Characteristics of foreshock and its identification

In the paper, we analyze 117 moderate-strong earthquakes occurred in Chinese mainland (MS≥5.5 in the east and MS≥6.0 in the west) since 1970, among them, 11 earthquakes (about 9%) have direct foreshocks and 63 earthquakes (about 51%) have generalized foreshocks. The predominant time interval between foreshock and main earthquake is no more than 30 days with a spatial distance less than 50 km and a magnitude difference over 1. From the digital seismic data in liaoning Province, we know that direct foreshock had an obvious shear-stress background before the MS=5.6 and MS=5.1 Xiuyan earthquakes occurred on Nov. 29, 1999 and Jan.15, 2000.