CHARACTERISTICS OF GROUNDWATER ANOMALIES OBSERVED IN ZHOUZHI DIGITAL OBSERVATION WELL BEFORE YUMEN M5.9 EARTHQUAKE IN GANSU

Yumen M5.9 earthquake occurred in Dec.14, 2002. Before the event, the water temperature, water level and outflow of Zhouzhi well, which is 1000km away from the epicenter, all showed clear short-term and imminent anomalies. The variation of these anomalies is introduced here. Compared with the historical analogue data of the station, the digital observation is more propitious to detecting short-term and imminent precursors as well as its whole variation.