Research On The Relationship Of Deep Groundwater Level Dynamic Response To Seismic Activity

At 14:28 on May 12th, 2008, Ms8.0 Wenchuan Earthquake happened in Sichuan Province, China, which shocked the people all over the world. This earthquake not only brought about catastrophic damage in the vicinity of the epicenter, but also caused the extensive damage in Sichuan Province and its neighboring provinces, with the most serious destruction and the largest spread since the founding of republic. Besides sorrow, people have deeper understanding to earthquake hazards, and have more intense hope for earthquake prediction to defend and relieve earthquake disaster. So, we must seize this natural test-Wenchuan earthquake, to do further study on seismic activity.Groundwater is one of the most active components in the earth's crust, and it widely distributes in 0~30km rock mass below surface. Because of its universality, fluidity and hard compressibility, when it exits the closed confined aquifer system, tiny stress and strain changes can inevitably change the pore pressure of the aquifer, and change the well water lever through groundwater seepage. Therefore, deep groundwater level dynamic can objectively and sensitively reflect the crustal stress and strain information, and it has clear physical meaning to study the earthquake preparation, occurrence and development by deep groundwater level dynamic.Based on projects of Scientific research to Wenchuan Earthquake of China Earthquake Administration and Well hydrogeology and tectonic environment condition investigation in the capital circle , using subsurface fluid monitoring data of Wenchuan Earthquake from subsurface fluid monitoring networks of China Earthquake Administration, taking Wenchuan Earthquake as research background, using the theory and method such as groundwater dynamics theory, linear elastic theory, porous tidal theory, tidal load-unload response ratio and wavelet transformation analysis and choosing Sichuan, Yunnan, Gansu, Shanxi Province and Chongqing City as research areas, the author systematically analyzes deep groundwater dynamic response at each stage of Wenchuan Earthquake (including pre-earthquake, coseismic earthquake and post-earthquake), refines deep groundwater level dynamical characteristics to non-tectonic stress (including air pressure and earth tide), seismic tectonic stress and seismic wave stress, initially establishes the response relationship between deep groundwater level anomalies and seismic activity, and also calculates coseismic crust stress state by using deep groundwater level anomalies.According to groundwater dynamics and porous linear elastic theory, without interfering factors such as rainfall and so on, mathematical model, which can express deep groundwater level dynamic response to the crustal stress-strain, is established as follows;Where, H~0(t )stands for trend item, H_p(t),H_θ(t )stand for groundwater level response to non-tectonic stress (including air pressure and earth tide), H_E(t )stands for groundwater level response to seismic activity,ε(t )stands for observation noise.As seen, the information inherent in deep groundwater level is rich and complex. Totally speaking, it consists of information part and noise part. For earthquake research, information part mean seismic groundwater level dynamic anomalies, the air pressure effect and earth tidal effect belong to noise part. Therefore, before analyzing the seismic groundwater level dynamic anomalies, groundwater level dynamic response to non-tectonic stress (including air pressure effect and earth tidal effect) should be analyzed. By linear correlation analysis, the air pressure coefficient and earth tidal coefficient of typical observing well water level can be calculated. The results show that, groundwater level response to non-tectonic stress is affected by lithology, buried depth, permeability of observing aquifer and other factors. Relatively, when limestone aquifer is deeply buried, groundwater level response to non-tectonic stress is comparatively obvious.By removing the noise part and extracting seismic information from deep groundwater level, it is found that deep groundwater level dynamic laws to Wenchuan Earthquake are complicated, with various abnormal shapes, different changing amplitudes, multi-stage temporal distribution, non-homogeneous spatial distribution, temporal and spatial distribution mobility and so on.Before earthquake, deep groundwater level dynamic contains information about earthquake preparation. Groundwater level mid-long-term anomalies mainly distribute in Sichuan-Yunnan region, most of which are type of breaking annual variation law and appear abnormal turning point within 17 to 47 months before earthquake. According to groundwater observing curves, mid-short-term anomalies only include Deyang well in Sichuan Province. So, the method of earth tide load-unload response ratio is used to analyze groundwater level mid-short-term and short-term anomalies, the results show that anomalies widely appear within 6 months before earthquake, and anomalies concentrate in Sichuan region within 1~2 months before earthquake. All of these indicate that spatial distribution of groundwater level anomalies has a shift from Sichuan-Yunnan region to larger scope to Sichuan regions, namely a trend from marginal areas to epicenter vicinity, which is consistent with the theory—regional seismic tectonic activity bears seismic source body, and seismic source body develops and generates earthquake. Meanwhile, seismic pregnant process results from interaction of seismic source tectonic activity and regional tectonic activity.Impending anomalies of groundwater level play a warning role to occurrence of earthquake, including weak oscillation and pulse type, as well as intense step-rise type. According to wavelet transformation analysis, oscillation and pulse type of groundwater level anomalies imply a similar form of precursor wave. Some well water level, such as Qingshui well, Gulanghengliang well and so on, can capture the information of precursor wave, which has a certain physical meaning to research the seismic source process. Also, impending step-rise anomalies last until post-earthquake, such as Qionglai well and Wudu well, which indicates that some sensitive well water levels can reflect rapid changes of seismic tectonic stress, and suggest the coming of large earthquake.Coseismic groundwater level anomalies can record earthquake, and distribute widely. In seismic source region, anomalies are mainly type of step and pulse, where aquifers are affected by the impact of Longmen Mountain fault rupture, seismic source tectonic stress field control the characteristics of groundwater level anomalies, so that, the aquifer solid skeletons occur elastic or plastic deformation, and groundwater level has sudden response to earthquake. In marginal region, groundwater level anomalies are type of oscillation, step and pulse.①Oscillation-type anomalies mainly distribute in Yunnan region in the SW direction of Longmenshan Mountain fault. Seismic wave propagation causes the periodic elastic deformation of aquifer solid skeletons, aquifer pore pressure change up and down, and groundwater alternately flows in and out between borehole and aquifer, so, groundwater level takes on up and down oscillation.②Step-type anomalies mainly distribute in Yunnan region in the SW direction of Longmenshan Mountain fault, and other 3 abnormal wells locate in Gansu-Shanxi region. The vibration of seismic waves bring about changes of aquifer medium pores, cracks or dynamic characteristics, result that groundwater movement changes and some sensitive well water levels change up or down.③Pulse-type ananomalies mainly distribute in Shanxi and Gansu region in the NE direction of Longmenshan Mountain fault. If wells locate in the region with regional tectonic stress enhancement, aquifer medium change or tectonic activity, seismic wave propagation or fault fracture can induce changes of stress on aquifers, result that aquifers are affected by both seismic wave and regional tectonic stress. So that, after coseismic pulse-type anomalies of groundwater level recovers, groundwater level takes on new up or down post-earthquake anomalies. Otherwise, when coseismic anomalies are pulse-type, they can recover post-earthquake.Post-earthquake anomalies of groundwater level have a certain significance to explore aftershock trend. The anomalies, that post-earthquake anomalies are obvious after coseismic anomalies recover, distribute in the region in the NE direction of Longmenshan Mountain fault. Meanwhile, strong aftershocks occur in the NE direction along Longmenshan Mountain fault, and the farthest aftershocks occur in the border zones among Sichuan, Gansu and Shanxi Provinces. So, where aftershocks occur is consistent with where above anomalies distribute, it shows that above anomalies has an important role in forecasting strong aftershocks, and post-earthquake anomalies reflect the adjustment process of regional stress.Coseismic effect is one of the most direct and effective means to reveal the crust medium response to stress-strain. Through coseismic anomalies of groundwater level, the crust stress field is inversely simulated. The results show that, the stress field changes greatly in the seismic source region, and the greatest stress value on aquifers reaches to 2.6MPa in Wenchuan-Qionglai region. In the NW direction along Longmen Mountain fault, aquifers are affected by tensile stress, and in the SE direction along Longmen Mountain fault, aquifers are affected by compressive stress.For Longmen Mountain fault, within a long period before earthquake, its hanging wall is obviously compressed, which is in contrast with the simulation results. It indicates that groundwater level changes indeed exit turning points from before earthquake to occurrence, and regional stress field changes from compression to tension, which are exactly consistent with the theories such as DD mode, IPE mode and fissure-pre-displacement mode. So, it provides new clues for earthquake forecast to find turning points of groundwater level.Until now, this research is the more comprehensive analysis and summary to the relationship between deep groundwater level dynamic and Wenchuan Earthquake activity. Its research achievement has a certain practical and theoretical significance to deeply reveal the seismic information implied in the deep groundwater level dynamic, and to further discuss mechanism between groundwater level dynamic and earthquake process.