Numerical Simulation Of Pre-seismic And Coseismic Continuous Response Of Well Water Level

Groundwater is sensitive to seismic and tectonic activity information.When the well-aquifer system was in a well-closed confined system,the system is a natural volumetric strain gauge.Well water level response mechanism of pre-seismic and coseismic is one of the hot issues seismic subsurface fluid.Pre-seismic and coseismic continuous response of well water level is a continuous reflection of the process of seismogenic and seismic rupture.The relationship of seismic activity and the water level can be further proved based on this continuous response.However,the factors affecting the pre-seismic and coseismic responses of well water level are complex,quantitative analysis of continuous changes of crustal stress,aquifer deformation,pore pressure changes,continuous changes of hydrodynamic conditions and dynamic changes of well water level during the process from seismogenic to seismic rupture was difficult in the study of well water level response to earthquake.At the same time,the research was separated in time due to the different physical mechanism and research purpose of preseismic and coseismic responses of well water level.Pre-seismic and coseismic response of well water level was regarded as continuous processes in this research.A series of physical processes of stress-strain-pore pressure-water level change before and during earthquake have been delineated.The corresponding relationship between the response process and hydrogeological(permeability and porosity)and rock mechanical parameters(Young’s modulus,Poisson’s ratio)were analysed quantitatively and physical mechanism of well water level preseismic-coseismic continuous response should be revealed.The research had certain practical significance for site environmental data acquisition,earthquake monitoring and prediction.Based on the previous studies in stages and single field,taking the near-field earthquake under static stress as an example,a mathematical model and a numerical model for the continuous response of well water level pre-seismic and coseismic based on hydraulic-mechanic coupling near fracture section are established.Based on parameter sensitivity analysis and simulation verification of typical well water level response of seismic,the results are as follows:(1)Conceptual model,hypothesis condition and mathematical model are constructed for the physical process of pre-seismic and coseismic continuous response of well water level.On the basis of considering velocity field,fault geometry,medium rock mechanical parameters and hydrogeological parameters,a numerical model of preseismic and coseismic aquifer pore pressure is established based on hydraulic-mechanic coupling.Stress accumulation before earthquake and evolution of strain and pore pressure of aquifer during fault rupture is simulated.A numerical model of aquifer-well water level response is constructed based on the consideration of aquifer properties,well structure,well flow and other factors.The response of well water level under pore pressure disturbance is simulated.Finally,using "pore pressure" as interface,the numerical model of pre-seismic and co-seismic continuous response of well water level near fracture section is constructed.(2)The pre-coseismic continuous response law of well water level is directly influenced by the accumulation law of stress field before earthquake,and is directly related to the adjustment value of pre-coseismic strain field.In the process of preearthquake accumulation to the moment of coseismic adjustment,the strain at the point position in the tensile strain region is worth releasing rapidly,and the pore pressure rises,and the well water level rises.The point strain in the compressive strain region is worth releasing rapidly,and the pore pressure drops,and the well water level drops.The water level changes before the earthquake and in the coseismic well show an upturn-sudden drop.After the adjustment of the strain value at the point site in the compression strain region,the compression continues to accelerate,the pore pressure continues to rise,and the well water level continues to rise.The variation of the water level in the well before the earthquake and in the coseismic well presents a rise-spike rise.(3)Through the sensitivity analysis of parameters of the numerical model of preseismic coseismic continuous response of well water level,it can be seen that: The poisson’s ratio of geologic body and the friction coefficient of fault directly affect the preseismic and coseismic strain field;Poisson’s ratio,Young’s modulus,porosity and permeability coefficient of aquifers influence the evolution of pore pressure of preseismic and coseismic aquifers;The evolution of pore pressure,permeability coefficient of aquifer,water storage rate and well diameter of monitoring well all affect the preseismic and coseismic continuous response of well water level.Aquifer parameters affect both the variation of water level and the recovery time of coseismic water level,and geological body geophysical parameters ultimately affect the variation of water level by changing the evolution law of strain field.(4)In order to verify the effectiveness of the numerical model in this paper for the actual well water level response,a numerical model for the pre-seismic and coseismic continuous response of Huize well water level to Ludian earthquake and a numerical model for the pre-seismic and coseismic continuous response Shuangqiao well water level to Tangshan earthquake were constructed.Through the design of different parameter schemes,and the identification and correction of site parameters,the simulated value of pre-earthquake to coseismic continuous response was well fitted with the measured water level,and the model was verified by the of coseismic strain and fixed water level.The numerical model of water level pre-seismic and coseismic continuous response of nearfield seismic constructed in this paper was "relatively reasonable" in describing the physical process.