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Study on Relocation and Activity Characteristic of Small Earthquakes in Pubugou Reservoir Area
详细信息   
摘要
The small earthquakes in and around Pubugou reservoir area are relocated by the double-difference method,and the earthquake properties of two cluster regions near water area are also analyzed. The 1834 earthquakes in the study area from 14 October 2006 to 31 December 2011are relocated, and we have gotten 1708 earthquake relocation results eventually,amounting to 93.1% of the total. After the relocation,the RMS average value of arrival time residual decreases from 0.93s to 0.21s,the average error is 0.5km in EW direction,0.6km in NS direction,and 2.9km in vertical direction.The epicenter distribution is more concentrated,and the dominant distribution is also more obvious,the cluster region of small earthquakes distributes in the middle part of reservoir area,near to and downstream of the dam.The average focal depth is 9.2km. Earthquakes with focal depths less than 10km amount to 59% of relocation results,and those within 15km amount to 72%,the deepest focal depth is 33.1km. According to spatial distribution of relocation results,we divide the study area into four main regions(A,B,C and D)to discuss respectively.The evolutions of earthquake activities with time show that the variation of earthquake activities in region C and D is relativity larger than that in region A and B; the earthquakes of region C and D are highly centralized,and the focal depth is obviously shallow. With respect to the origin time of earthquakes,the earthquakes in region A and B occurred randomly in 24 hours of a day,which is accord with the time characteristic of natural earthquakes,while the occurrence time of earthquakes in region C and D had obvious regularity,most of them occurred in daytime,and focused on pre-and post 12’clock at noon and 18’clock in the afternoon.The results of field survey show that the seismic cluster area in region C is in accord with the construction sites of Ya′an-Xichang expressway and new Hanyuan county,and the cluster in region D is in accord with the quarry of Kaergou and Jiali’ega which are all near the reservoir dam. By synthesis analysis,we think that small earthquakes of region C and D may not be tectonic earthquakes; moreover,most of them occurred before November 2009,a time before the impoundment of Pubugou reservoir. So the earthquakes are not related with the impoundment either,but possibly the explosion ones caused by all kinds of construction.

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