Determination of earthquake epicentroids by inversion of gravity variationdata in the BTTZ region, China

During the last decade and a half, precise measurements of gravity variatio ns 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). Th e model consists of the dilatancy-diffusion, the dilatancy-instability, and the fault-zone dilatancy models. Gravity variations in space and time asso ciated 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 'hypocent roid', the projection of which on the earth's surface is defined as 'epicen troid'. This paper attempts to confirm the above stated concept of 'epicent roid' associated with the fourteen earthquakes of magnitude mainly 4-5 whic h occurred in 1981-95 in the BTTZ region by means of a proposed least-squar es inversion method, using observed gravity variation data. On the basis of the 1976 Tangshan earthquake, the 1989 Datong earthquake, and these fourte en earthquakes, an empirical relationship between the maximum variations of gravity and magnitude was first established. Iterative inversions of obser ved gravity variation data were then carried out. Inversion results show th at (1) the separation of the epicentroids and epicenters range from 0 to ab out 40 km, (2) the epicenters of these fourteen earthquakes are generally l ocated at the terminations of the faults or at the intersections of the fau lts, whereas the epicentroids are inevitably located within intact tectonic blocks, and (3) the epicentroid of an earthquake at least in the BTTZ regi on as attested appears to be determinable. (C) 1999 Elsevier Science B.V. A ll rights reserved.