Dynamic Loading And Physical Dynamics Process Of Groundwater

Observations of borehole water level and water temperature in the physical dynamics of groundwater are the important observed components of precursores for earthquake prediction. In this paper, by analyzing the dynamical loading of far-field seismic wave and artifical explosion, the dynamic response characteristics of borehole water level and water temperature in well-aquifer system is investigated and deeply studied in order to reveal the relationship between physical dynamics of groundwater and crustal stress and strain in the process of earthquake gestation.In this study, we made a statistic of types of coseismic effects and characteristics of groundwater level and water temperature in the boreholes of earthquake ground water net between 2004 Sumatra earthquake with magnitude 8.7 in Indonesia and the 2008 Wenchuan 8.0 earthquake in the Chinese mainland. Through these work, we proposed the "thermal convection - conduction model" and interpreted physical mechanism of the borehole water level shock as well as water temperature decline. Using explosion test results, which was generated by 600kg -weighed explosives in the depth of 10m in underground explosion field, we analyzed the impact characteristics of explosion dynamic loading on the borehole flow, and compared them with seismic wave effects. By the statistical analysis, numerical simulation and field tests above-mentioned, the key findings are obtained as follows.1. It is found that in Chinese mainland 78 out of 103 boreholes tracked records of water level coseismic effect associated with the Sumatra 8.7earthquake, while water level in 196 out of 218 boreholes showed coseismic responses to Sichuan 8.0 earthquake. Moreover 69 out of 113 boreholes recorded cosesimic water temperature effect related to the Sumatra earthquake, and 132 out of 216 boreholes demonstrated cosesimic water temperature responses to Sichuan 8.0 earthquake. Through comparison and analysis of the two earthquakes related responses, it found out that the proportion of borehole numbers with borehole temperature decreasing is higher than that of temperature-increasing ones, with oscillations type higher than other types. The impact of Wenchuan 8.0 earthquake on some areas of China is greater than Sumatra 8.7-magnitude earthquake, indicating that the stress state in these areas has changed, which has inductive indicating significance to the earthquake tracking.2. Borehole water level oscillations with rapid decline of temperature are mainly caused by the vertical seepage in the aquifer surrounding the borehole, and the slow recovery of the water temperature after the transportation of seismic waves are contributed to the surrounding rock thermal conduction. Using "thermal convection - conduction model" to perform the simulation , it shows that the mixture rate (θ) of the aquifer water with the borehole water has directly impact on the decline of water temperature, with greaterθresulting in higher mixing velocity. Besidesθ, temperature decrease depends on the initial mixed borehole temperature T0 . It is seen from the calculated results that the main factor impacting on water temperature recovery velocity is the size of vertical seepage mixture area around the borehole. A key finding is that ,for different wells - aquifer systems, the decline amplitude of well water temperatures is dominated by hydrological environment of the well itself. While, for the same well - aquifer system, the exponential statistic relationship between the intensity of seismic wave as well as water level oscillation has been obtained.3. In the explosion test, five sets of strong motion seismograph are installed on the earth surface with one earth acoustical instrument installed in the well. With the strong motion records, the additional stresses induced by SH waves in the depth of 150m have be obtained . The tangentialσt, radialσn, and vertical stressσu are 318.2Pa, 735.7Pa and 2851Pa, respectively. Results show that the acoustical wave pressure received in the depth of 150m from the well bottom reaches approximate110Pa. While Energy induced by explosion is equivalent to ML1.3 earthquake (MS 0.37), which can simulates effects of medium and strong earthquakes occurring beyond 100km on fluid observation points. It proves that the flow of wells is instantaneously enhanced by seismic waves and elastic impacts due to explosion dynamical loading. While porous pressure diffusion induced by seismic waves or explosion contributes to the considerable increase of well flows after two days.