Longtan Reservoir Induced Seismicity Characteristics And Mechanism

Studying on the characteristic and mechanism of reservoir-induced earthquakes is the important foundation of conducting risk evaluation. Before the impoundment of the Longtan reservoir, Longtan Digital Seismic Network has been operated. After the impounding, a great deal of earthquakes occurred around the reservoir. So Longtan reservoir is chosen as the objective region to study in the thesis. Earthquakes occurred around Longtan reservoir from October 2006 to the end of 2007 recorded by Longtan Digital Seismic Network were relocated firstly by using the hypoDD relocation method and waveform cross-correlation technique. Then, the mechanisms of some earthquakes were solved and the stress field was inversed. Based on the analysis of the earthquake distribution occurred around Longtan reservoir, along with the impounding process, water level, geological structure, lithological characteristics et al., some significant conclusions have been drawn.Compared with the location results by using DD method, especially with the catalogue from the Longtan Digital Seismic Network, the relocation accuracy was obviously enhanced by using the DD method combining with the WCC technique. Having analyzed the seismic activities in reservoir region on this basis, the characteristics of Longtan reservoir are indicated as below:Before the impoundment, seismic activity level around Longtan reservoir was very low, there were few earthquakes occurred. After the two times of annual impounding, seismic activity level around the Longtan reservoir obviously enhanced, displaying the quick response.The focal depths of earthquakes around Longtan reservoir region are relatively shallow, concentrating on the depth ranging approximately from 7km to 11km, and clustered on 5 limited small regions (A,B,C,D,E) near the riverside asymmetrically. Although there are some faults on or around every cluster, earthquake activities seem not be controlled by the faults.After the impoundment, earthquake activities on every cluster region obviously enhanced, but the activity level and response time to the impounding process showed different features. Among them, the cluster B, which is near both the deepest water depth and the dam, was the fastest response region with the highest activity level.Mechanisms of 73 reservoir earthquakes were obtained by using the first P wave polarization data and amplitude ratios of SH wave to P wave, and the stress field was inversed further. The result demonstrates that the main compressive stressσ1 is nearly horizontal, trending between the NWW-SEE and NE-SW direction. Under the collective action of this stress field and water loading, most of the faulting types are thrust mechanism even though they are complicated and showing thrust fault, slip fault as well as the normal fault type. This is consistent with the faulting types of faults whose scales are relatively larger than that of the reservoir.The conclusion that the increase of shearing stress strength resulted from Longtan reservoir's water loading is not the main causation of the induced earthquakes around Longtan reservoir can be drawn based on the following analysis. Most Longtan reservoir earthquakes are thrust faulting, and there exists responding time difference of induced earthquakes on each cluster. With the high permeability of the stratum under the reservoir, cluster B shows the highest earthquake activity level and extremely rapid quickly response to the impounding, Whereas the cluster A shows high earthquake activity level but relatively late response owing to the relatively weak permeability of the stratum under the reservoir in the region where cluster A located. Therefore, the main reason of the distinctive increase of reservoir induced earthquake activity is suggested to be resulted from the increase of the pore pressure under the effect of both water loading and permeating, as well as the decrease of the friction strength due to the fluid lubricating and weak action on the fault plane.