MODELING GRAVITY VARIATIONS CAUSED BY DILATANCIES

One postulated earthquake mechanism is the dilatancy model. The dilata tions due to deformation, microfracture (fracture), and stress relaxat ion, either accompanied by fluid invasion or not, change the density, the geometrical configuration in space, and the porosity and the conte nts of the stressed volume of a tectonically active region. Observed g ravity therefore is expected to respond to the sequence of events in t he deformation cycle of dilatation. In this paper, a formulation of th e gravity variations associated with the dilatation processes based on a combined dilatancy model is given. The combined dilatancy model con sists of the dilatancy-diffusion, the dilatancy-instability, and the f ault-zone dilatancy models. For numerical calculations, the total tect onically active region involved is represented by a vertical finite, r ight circular cylinder, within which the fault-zone volume of the pote ntial hypocentral region represented by a disk is embedded. The cylind er 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. T he 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 ea rthquakes of magnitude 4-5 that frequently occurred through the last d ecade or so in the Beijing-Tianjin-Tangshan-Zhangjiakou (BTTZ) region in China. The results of the calculations show that the variation form s of gravity changes are a function of space and time, depending upon the size and shape of both the total tectonically active region and th e fault-zone volume. Therefore, the calculation of theoretical gravity variations for a specific region comparable to that of the BTTZ regio n, may be made on the basis of a thorough prior knowledge of the geolo gical setting and the tectonic stress distribution of that region.