Crustal S-Wave Attenuation Tomography In Zipingpu Reservior Area

Amplitude attenuation of seismic wave propagated in the crust are closely related the broken level, activity intensity of structured fault zone, as well as other fluids activities,such as magma, water, oil and so on. Experimental studies have shown that the presence of fluid will affect the characteristics of seismic parameters in the rock. Chen Yong et al (2009) found that seismic wave attenuation depends on the microscopic nature of rock, such as density, distribution, structure of rock crack and pore fluid interaction, etc. For shear wave,Wang Daxing et al (2006) found that energy dissipation caused by the viscosity movement of pore fluid in rocks is the main factor causing energy attenuation. Liu Jianhua et al (2004) on the physical mechanism of seismic wave attenuation studies suggested that seismic wave attenuation in crust media was mainly due to a large number of cracks in the crust and full of water or part of the crack in the water. When seismic wave propagated in fluid motion, it will lead to attenuation of seismic waves.In the specific geological environment, such as reservoir storage, because water reservoir increase the load range of the crust and reservoir water filter along the fracture zones, joints and fissures, increasing pore water pressure underground rock, changing the original crustal structure of the medium composition and physical properties, fault plane may reduce the friction coefficient and the shear strength of rock and natural tectonic earthquake may be the energy savings for early release. The cracks, fluid content etc in the rock are closely related to the attenuation of seismic waves, therefore, by studying the Q values And dynamic variation reflecting seismic wave attenuation, we can further understand and explain the earth's interior structure, characteristics and changes of the medium, can further explore reservoir water penetration and its effect on the impact of the shallow crust in the specific geological environment (water storage areas) to help understand the construction and deep dynamic activity characteristics of water storage areas.In this study, first, static and dynamic crustal Qs value distribution (1～15km) of Zipingpu Reservoir area are imaged through attenuation tomography by the s-waveform data relocated by double-difference location method from August 16, 2004 to May 11, 2008. Second, Zipingpu Reservoir area are subdivided into four regions of different sizes, and each partition is calculated monthly moving average Qs value. We also did a survey of earthquake frequency of different magnitude of monthly slide and respectively analyzed the correlation of Qs value, frequency of earthquakes with storage time and water level to discuss the relationship between the s-wave attenuation, seismic activity with the water level. Finally, with experimental results of rock lithology, faults, and hydro-geological surveys, we discussed the permeation conditions and impact on seismic wave attenuation in Zipingpu Reservoir area. Meanwhile, the physical variety of medium are discussed about the cause of earthquake occurrence.The results show that:(1) Crustal s-wave attenuation inhomogeneity changes significantly in Zipingpu Reservoir area, namely, there are low Qs (high attenuation) annular regions around the reservoir, including five low-Qs reservoir areas: northeast, southeast, southwest, northwest and the middle regions. Among them, the reservoir northeast, the middle of the reservoir area, the southwest regions have more favorable conditions for reservoir water infiltration. Qs value is closely related with water infiltration in these regions. Our preliminary view is that the reservoir water may infiltrate along the field of the broken rock band, joints, fissures underground areas on both ends and in the middle of Tongjichang faults, leading to penetrate the rock pores filled with fluid. Then internal friction increases, but seismic wave greatly attenuates and Qs value drops significantly.(2) Study on relationship between monthly sliding average Qs and the water level, it shows that Qs value decreases as the water level increases in the Zipingpu Reservoir area, particularly Qs value decreases sharply in the middle region. Possible reason is that the reservoir water level gradually increases with increasing over storage time. When the reservoir water gradually broken down along the rock band, joints, fissures underground areas to penetrate the rock pores filled with fluid, internal friction increases, and the seismic wave attenuates significantly, resulting in a substantial decline in the value of Qs. It is the most advantageous place of reservoir water infiltration in the middle reservoir area passing through by Tongjichang secondary faults, and seismic wave attenuation reflects fastly and most obvious.Study on relationship between monthly sliding earthquake frequency of different magnitude and water level, it shows that the storage of Zipingpu Reservoir increased seismic activity in the area. Small earthquakes with ML0.0~3.0 were increasingly active after the reservoir was filled 3 months. When the reservoir was filled 14 months later and the water level reached 840 meters or so, small earthquakes with ML0.0~1.0 was the most active and then started to diminish. Small earthquakes with ML1.0~3.0 activity increased gradually as the water storage. This may be due to reservoir water infiltration and increasing water pressure in the rock gap. When full of water or part of water in the rock cracks, as the effect of humidification and lubrication of water on the rocks crack boundary, reducing the shear strength of the seismogenic fault, faults slide easily to induce earthquakes.According to results of previous studies and research on seismic activity with migration features of storage water in this paper,we suggests that reservoir earthquake swarms have been induced in southwest region and parts of the reservoir earthquakes have been induced in the northeast and southeast regions. Group of three small earthquakes in northeast, southeast and southwest regions are basically converged in the transition regions between high Qs value(low attenuation) and low Qs value (high attenuation). This is perhaps as a result of high attenuation of seismic wave. It is not easy to accumulate energy. In the transition area of high and low Qs value, the media are in the state of "soft" and "hard", so it is possible to accumulate strain energy and triggers earthquakes.