Analysis On The Mechanism Of Co-seismic Response Of Ground Water Beneath The Northern Xiannvshan Fault In The Three Gorges Reservoir Are

Earthquake-induced groundwater response is one of the hotspots in seismic hydrology research.Studying coseismic water level response provides an important way for us to understand the evolutionary relationship between tectonic activity and hydrogeological processes from the temporal and spatial scales.The predecessors have proposed many mechanisms to explain the co-seismic groundwater response,but there are still many disputes about the existing mechanism.The Xiannv Mountain Fault is an important seismogenic fault in the Three Gorges reservoir area.After the Three Gorges Project began to store water,a large number of seismic activities occurred around it.By analyzing its co-seismic groundwater level response,and then studying its tectonic activities and hydrogeological processes,it is of great significance to understand the mechanism of earthquake occurrence in this area.Based on the Three Gorges underground fluid monitoring well network in China,the response data of three underground fluid observation wells in the Xiannvshan fault zone to the near-field and mid-far field earthquakes in recent years were collected.Based on the basic principle of coseismic response and solid tide,this paper analyzes the collected well water level data from the perspective of “multiple wells to multiple earthquakes” and systematically studies the variation characteristics and influencing factors of coseismic water level.The tidal analysis of the water level observation data was carried out by Bay-tap software,and the permeability change before and after the earthquake was obtained.Combined with the current theory of coseismic response mechanism,the internal mechanism of the coseismic reaction of several earthquake water levels was discussed in depth.The main research contents and main understandings of this paper are as follows:(1)Based on the data of three underground fluid observation wells and seismic data since the impoundment of the Three Gorges Reservoir,several typical earthquakes with water level co-seismic response in Xiannushan fault zone are selected,and the co-seismic response characteristics of far-field and near-field earthquakes are analyzed through the co-seismic response images of the water level of the underground fluid observation wells with different earthquakes.The results show that the three wells present complex characteristics of co-seismic water level response,and the magnitude of co-seismic water level response is affected by the location of well point structure and the characteristics of well bore structure.(2)Based on the data of co-seismic response of water level,the energy threshold of co-seismic water level response triggered by three underground fluid observation wells and the correlative equation of water level response amplitude with magnitude and well spacingare obtained by using the formula of seismic energy density and the method of multiple linear regression.The results show that the seismic energy density of Dahekou well and Zhouping well is less than 10-4J/m3,while that of Guojiaba well is 10-2J/m3,which indicates that Dahekou well and Zhouping well are easier to trigger the co-seismic response of water level than Guojiaba well.(3)The pore elastic response of well-aquifer system,the content of tidal analysis and Baytap-G analysis principle are introduced in detail.At the same time,based on Hsieh formula,the aquifer parameters are inverted by using the response of water level of confined well to tidal stress.Baytap-G software is used to invert aquifer parameters.The comprehensive analysis of tidal amplitude and phase difference of three wells shows that the coupling relationship between water level and aquifer head in Dahekou and Zhouping wells is transitional zone and undrained zone respectively.The solid tide effect of Guojiaba well is poor,so it can not be used for tidal analysis.Before and after the Wenchuan earthquake in Dahekou well,the hydraulic conductivity is between 2.5 *10-7 m2/s and 5 *10-7 m2/s.It is presumed that the permeability of backfill near the well bore has changed.(4)The strain sensitivity coefficient of well water level is obtained by maximum diurnal tide difference and maximum strain value,and the static volumetric strain of the aquifer where the well hole is located is calculated by Coulomb,and then the variation of well water level under static strain is estimated.Through tidal analysis of well water level,the strain sensitivity coefficient of well water level is obtained,and the static volumetric strain of the aquifer where the well hole is located is used to estimate the variation of well water level under static strain.The results show that the estimated variation of water level is usually smaller than the actual variation of water level,and the direction of the estimated variation of water level is opposite to the actual variation.The analysis shows that the co-seismic water level response of four near-field earthquakes may be the result of the interaction of static and dynamic strains.Finally,based on the current theory of co-seismic response mechanism,the intrinsic mechanism of the co-seismic response of typical earthquakes is discussed.The analysis shows that the increase of permeability caused by earthquakes is a possible mechanism to explain the co-seismic water level changes of mid-and far-field earthquakes.The near-field co-seismic water level response may be affected by both static strain theory and permeability enhancement mechanism.