Study On 3D Velocity Structure Tomography Of Reservoir Area Based On Dense Observation

In this paper,the double difference seismic tomography method is applied to the phase arrival times of seismic events recorded by JinshaJiang River seismic monitoring network and the Three Gorges seismic network,to determine the hypocenter parameters of events as well as detailed 3D velocity structure at regions downstream the Jinsha River and in the Three Gorges reservoir area.The data used at regions downstream the Jinsha River(Baihetan-Wudongde section)was recorded by42 stations of The JinshaJiang River network from August 2013 to November 2016.At present,Baihetan and Wudongde Reservoirs aren't in the state of impoundment.The observation data used in the Three Gorges Reservoir area from March 2009 to June 2010 belong to the storage period of the Three Gorges Reservoir,The geologic structure and the water storage conditions of the two reservoirs are different.This paper analyzes and compares the 3D velocity structure and seismic distribution of the Baihetan-Wudongde section and the Three Gorges reservoir area.The inversion results with a resolution of 0.1° are obtained in the Baihetan-Wudongde reservoir area.At 2 ~ 6km,Vp and Vs present low velocity anomalies along the northern segment of Xiaojiang fault,and the Vs anomaly is especially remarkable.On both sides of the Xiaojiang fault,there exist obvious P and S wave low velocity areas.These low velocity areas correspond to the terrain,lithology distribution and the watershed of Jinsha river at shallower layer in study area.Starting from 6km,a northeast high Vp band along Zhaotong-Ludian and Huize-yiliang fault forms on the eastern side of the northern segment of Xiaojiang fault.Vs also shows the high value in the area bounded by Lianfeng fault,Baogunao-Xiaohe fault and Huize-Yiliang fault.This is related to the intrusion of a large number of basic and ultrabasic mantle-derived materials into the crust caused by Emeishan basalt eruption.The relocation result shows that : all the focal depths in study area are shallower than 20 km,and the focal depth values are mainly concentrated within 15 km.The earthquakes are mainly distributed along the fault zone and are concentrated in the northern segment of the Xiaojiang fault,the north-west-west branch of faults on the east and west sides of the Xiaojiang fault,andthe source area of the Ludian earthquake(Zhaotong-Ludian fault zone).These tectonic earthquakes mainly occur in areas where the velocity structure of medium changes.Take the Ludian earthquake swarm as an example,The seismic source region located in the relatively high velocity area with low velocity anomalies in the deeper part,and the different velocity structures between the southwestern and northeastern segment of the Zhaotong-Ludian fault,may provide the seismogenic environment.In the Three Gorges reservoir area,the resolution of the inversion result is 0.05°.The following conclusions are drawn : The heterogeneity of medium is obvious,reflecting the complex media structure in the reservoir area.The Vs distribution is bounded by the Xiangxi River between the Zigui Basin and the Huangling anticline.It shows a high Vs distribution to the east of the Xiangxi River including the reservoir dam.To it's west,there are obvious large-scale low Vs distributions,including the Zigui basin,the Gaoqiao fault,and the Yangtze River.The distributions of low Vp and Vs in the Zigui Basin show that the sedimentary thickness of the basin is between 6and 8 km.The low Vs anomaly in the margins of the basin and Xiangxi River is significantly higher than Vp.It can infer that fluid diffusion and infiltration may occur at the edge of the basin.The seismic events in the reservoir area,which clusted in the Badong area,along the Yangtze River coast west of Xietan,and the Xiannvshan area,form a A-frame pattern,which is consistent with pervious study.The lower boundary of focal depth is deeper from west to east,with a dominant distribution between 0 and10 km.The three near east-west strip-shaped earthquakes in the Badong area are related to the easy fluid penetration of the lithological conditions in the Badong area.The earthquakes at the intersection of the northern part of the Xiannvshan fault,the Jiuyuxi fault,and the Yangtze River include both the reservoir-induced earthquakes and tectonic earthquakes.Analyze and compare the inversion results of the two reservoir areas we can find:(1)The shallow velocity structure in the two reservoir areas is directly related to the lithology,geological structure,and hydrological conditions in the reservoir area.For example,the characteristics of low velocity and high velocity ratio along the main river can be seen in both study areas.The state of impoundment may affect the distribution of this velocity feature in depth,the main rivers in the reservoir areawithout impoundment only affects the medium structure in the shallow 2–4 km,but low velocity and high velocity ratio distribution character is visible in the entire upper crust of the impounded Three Gorges reservoir area.(2)At regions downstream the Jinsha River the tectonic seismicity is mainly concentrated along the fault,and the epicenter is mostly located in the area where the velocity structure of the medium changes.However,the seismic activity in the Three Gorges Reservoir area that is basically consistent with the distribution of seismic tectonic belts in the area.The focal depth is shallow with predominant depth about 5km.The occurrence of shallow earthquakes may be related to fluid infiltration in the reservoir area.Seismic activities that occur in the area where the fault zone meets the river include both reservoir-induced earthquakes and tectonic earthquakes.(3)It is suggested that regions downstream the Jinsha River(after impoundment)in the future needs to pay more attention to the reservoir section where the lithology is bennificial to fluid penetration,shattered fault zones and fractures that act as water conduits,as well as where fractures and rivers meet,which may induce clustered earthquakes.