Study On Characteristics Of Seismic Activity And Source Parameters In Nuozhadu Reservoir Region

In recent years, reservoir’s influence on the seismic activity is growing concern with large reservoirs built all over the world and if some of the occurrence of earthquake is related to the reservoir nearby is becoming a hot issue to discuss. Nuozhadu Reservoir, located in Yunnan Province and beginning to fill in November 2011, is one of the major projects to promote the local economic development and to reduce the pressure of the ecological environment in eastern region while it is in the area of high seismic in China. We study the earthquakes of Nuozhadu Reservoir not only for the importance of scientific research,but also for the economic and earthquake relief.We use earthquakes data recorded by Nuozhadu reservoir network, Xiaowan reservoir network and Jinghong reservoir network from January 1st, 2011 to March 31 st, 2014 in the area of 98.5°~102.0°E, 21.0°~24.5°N to study the seismic activity features and the characteristics of source parameters in Nuozhadu Reservoir area. We use double difference location method to relocate 15110 earthquakes(ML≥0) at first, and then calculate source parameters of 114 earthquakes including seismic moment, corner frequency, stress drop and apparent stress. The calculation process is as follows:(1) we correct the observed spectrum to obtain the seismic source spectrum considering the influence of the geometry effect, inelastic attenuation, site effects and instrument response on the earthquakes waveform;(2) the source-spectrum flat level and corner frequency are calculated by genetic algorithm according to the Brune model;(3) we calculate earthquake radiated energy by integrating the velocity spectrum and apply a high frequency compensation to it;(4) seismic moment, corner frequency, stress drop and apparent stress can be obtained using the equations and the above results. To ensure the reliability of the results, we select the earthquake records with the local magnitude at least 1.0 and the signal-to-noise greater than 2, and each event must be recorded at least 3 stations.This paper analyzes the variation of earthquake frequency, local magnitude and spatial distribution inside and outside the reservoir area within 10 km range from the edge of the reservoir before and after storing water, as well as the distribution of apparent stress and the relationships between the source parameters. The results are shown as follows:1. After impounding, the seismic activity dramatically increased inside the Nuozhadu Reservoir area. The maximum local maginitude rose from ML 2.2 before storing water toML 4.5 after filling, and the earthquakes of ML≥1.0 increased obviously. There is a time delay of 1 to 3 moths between water level change and seismic activity increase.2. In the reservoir area, the seismic activity was low before storing water, while the earthquakes increased and located concentrated in the vicinity of the fault zone after impoundment, with the hypocentral depths migrated to shallow. Seismic activity within 0 ~ 4km depth increased significantly.3. The area within 10 km range from the edge of the reservoir and within 10 km depth is most affected by Nuozhadu Reservoir, while the other area is less affected.4. The seismic activity characteristic in different reservoir segments is different. The specific reason is complex. It is generally related to the wate lever changes, and may be affected by geological conditions, hydrological conditions or water permeability of rock. We must discuss separately.5. No matter in or out of the Nuozhadu Reservoir area, corner frequency decreases with seismic moment and local magnitude, while stress drop and apparent stress increase. The relationships of the source parameters inside the reservoir area after impoundment are different from that before storing water and outside the reservoir area after storing: with the same seismic moment or local magnitude the value of corner frequency, stress drop and apparent stress are lower, and the relationships is more linear, and the ratio of stress drop and apparent stress is also slightly lower.6. Apparent stress in deeper depth is larger in the same period. In the same depth, the longer impoundment the less apparent stress. The difference between the apparent stress inside and outside Nuozhadu Reservoir is largest in 4km depth, and gradually decreases as the depth increase. Below the depth of 10 km the difference disappears.In contrast, seismic activity and characteristics of source parameters outside the reservoir area changed less after impoundment, so the influence of the reservoir is little.