Preliminary Study Of Earthquakes In The Zipingpu Reservoir Area, Sichuan, China

Zipingpu Reservoir is the water regulation works of Chengdu city and Dujiangyan irrigated area. We study the earthquakes of Zipingpu Reservoir not only for the importance of scientific research, but also for the economic and earthquake relief.We use double difference location method to relocate 1569 small earthquakes recorded by 8 seismic stations of the Zipingpu reservoir network from August 16, 2004 to May 10, 2008. The results show that after relocation, the earthquake distribution is from spatial discretization and gridding to concentrated distribution, and the small earthquakes relocated are mainly concentrated in central Beichuan-Yingxiu fault, frontal Guanxian-Jiangyou fault and retral Wenchuan-Maowen fault of Longmen Mountains. Most of the small earthquakes are located in the Beichuan-Yingxiu fault and converge to both ends of Zipingpu reservoir. The earthquakes in this region are composed of three earthquake-intensive areas, which are respectively located in the southwest area, the northeast area of Zipingpu Reservoir and Front Range fault zone. Most relocated earthquakes occurred at the depth of 5~15km,and a few at depth of 15~30km, and the mean depth is 7.8km. Seismic frequency of Zipingpu Reservoir region has a correlation with the reservoir water level. The average root-mean-square of travel time residual is 0.048s; the horizontal average error is estimated to be 0.63km, and the vertical average error had been controlled less than 0.51km; This paper also discussed the relation between location image and the geological structure.At the same time, we find that the earthquake distribution in the focal profile of relocated earthquakes has similar geological characteristics with the central Beichuan-Yingxiu fault and the Front Range fault. The earthquakes in the southwest area of Reservoir are significantly increasing in the first year after storing water, and many earthquakes occurred during the two highest water level periods. It shows that earthquakes in the southwest area are most affected by the Reservoir. The seismic activities in the northeast area of Zipingpu Reservoir were weakened after storing water. There are nearly no earthquakes in the Front Range fault zone until the Dujiangyan earthquake swarm occurred on February in 2008. These results suggest that the southwest area of Zipingpu Reservoir is the most affected area by the water storing, and earthquakes in this area should be reservoir-induced earthquakes. Earthquakes in the Front Range fault zone should be structural earthquakes.According to the multi-station method and using digital seismic records in Zipingpu Reservoir, we studied the average seismic velocity ratio. The results show that the wave velocity ratios in the southwest area of Reservoir are most between 1.6 and 1.8, and its average seismic wave velocity ratio is 1.73. The wave velocity ratio in the southwest area has remained low state from November 2006 until the end of 2007, and then it slowly returns to the mean level in early 2008. Larger earthquakes in the southwest area of Zipingpu Reservoir are corresponding to the unloading process of the reservoir and the falling of the wave velocity ratios. The average wave velocity ratio is 1.77 in the northeast area of Zipingpu Reservoir. One month after Reservoir filling with water, the wave velocity ratios in the northeast area begin to decrease. While calculating Vp and Vs of the southwest area of Reservoir by using of the accurate location results. The average of Vp and Vs in the southwest area of Reservoir are separately 4.61km/s and 2.80km/s, and The average of Vp and Vs in the northeast area of Reservoir are separately 4.95km/s and 3.02km/s...