Study On Permeability Changes In Fault Zones Caued By Earthquakes Based On The The Tide Analysis Of Water Level

The hydrological responses to earthquake?e.g.,changes in groundwater level,water temperature,chemical composition and stream flow?are often attributed to the permeability changes in the shallow crust.There are many researches on the effort and mechanism of crustal permeability changes induced by static and seismic waves?dynamic stress?.As probes for the change and recovery of fault stability,the permeability in fault zones has been widely studied.Highly fractured rocks in fault zone,caused by strong tectonic activities,supply the main fluid flow paths.Many studies focus on the permeability changes in fault zone.In natural systems,strain amplitude as small as 10^-6 can change the groundwater level in well with confined aquifer.Monitoring the permeability changes,based on the tidal response of water level,provides a way to obtain the changes of rock deformation in fault zones.The research on the earthquake-induced permeability changes in fault zones,based on the tide response of water level,has important implications.On one hand,it provides a way to obtain the spatial and temporal changes of fault zone properties in response to earthquake and other tectonic activities,on the other hands,it contributes to the reasonable exploitation and utilization of underground resources?e.g.,underground water,geotherm,oil and gas?,and secure storage of nuclear waste and toxins.In this paper,based on the tidal responses of water level,the transmissivity was calculated according to the relationship between tidal parameters?phase and amplitude?and hydrometrical parameters in Hsieh model?1987?and Roeloffs?1996?model.Then,the relationship between permeability changes in fault zones and the static stress/seismic waves was thoroughly analyzed from several aspects,such as:the co-seismic changes?increase or decrease?in permeability,co-seismic changes in horizonal and vertical permeability,the inter-seismic changes in permeability trend.Main conclusions were obtained as following:?1?The co-seismic changes in permeability of three wells in the Tan-Lu fault zone were analyzed,the results demonstrate that seismic waves could not only increase but also decrease the permeability.The understanding of the decrease in permeability induced by seismic waves was proved by directional field data.In addition,we found that the earthquakes that decreased the permeability were distributed in certain azimuthal zones for three wells.Due to the azimuthal distribution of distant earthquakes,taking the mechanism of fracture unclogging as a reference,the decrease in permeability could be attributed to the clogging of fractures,by the wave-induced removal of deposits and particles that block the flow paths.Furthermore,we discussed the possible process of particle mobilization in a natural fracture network created by seismic waves,which may be used to explain the azimuthal distribution of the earthquakes that decrease the crustal permeability.The fracture development of the shallow crust may affect the azimuthal distribution of the earthquakes that increase or decrease the permeability.?2?The tidal response parameters?e.g.phase and amplitude?of four wells in the Xiao-Jiang fault zone were analyzed.More correctly discriminant condition of tide flow type was proposed,that was not only consideration the of negative or positive phase but also consideration of the positive or negative correlation between the change of phase and amplitude.According to the discriminant condition,there was the dominant vertical groundwater flow in an unconfined?or semiconfined?well-aquifer system in Zhaotong well,there was the dominant horizonal groundwater flow in a confined well-aquifer system in Qujing and Dongchuan well,while there was a primary vertical and secondary horizonal groundwater flow in an unconfined?or semiconfined?well-aquifer system in Mile well.The effect of earthquakes on the horizonal or vertical permeability had been analyzed.It showed that,the earthquake temporarily increased the co-seismic vertical permeability in Zhaotong well,it temporarily increased the co-seismic horizonal permeability in Qujing and Dongchuan well,especially the Ludian earthquake permanent enhanced the horizonal permeability in Dongchuan well.The seismic waves temporarily increased the horizonal permeability in Mile well,the dominate vertical groundwater flow recovered after earthquakes.We discussed the possible process of particle mobilization and fracture aperture variations in a natural fracture network created by seismic waves,which may be used to explain the transformation from vertical flow to horizonal flow in Mile well induced by seismic waves had been discussed.?3?The spatial and temporal tendency changes of the water level and permeability of four wells in the Hua-Yinshan fault zone were analyzed.The results showed that the trend of water level and permeability are consistent in Beibei well.The permeability in Beibei well and Dazu well was tendency decrease.Combine the changes of GPS baselines,it could be attributed to the enhancement of regional compressive stress.The water level in Rongchang and Nanxi wells was tendency increase.That could be attributed to the rain in the region.The study shows that the change in permeability trend to some extent response to the adjustment of regional stress and strain,excluding the nontectonic factors such as rain,air pressure.Accordingly,during 2008-2018,there was an obvious regional stress adjustment process in the northeast part of Hua-yinshan fault stress,but that was not obvious in the southwest part of the fault.During 2008-2016,the compressive stress enhanced in the northeast part of the fault,meanwhile,the tensile stress changed from weakness to enhancement in this part.During 2016-2018,the compressive stress was gradually weakening,but the tensile stress was gradually enhancing in this part.In this study,based on the tidal response of the water levels in fault zones,we focus on the relationship between permeability changes and earthquake activities.The research achievements could provide theoretical basis and analytical tools for an enhanced understanding of the fluid flow in the fault zones,an overall comprehension of the effect of earthquakes on the permeability in the shallow crust,and provide a new angle of view for further research of interaction between fluid and earthquakes.