CHARACTERISTICS OF SEISMICITY OF THE XIAOWAN RESERVOIR IN AN AREA OF ACTIVE TECTONICS FROM DOUBLE-DIFFERENCE RELOCATION ANALYSIS

Abstract

We simultaneously relocate the M ≥ 1.0 earthquakes occurred from July 2005 to December 2014 in Xiaowan reservoir area, which is located in an active tectonic region in the middle and lower section of Lancang Jiang in western Yunnan province of China, using cross-correlation (CC) and double-difference (DD) method. The relocated epicenters are distinctly clustered in 8 subareas (subarea A to H), and the earthquakes located inside the Xiaowan reservoir area are generally shallower than that outside of the reservoir area. The relocated focal depths, the relation between the seismicity and water level, and the calculated b values in different subareas indicate that the characteristics of seismicity within Xiaowan reservoir area are quite complicated. The earthquakes occurred outside of the reservoir areas are likely to be tectonic earthquakes irrelevant to water filling of Xiaowan reservoir, and there are both reservoir-triggered seismicity (RTS) and tectonic earthquake activities in Xiaowan reservoir area. The reservoir-triggered earthquakes (RTE) are mainly located along the Heihui Jiang (subarea A) and Lancang Jiang (subarea B and C), and could be classified into rapid response RTS. In subarea A, a significant “migration” of hypocenters is observed in the first filling cycle along the Heihui valley and Qiaoxi-Weihou fault zone, which probably indicates the water permeation process in this area. According to the distribution of active faults, rock properties and relocated focal depths in the reservoir area, we suggest that the karst action and fluid permeation caused by water filling, and fault movements are the key factors of RTS in Xiaowan reservoir. Further studies are needed to better understand the physical process of RTS and tectonic seismicity in some subareas in the reservoir.