| With increasing construction projects of hypo-junctions, the problem of Reservoir-induced Seismicity(RIS) has attracted more attention. The Three Gorges Reservoir(TGR) is a world-class water conservancy hub project, for which geological and geophysical research associated with RIS is very important. During the implementation of the key project “Exploration Technology of Reservoir-induced Seismicity Conditions”, a dense seismicity monitoring station network consisting of 26 stations using Reftek130 broadband recorder and L-22 short-period seismometers was constructed by Institute of Geology, China Earthquake Administration(IGCEA) in 2009. It has recorded 5275 quakes in the TGR region from March 2009 to December 2010. Based on these rich and high-resolution data, using the double-difference tomography method, this work studied the precise relocation and three-dimensional velocity structure imaging of this region.The double-difference tomography method(Tomo DD) combines both absolute travel time data and catalog or waveform cross-correlation differential data to conduct joint inversion hypocenter parameters and velocity structure simultaneously. Compared with the conventional local earthquake tomography(LET) method, Tomo DD can image finer velocity structure in the source region. The Tomo DD method uses absolute travel time data and differential data, which effectively overcomes the limitations of hypo DD that ignores the influence of velocity perturbation bringing about relative relocation and LET that requires high precision hypocenter parameters. Tomo DD uses grid nodes to get a discrete model space, using the pseudo ray-tracing method to search raypath and calculate minimum traveltime, and makes inversion with smoothness constrained by LSQR. At last this method determines velocity structure outside the source region by absolute and fine velocity structure in the source region with differential data.Tomo DD algorithm with cross-correlation of waveform method can obtain the medium velocity structure at a high accuracy. This study used a third-order spectrum domain BCSEIS algorithm to analyze the waveform cross-correlation of seismic data from the dense network, getting the standard cross-correlation and BS verification cross-correlation delay times of observational data. The results show that the effective distance of waveform cross-correlation for seismic event-pair is about 6km. In this research, combining absolute travel times catalog difference data and waveform cross-correlation data, the Tomo DD method is employed to obtain the source location parameters and 3D velocity structure of TGR, which provide basic information for better understanding the relationship between reservoir water and velocity structure, and for studying of coupling between source location and velocity structure and assessing the risk of RIS. Results of this study are summarized below.1) The velocity lateral heterogeneity of TGR from head of dam to Badong is remarkable. A horizontal velocity profile shows that the velocity change area is mainly concentrated in the vicinity of the Yangzi River and its tributaries, especially at depth Z=2km and Z=5km lateral heterogeneity is most profound. With the increase of depth, such lateral heterogeneity becomes weak. In the shallow subsurface 0-5km, three are three high-velocity local areas on the Gaoqiao fault, Niukou fault and northern part of Xiannvshan fault, respectively. While in the southeast of the Zigui Basin and Yandu River area, velocity is obviously low. Within the 5-8km range, the high-velocity areas shift to the north bank of Badong and along the Yangzi River with a bigger area, and become into east-west trending strips.. In the north of Badong on both sides of the Shennong Stream and in west of Xietan exist nearly east-west trending banded wave velocity anomaly zones. In the vicinity of the Xiangxi River also exists a planar wave velocity anomaly zone which corresponds to a high Vp area, and 10 km to the Zigui Basin, a S-wave velocity uplift is seen.2) The earthquakes hypocenter are mostly distributed along the high to low velocity transition zones. Velocity structure from inversion and distribution of microseismic are largely consistent. The projected epicenters of different depths are closely related with surface fracture faults and velocity structure. In the Badong area, the relocated epicenters of quakes are roughly consistent with the east-west trending faults, showing a linear distribution along the faults, in accordance with the strike of strata. High velocity characterizes of this area, where quakes are mainly concentrated on the transition zone of velocity on vertical profiles. In the Xiannvshan and Jiuwanxi areas, mircoseismic events are mainly distributed near the faults, extending downward to below 8km, most of which are located on the transition zone showing the coupling relationship between microseismic distribution and fault structure and velocity. While in the west of Xietan, there exits a 10km-long east-west trending seismic belt independent of the surface structure. In the north of this belt, low and high velocity appears on the north and south of the Zigui Basin, respectively.3) The reservoir water loading of the Three Gorges Reservoir has a certain effect on the velocity change. During the TGR water loading process, although the highest water level changed only 4m, within the 40 m range the water loading may caused large variation of P-and S wave velocity structure. Even considering the error of numerical faults, it also shows that the effect of water on hydraulics structure is larger than previously recognized.4) Small earthquakes in TGR distribute in strips, showing a steep form on vertical profiles. The quakes occurred along high-low velocity transitions of P-and S waves. For instance, the December 16, 2013 Badong Ms5.1, March 26 and 29, 2014 the Zigui Ms4.3 and Ms4.7 quakes may be attributed to discontinuous structural planes and rock strength weakening. North of the Badong and Banqiao River areas, earthquakes distributions are consistent with the strike of the fault, and the planes of micoseismic velocity gradient zones reveal the high-angle feature of the Xiannvshan and Jiuwanxi faults. |
PDF Full Text Request