| Groundwater is an important component of the earth system, of which the movement and change are closely associated with human living environments. Earthquakes are sudden and frequent events of on the earth. The relationship between earthquakes and groundwater change has been the subject that scientists make great efforts to study for tens of years. Coseismic effects of well-water level and temperature caused by earthquakes is currently the outstanding topic receiving much attention. In this thesis, focused on this problem that, an attempt is made to reveal the distribution rule in space and time, influence factors, mutual relations and formation mechanisms of coseismic variations of groundwater level and temperature. It is of great significance in both theory and practice to clarifying relationship between earthquakes and groundwater variation, studies of crustal movement, reduction of secondary disasters, tracking following earthquakes, and earthquake precursor retrospection. The observation network of groundwater level and temperature in China has experienced prelimilary constructions in 70's and 80's, developments in ninth 5-year plan and tenth 5-year plan, and the numbers of observation wells have reached 420 and 277 respectively. Especially, the sample rate of digital observation apparatus, has reached one value per minute, is rapid, continuous, and convenient, which lays solid data foundation for a detailed study on the coseismic effects of well-water level and temperature.Fist, this thesis makes a systemic investigation is to the current research status on coseismic variations of groundwater level and temperature. It finds that there are many existing problems. For example, most work is of qualitative analysis, lacking further quantitative analysis. There is no report on systemic research on the relationship between groundwater level and temperature in coseismic variations. Researches on the influence factors on water temperature are rare. The understanding on the mechanism of coseismic variations of groundwater level and temperature is still controversial.Targeting the problems above, some representative wells are selected for a detail analysis of the continuous observation data of several years, and for the study of the coseismic variation rules of groundwater level and temperature under one well-multi earthquakes conditions. The observation wells in the database of national precursor network are selected according to specific principles. The observation data of groundwater level and temperature of the selected wells are then colleclected systemically, which are around the time when the Wenchuan Ms 8.0 (2008) event occurred, for studying the coseismic variation of groundwater level and temperature in the case of one earthquake-multi wells, and the combination type of groundwater level and temperature variations in the same well. Those wells within the range of 1000 km with the center of Wenchuan epicenter are selected to collect their observation data around Sumatra Ms8.5, Indonesia (2007) event. Next this thesis analyzes and compares the differences and similarities in coseismic variations of groundwater level and temperature caused by nearby and remote earthquakes and their influence factors. Finally taking the Tayuan well in Beijing as the test site, apply a comparison observation on groundwater level, dynamic variations of groundwater temperature at different depths, discuss the influence factors for variations of groundwater temperature. And models are constructed to perform numerical modeling and explore the mechanism of coseismic variations of groundwater level and temperature.The main work in the thesis can be concluded as following.1 Coseismic variations of groundwater levelCoseismic variations of groundwater level can be divided to oscillation type and step type (including ascent and descent). The groundwater oscillation is the elastic response of aquifer to earthquake waves, which has been well studied. So this work concerns the coseismic variations of the step type. Two representative wells, the Dazhai well in Simao, Yunan Province and Xin 04 well in Urumchi, Xinjiang are selected firstly for the study of ascending and descending characteristics in coseismic variations of groundwater level. Then, the coseismic variation data of groundwater levels before and after the Wenchuan Ms 8.0 (2008) and Sumatra Ms 8.5, Indonesia (2007) events are collected for analysis and comparison of the differences and similarities in coseismic variations of groundwater level and temperature caused by nearby and remote earthquakes and their influence factors.The research results of the case one well-multi earthquakes indicate that the coseismic variation directions of groundwater level at a same well do not change with different earthquakes. It keeps ascent or descent for any orientations, distances, magnitudes and mechanisms of the earthquakes. The amplitudes of coseismic variations of groundwater level depend upon the seismic magnitudes and well-earthquake distances, of which the three are well correlated.The coseismic variation of groundwater level in the Simao well is always ascending with growing amplitude with increasing magnitudes of earthquakes and declining amplitude with increasing well-earthquake distances. At the same time, it is also affected by seasonal rainfalls, regional tectonic settings and change of local stress state. The magnitude of coseismic variations of groundwater level will deviate from the general statistic relation when earthquake waves cross the Red River fault or a shock takes place near the well. Analysis suggests that the decrease of permeability may be the primary reason for coseismic ascending of groundwater level. The coseismic variation of groundwater level in the Xin 04 well is always descending, with growing amplitude with increasing magnitudes of earthquakes and decreasing amplitude with increasing well-earthquake distances. Meanwhile it is also affected by local stress status. When three earthquakes with magnitudes around Ms. 5.0 occurred near the Xin 04 well, the coseismic response ability of groundwater level decreased notably, which may be due to pore shrinking, fault locking, and stream blocking. The Xin 04 well penetrates a fault fractured zone on which will open-close action or creep would occur when it is triggered by S waves or surface waves of strong shocks, where the outflow of well water may be the reason for coseismic descending of groundwater level.Wenchuan Ms 8.0 earthquake (2008) was the most remarkable one that occurred in China when the projects of groundwater level and temperature in ninth 5-year plan and tenth 5-year plan finished. So Wenchuan earthquake is selected to analyze the effect of one earthquake-multi wells. The wells recorded in the precursor networks are selected firstly. With giving attention to the combination analysis of coseismic effects of groundwater temperature and coseismic variations of groundwater level and temperature in same well, the observation wells are selected following such principles:①the well station must have the two observation items as groundwater level and temperature, and work normally before, during and after the earthquake on May 12, 2008.②the Wenchuan earthquake has caused coseismic variations of at least one item of groundwater level and temperature.③the dynamic background of groundwater level and temperature is relatively stable. According to these principles, 96 wells are selected from the recorded wells in the database of precursor networks, and the coseismic variations of groundwater level in these wells are classified, the statistics shows that the coseismic variations of groundwater level caused by the Wenchuan Ms 8.0 earthquake (2008) are dominated by water ascent.Among the above 96 wells, 32 wells in the area of 1000 km centered by the epicenter of the Wenchuan event are selected to collect their responses to Sumatra Ms 8.5, Indonesia (2007), in order to have further comparison of the differences and similarities in coseismic variations of groundwater level and temperature caused by nearby and remote earthquakes. The result shows that 6 wells which exhibited ascent or descent related with the Sumatra Ms 8.5, Indonesia, 2007 kept same change when the Wenchuan event occurred. And among the 18 wells with oscillation variations of groundwater level associated with the Sumatra event of 2007, except 4 wells had oscillations related with the Wenchauan shock, the other 14 wells showed ascent or descent changes when the Wenchuan event took place. In the rest 8 wells which looked like quiet re during the remote Indonesia remote earthquake, only one well kept invariant while the other 7 wells showed coseismic ascent or descent.The comparative analysis suggests that the proportion of coseismic ascent of groundwater level is far greater than that of coseismic descent for either remote or nearby earthquakes. The number of wells with coseismic stepwise changes of underground water caused by nearby earthquakes is larger, while that of wells with oscillation or without change is small with respect to that by remote events. The directions of coseismic ascent or descent of groundwater level do not depend on distances, magnitudes, mechanisms or azimuths of earthquakes, instead are controlled by local geological settings and hydrologic conditions. If the energy of seismic waves is strong enough, some wells that only have oscillations or no coseismic responses originally will show stepwise variations of groundwater level.2 Coseismic variations of groundwater temperatureThere is ascent or descent in coseismic responses of groundwater temperature, but no oscillation is observed. Such changes are closely related with coseismic variations of groundwater level. The case study of one well-multi earthquakes suggests that coseismic variation of groundwater level seems to be the necessary condition for coseismic variation of groundwater temperature. It means that the earthquakes that have coseismic variations of groundwater temperature also show coseismic variations of groundwater level, while the events that have coseismic variations of groundwater level are not always accompanied by coseismic variations of groundwater temperature. In the research, the coseismic variations of groundwater level and temperature can be divided into four main types as ascending groundwater level and ascending groundwater temperature, descending groundwater level and ascending groundwater temperature, descending groundwater level and descending groundwater temperature, and oscillating groundwater level and descending groundwater temperature. In the thesis, the Simao well inYunnan, Xin 04 well in Xinjiang, Zuojiazhuang well in Beijing, and Tayuan well in Beijing are selected as the representing four cases for further analysis and discussion. Then the response data of groundwater temperatures before, during and after the Wenchuan Ms 8.0 (2008) are collected systemically from the 96 wells which are selected from the national underground fluid observation networks. 32 wells in the area of 1000 km centered by the epicenter of the Wenchuan event are selected to collect their responses to Sumatra Ms 8.5, Indonesia (2007). The differences and similarities together with their influence factors in coseismic variations of groundwater temperature caused by nearby and remote earthquakes are analyzed and compared.Type of ascending groundwater level and ascending groundwater temperatureTaking the Simao well in Yunnan as an example, the coseismic variations of both groundwater level and groundwater temperature are of ascent, which start nearly at the same time. The amplitude of coseismic ascent of groundwater level has a magnitude of tens of centimeters with minimum value as 12 cm and maximum value as 60.6 cm. The amplitude of coseismic ascent of groundwater temperature ranges from tens to hundred 10-4℃. The ratio between them is several℃/100m which is roughly the same as the normal geothermal gradient of 2-3℃/ 100m. The measurements when the observation system was installed show a positive gradient area where water temperature increases with depth near the groundwater temperature sensor. The long-term dynamic comparison of groundwater level and temperature in the Simao well also shows changes in a same direction. Considering that the ratio between the scopes of groundwater level and temperature is equivalent to the order magnitude of groundwater temperature gradient, it is indicated that coseismic ascent of groundwater level is the direct reason for coseismic ascent of groundwater temperature.Type of descending groundwater level and ascending groundwater temperatureAt the Xin 04 well in Xinjiang, the coseismic variations of groundwater level are of descending-resuming type, while that of groundwater temperature mainly displays as stability-ascent-recover. The start time of coseismic ascent of groundwater temperature is comparable to the time when groundwater level changes from descending to resuming. The maximum amplitude of coseismic descending of groundwater level is 58 mm. When the coseismic step of groundwater level is greater than 10 mm, the coseismic variations of groundwater temperature also take place with corresponding amplitude of groundwater temperature ascending as tens of 10-4℃. The ratio between them can reach 10℃/100m which is greater than normal geothermal gradient of 3℃/100m. Moreover, the starting time of coseismic ascending of groundwater temperature is not synchronous with the time when groundwater level begins descending, but basically equivalent to the time when groundwater level begins recovering after descending.Further analysis indicates hot aquifer may exist below the groundwater temperature sensor in the Xin 04 well. This hot aquifer will act together with the fault and control the variations of groundwater level and temperature in the well. When the well water is triggered by seismic waves and leaks out, the temperature ascent caused by groundwater level descent will counteract the groundwater temperature ascending caused by hot water supplied by aquifers and come to a balance, so the temperature will not change evidently. After the seismic waves are over, the leakage of well water decreases, while the hot water in aquifers will flow into the well continuously, which results in ascent of well water level and temperature. When the well water level stops ascending, the hot water in aquifers will also cease to flow into the well, and the well water temperature resumes to normal status under the control of the geothermal gradient.Type of descending groundwater level and descending groundwater temperatureIn the Zuojiazhuang well in Beijing, the temperature sensor is located at the positive gradient section sealed by well casing. The coseismic variations of groundwater level and temperature are of synchronous descending. The recorded amplitude of coseismic variation of groundwater level is in the range of 1-2 m; the recorded coseismic variation of groundwater temperature is in the range of 0.03-0.04℃; the ratio between them is in the range of 2-3℃/100 m. The mechanism of coseismic variation of groundwater temperature is similar to that observed at the Simao well in Yunnan Province, which suggests the influence of the coseismic descending of groundwater level on that of groundwater temperature in a positive gradient status. Type of oscillating groundwater level and descending groundwater temperatureIn the case of the Tayuan well in Beijing, the coseismic change is always oscillating, while that of groundwater temperature is usually of a descending-ascending-resuming process and independent of azimuths and mechanisms of earthquakes. The coseismic variations of groundwater temperature often occur at the time when seismic waves arrive and after the water level seismic wave starts. The recorded maximum coseismic variation of groundwater temperature can reach 0.097℃.Further analysis suggests that the water body in the well will be excited by oscillation, resulting in accelerated convection and mixture, which is the main cause for coseismic descending of groundwater temperature in the Tayuan well. The water temperature sensor is commonly placed at a relatively deep section of the well with relatively high temperature in normal conditions. When the seismic waves come, the water body in the well will accelerate convection and deep hot water ascends, while the shallow cold water falls down, so the water temperature sensor will observe temperature descending firstly. With the water level seismic waves calming down, the well water temperature around the sensor will resume ascending gradually.The amplitude of coseismic descent of groundwater temperature increases with the growing magnitudes of earthquakes and decreases with the increasing well-earthquake distance. The coseismic variations of groundwater temperature caused by individual earthquakes east of the well are smaller than theoretic values, which may be due to the influence of the Huangzhuang-Gaoliyin fault there.Classification statistics of coseismic variations of groundwater temperature caused by Wenchuan Ms 8.0 earthquake (2008) is made. Among 96 observation wells that have coseismic effects of groundwater level and temperature, there are 66 wells that exhibit coseismic variations of groundwater temperature, in the 66 wells, 65 wells show corresponding coseismic variations of groundwater level, the well that no corresponding coseismic variations of groundwater level were observed may be caused by some special reasons. In the classification statistics of coseismic variations of groundwater level and temperature in same well, there are 77 wells that show coseismic ascending or descending steps of groundwater level, of which 31 wells show changes of groundwater level and temperature in same directions; 23 wells with changes in opposite directions; and 23 wells without variation of groundwater temperature. The wells with changes of groundwater level and temperature in same directions are predominant. Among the 18 wells with oscillation of groundwater level, 7 wells show no changes in groundwater temperature; 2 wells show temperature ascending; 9 wells show temperature descending; and descent dominates the groundwater temperature.Among the 32 wells with a range of 1000 km centered by the epicenter of the Wenchuan Ms 8.0 event in 2008, the coseismic variations of groundwater temperature by this event and the Sumatra Ms 8.5 of 2007 are compared. It indicates that there are more fewer wells without such changes, with respect to the case of remote earthquakes. If the energy of seismic wave is strong enough, some wells without coseismic responses originally will show water temperature steps. Of course, the properties of most coseismic ascending or descending of groundwater temperature will not change with the variations in distances, magnitudes, mechanisms or azimuths of the earthquakes. The two wells that occurred character change in coseismic ascending or descending of groundwater temperature were caused by artesian flowing change or the coseismic variation of groundwater level changing from oscillating to step.Case comparison of one earthquake-multi wells and nearby and remote earthquakes also indicate that coseismic variations of groundwater temperature occur on the premise of coseismic variations of groundwater level. The properties of coseismic ascending and descending of groundwater temperature will not change with different earthquakes. The amplitude of coseismic variation is related with such factors as magnitudes, well-earthquake distance, season, geothermal gradient, and location of temperature sensor.3 Test of well water temperature sections and numerical modeling of dynamic relation between groundwater level and temperatureThe Tayuan well in Beijing is selected as the test site for discussing influence factors of well-water temperature variation. With a new high-precision temperature meter, detail measurement of water temperature gradients and dynamic continuous measurements of groundwater temperature at different depths have been carried out. The dynamic data of groundwater temperature at different depths in the Tayuan well are obtained with new sensor that allow a comparative study with data obtained by intrinsic groundwater level and temperature meter (which was installed in July, 2001). At the same time, the observed data from the Tangshan mine well and Taihe well in Xichang are also referenced to analyze the influences of groundwater gradient on tide and postseismic effects.From the detailed measurements of water temperature gradient, it exists a negative gradient section at 105-180 m away from the mouth of the well. the occurrence of negative gradient section may be caused by cold fluid existing at or flowing into local sections The test results at different depths indicate that water temperature gradients are large and the tidal effects are not evident at the shallow part of the wells. The groundwater level and temperature show change in same directions in the positive gradient section of deep groundwater temperature; and in opposite directions in the negative gradient section. The larger the groundwater gradient is, the bigger the amplitudes of tidal effects become. Differences of water temperature gradients around the sensor and its relative position to aquifer are the important factors that cause complex dynamic variations of water temperature.Control equations of fluid motion and heat exchange equations are introduced in this study. COMSOL Multiphysics is a professional FE numerical analysis software that is developed for describing and modeling varies of physical phenomena; it can model those physical procedures expressed with partial difference equations. Numerical modeling of coseismic variations of groundwater level and temperature is carried out with COMSOL 4.1 software. The ascending and descending or oscillating of groundwater level are realized with Arbitrary Lagrangian-Eulerian (ALE) method in COMSOL software.A three-dimensional model is constructed to model and explain the phenomenon of water level oscillating and water temperature descending. The geometry of the model includes 0.25 m of well radius (r) and 100 m of well depth (H). The temperature difference between the top and bottom of the well is 3℃. Because the borehole and the water comprised in it construct an axial symmetry structure, the three-dimensional model is simplified to a two-dimensional axial symmetry model. Compressive waves composed of sine waves of 5 s and 20 s are exerted at the side wall of the bottom section of the well to simulate the effect of seismic waves on well groundwater. The results show that obvious groundwater level oscillation appears on the upper surface of well water; the groundwater temperature oscillates downward in the middle part of the well where compressive waves are exerted, which is coincided with many observations of groundwater level oscillations and coseismic descending of groundwater temperature. Because of the delayed effect of water temperature sensor and sample rate of one value per minute, it is difficult to observe the oscillation effects of groundwater temperature.By systemic test observations and analysis of coseismic variations of groundwater level and temperature, this thesis has obtained some insights presented below.(1) The coseismic ascending or descending of groundwater level and temperature at the same well is mainly controlled by local geohydrologic conditions, and will not change with the orientations, magnitudes, distances, and mechanisms of earthquakes. Those wells with groundwater ascending are always in an ascending state, while those with groundwater descending wells are always in a descending regime.(2) The ascending or descending amplitude of groundwater level in a single well increases with growing magnitudes of earthquakes and decreases with increasing well-earthquake distances. There is a good correlation among them. Moreover, such factors as seismic waves crossing faults or coming from a special direction, and seasonal rainfall will cause the amplitude of coseismic variation deviate from general statistics relation. Earthquakes preparation near the well may probably reduce coseismic response capability.(3) Coseismic variation of groundwater level is the necessary condition for coseismic variation of groundwater temperature at the same well. The coseismic response of groundwater temperature usually appears at the time when seismic waves arrive and coseismic variations of groundwater level begin. The amplitude of coseismic variation of groundwater temperature is affected by such factors as magnitude, well-earthquake distance, seasonal rainfall, geothermal gradient, locations of water temperature sensor.(4) The coseismic variations of groundwater level caused by a same earthquake are mainly ascending, which is true for either nearby or remote earthquakes and may be caused by permeability decrease. Most wells with coseismic descending of groundwater level may penetrate fault fracture zones. The seismic waves will cause the faults to open and close or creep. The leakage of well water may cause coseismic descending of groundwater level.(5) Compared with remote earthquakes, the number of wells with coseismic steps of groundwater level and temperature caused by nearby earthquakes is much larger, while the number of wells with oscillations or without changes is less. If the energy of seismic waves is strong enough, the groundwater level and temperature of some wells that only oscillate or show no coseismic responses originally will show steps, but the seismic waves cannot change the step character of groundwater level and temperature. The character change of coseismic ascending or descending of groundwater temperature in individual well was caused by variation in water artesian flowing or coseismic variation of groundwater level changing from oscillation to step.(6) Differences in groundwater temperature gradients around the water temperature sensor and its relative position to aquifer are important factors causing complexity in dynamic variation of groundwater temperature. The groundwater level and temperature show syn-directiion change at deep sections of positive gradients but reverse- direction change at sections of negative gradients. The larger the groundwater gradient is, the bigger the amplitudes of tidal effects and coseismic variations become. The occurrence of negative gradient section in Tayuan well may be caused by cold fluid flowing into local sections of the well.(7) In the same well, the relation of coseismic stop of groundwater level and temperature shows that changing in same direction is dominant to changeing in opposite directions. For wells with coseismic oscillations of groundwater level, the deep groundwater temperature variations are mainly of coseismic descending-resuming. The reason is that the water body in the well is triggered by oscillations so that water convection and mixture are acceleratedBecause some objective factors such as limited time available, large amount of work, numerical modeling software that can be used only lately, and rebuilding of the test well (Tayuan well) , there are many problems that exist in the article or remain unsolved. For example, each well has its special characters; the detailed data of geohydrologic condition, structure of borehole, geothermal grident, etc. are needed to have thorough analysis of observations, but these data of many wells are incomplete. There are limited examples for the case studies of one well-multi earthquakes and one earthquake-multi wells. The movement of fluid is complex; some phenomena are hard to be modeled. These problems will be studied further in the future.
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