilatation processes based on a combined dilatancy model is given. The combined dilatancy model consists of the dilatancy-diffusion, the dilatancy-instability, and the fault-zone dilatancy models.For numerical calculations, the total tectonically active region involved is represented by a vertical finite, right circular cylinder, within which the fault-zone volume of the potential hypocentral region represented by a disk is embedded. The cylinder has a radius of 40 km and thickness of 15 km with the bottom of the cylinder coinciding with the depth of a tectonically detached zone. The disk is 2 km thick and 11 km in radius with the bottom of the disk at a depth of 15 km. This representation is compatible with that of earthquakes of magnitude 4-5 that frequently occurred through the last decade or so in the Beijing-Tianjin-Tangshan-Zhangjiakou (BTTZ) region in China. The results of the calculations show that the variation forms of gravity changes are a function of space and time, depending upon the size and shape of both the total tectonically active region and the fault-zone volume. Therefore, the calculation of theoretical gravity variations for a specific region comparable to that of the BTTZ region, may be made on the basis of a thorough prior knowledge of the geological setting and the tectonic stress distribution of that region.
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