Well Water Level And Aquifer Permeability Changes In North China Plain Induced By Far-Field Large Earthquakes

Changes in groundwater level,streamflow,water temperature,and chemical composition are the most widely reported hydrological and hydrogeological responses to earthquakes.Specifically,the well water level and aquifer permeability changes are the hot spots in the field of underground fluids,as they impact on the safety of water resources and underground waste disposals.Some reports provide quantitative information on the relationship between water cycle and earthquakes in the near filed,while study cases in the far field are rarely performed.On the basis of the ten Ms 8.0 or larger earthquake event sequences occurred worldwide from 2008 to 2015?2008Ms8.0 Wenchuan earthquake;2009 Ms8.1 Samoa earthquake;2010 Ms8.8Chili earthquake;2011 Ms8.7 Japan earthquake;2012 Ms8.6 Sumatra earthquake;2012 Ms8.2 Sumatra earthquake;2013 Ms8.0 Okhotsk Sea earthquake;2014 Ms8.2 Chili earthquake;2015 Ms8.2 Nepal earthquake;2015 Ms8.3 Chili earthquake?,and the response of groundwater level of nine monitoring wells in the study area had been collected?CP well,XZ well,SZ well,GC well,XUZ well,XH well,JH well,JZ well,and LK well?.This paper compared the long-term record of groundwater level fluctuations,phase and amplitude shifts,and permeability changes before and after the above-mentioned earthquakes.The analysis provides us a good opportunity to explore why and how the well water level and aquifer permeability change in the North China Plain.The main purpose of this study is to analyze the possible mechanism of earthquake related well water level and aquifer permeability changes,and whether these changes affect groundwater resources and hydrogeochemical processes.The research methods in this study includes:spectral analysis,Cooper?1965?analytical model,tidal analysis,Hsieh?1987?analytical model and Doan?2006?analytical model.Spectral analysis is used to extract the influencing factors of well water level changes when they are affected by the surroundings except earthquakes.The theoretical amplification factors which represent the amplitude of well water level and the change of pressure head of aquifer are calculated by analytical equation developed by Cooper?1965?.Then,the pressure head fluctuations of aquifer are calculated based on the recorded amplitudes of well water level.By this means,the changes of pressure head in the aquifers can be reproduced.Tidal analysis,Hsieh analytical model and Doan analytical model are used to calculate phase and amplitude shift for the further estimation of the related aquifer parameters.The main conclusions of this study are as follows:?1?The multi-year and annual periodic variation of well water level without earthquake impact was mainly affected by lateral recharge groundwater and manual exploitation,which exhibited runoff-mining type,runoff type and mining type.The diurnal variation of well water level without earthquake impact was mainly affected by M2 and O1 waves of lunar gravity,which exhibited 24h and12h periodic characteristics.?2?Far-field large earthquakes caused various change forms of well water level in the North China Plain,which could be divided into the following six types:oscillation,gradual rise,gradual drop,step rise,oscillation-gradual rise,and oscillation-gradual drop.Among them,the oscillation was the most common form of well water level changes.It had 34 times of occurrences,which take up 73.6%of the total times of well water level changes,and well water level amplitudes are between 0.001m and 1.05m.The normalized energy trigger threshold of well water level changes in North China Plain was1×10^-3-5×10^-3Jm^-3,which was consistent with the results of previous studies in China mainland but not with the results in other areas of the world.It was speculated that the reason was the differences of geological structure conditions,aquifer properties and well structure in different research areas.?3?The mechanism of well water level changes was that the change of pressure head in aquifer.This change is induced by seismic wave,which causes a pressure difference between the aquifer and well.Groundwater is thus driven to flow into and out of well,resulting in the oscillation of well water level.From 2008 to 2015,the hydrostatic pressures of different aquifers were different.Among them,the hydrostatic pressure of GC well was the highest,ranging from 26.24 to 26.34MPa,and the water column height was 2677.7 to2687.8m.The hydrostatic pressure of CP well was the lowest,ranging from1.22 to 1.46MPa,and the water column height was 124.5 to 148.9m.However,the variation range of hydrostatic pressure was small,which was less than0.5MPa in all wells.Affected by the far-field earthquakes,the theoretical amplification factors between well water level oscillations and aquifer pressure head variations were between 0.0009-0.0088.Further calculation indicated that the variations of pressure head in aquifers were between 0.029 and2.8MPa,which were larger than hydrostatic pressure variations in most cases.The calculations of hydrostatic pressure and co-seismic pressure head changes reproduced the fluctuations of pressure head in aquifers caused by far-field large earthquakes.?4?Permeability changes caused by far-field large earthquakes can be divided into temporary changes and long-term changes.The post-seismic phase of 4wells?CP,SZ,GC,and LK wells?increased 3.866,20.851,13.892,0.932,1.616 degrees and the transmissivity increased 14.724,33.192,20.664,1.584,10.08 m²/d,respectively.All of them returned to the pre-earthquake levels within 15 days after the earthquake,indicating temporary changes of permeability.While the post-seismic phase of 3 wells?XZ,JH,JZ wells?increased 20.851,3.185,4.862 degrees and the transmissivity of JH well and JZ well increased 4.356 and 1.152 m²/d,respectively?the transmissivity of XZ well can not be calculated because of uncertainty of flow direction?.The permeability values returned to the pre-earthquake level in 500,150 and 60days after the earthquake,indicating long-term changes of permeability.In addition,the permeability changes and aquifer boundary conditions were significantly related.If the aquifer was well-confined,and the groundwater was mainly horizontal flow,the post-seismic permeability was increased,while the aquifer was not well-confined,and the groundwater was mostly vertical flow,the post-seismic permeability was decreased.The clogging/unclogging model and the fracture of the aquifer/aquitard model can explain the changes in aquifer permeability.