Water-level Changes Induced By Earthquakes And A Preliminary Study Of Their Mechanism

Groundwater-level changes caused by earthquakes can also be called water-level variations after earthquake or post-seismic effect. They can appear at many wells in a large range as excited anomalies in response to major earthquakes. The study of post-seismic water-level variations has great theoretical and practical implications for mitigation of secondary hazards, research of crustal motion, chasing after-shocks, and tracing back to earthquake precursors. Analysis of record data show that water-level variations after earthquakes can be classified into two types in form: fast oscillational change and continuous monotonous changes. The former is of a water-earthquake wave which has been extensively studied. This thesis focuses on the second type of water-level variations after earthquakes.First, this work studies the case of one well and many earthquakes. It means water-level changes at one well caused by varied events. A well at Simao of Yunnan province and a Xin-04 well at Wulumuqi, Xinjiang are chosen as the study targets,which represent water-level rise and water-level fall after events, respectively. During observations of several decades, a lot of major earthquakes occurred around these two wells, so that there are enough record date available for the research. This word analyzes the features of water-level changes after earthquakes at these two wells, studies the quantitative relationship between the water-level change amplitude, earthquake magnitude, and epicenter distance, and discusses the possible factors affecting water-level changes and tectonic stress settings. Then this thesis makes an integrated study on the case of one earthquake and many wells, by taking the example of water-level variations in Sichuan and Yunnan, associated with the Ms8.1 Kunlun earthquake of 2001. The finite element method is used to explore the causes of water-level variations after the earthquake.The Simao observational well is located at the middle of Yixiangbazi, about 11 km southeast to the Simao town. Since 1990,obvious variations of water-level variations(more than 8cm)at this well have been recorded which are presumably associated with eight times of earthquakes greater than M6. The common characteristics of these variations are that the water-level always went up and reached the peaks quickly after earthquakes, and then fell down slowly. These features exist no matter where earthquakes occurred and of what mechanisms they are. It took severaldays or weeks for the water-level to return the value before the earthquakes, dependent on the maximum amplitudes of their rise. Analysis of statistical regression yields the following relationship between the rise amplitude ( h),magnitude(M),and distance between the well and epicenter (D)for the Simao well:When h 8 cm, the relationship between M and D is further obtained:Although some earthquakes meet the above relationship, they did not cause water-level changes greater than 8cm at Siamo. The reason is probably complicated. For instance, an Ms 6.2 event occurred 38km to the Simao well on January 27 of 1993. Following this event four earthquakes in 1994-1995 did not produce rising change of water-level at the Simao well, implying that the response of well water-level might be affected by the local stress state. Further analysis indicates that earthquakes at Dayao,Yaoan, and Wuding of Yunnan caused small rising values of water-level at Simao. It is likely due to that the Honghe fault has a major attenuation or screening role on the seismic wave.The Xin-04 well is located at the yard of the shuimegou hot spring in Wulumuqi city. It is on the hangwall of the Shuimegou-Baiyangnangou fault which is trending NE and of compression with strike-slip. At this well the post-seismic water-level change is characterized by descending first and then rising recover. Since 1990,seven events of M = 6.5 produced marked water-level descending(more than 3mm) at this well. Thy meet the following relationship:When h 3mm, there is M7.045+4.645LgDFurther analysis shows that the seismicity has major differen...