During the last decade and a half, precise measurements of gravity variations have been carried out in the Beijing-Tianjin-Tangshan-Zhangjiakou (BTTZ) region in China. A possible seismogenic mechanism of a combined dilatancy model (CDM) for the BTTZ region has been proposed by Kuo and Sun (1993). The model consists of the dilatancy-diffusion, the dilatancy-instability, and the fault-zone dilatancy models. Gravity variations in space and time associated with the model involve a tectonically stressed volume surrounding a small volume of an impending rupture (fault) zone. They refer to the center of effective mass of the total stress volume which is defined as 'hypocentroid', the projection of which on the earth's surface is defined as 'epicentroid'. This paper attempts to confirm the above stated concept of 'epicentroid' associated with the fourteen earthquakes of magnitude mainly 4-5 which occurred in 1981-95 in the BTTZ region by means of a proposed least-squares inversion method, using observed gravity variation data. On the basis of the 1976 Tangshan earthquake, the 1989 Datong earthquake, and these fourteen earthquakes, an empirical relationship between the maximum variations of gravity and magnitude was first established. Iterative inversions of observed gravity variation data were then carried out. Inversion results show that (1) the separation of the epicentroids and epicenters range from 0 to about 40 km, (2) the epicenters of these fourteen earthquakes are generally located at the terminations of the faults or at the intersections of the faults, whereas the epicentroids are inevitably located within intact tectonic blocks, and (3) the epicentroid of an earthquake at least in the BTTZ region as attested appears to be determinable. (C) 1999 Elsevier Science B.V. All rights reserved.
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