Co-seismic Water Level Steps Of The Anxi-1Well,Fujian Province Induced By Local And Distant Earthquakes

Co-seismic well water level responses have been reported all over the world. A large number of records show that earthquake-induced water level changes in morphology involve two kinds:rapid shock and continuously monotonic change. Many researchers have found that regardless of the focal mechanism, the water level of the same well always respond to earthquakes in a fixed pattern, and there exists a quantitative relationship between the sizes of changes, the magnitude of each earthquake and its distance from the well. However, in recent years, it has been found that even equidistant from the well, earthquakes with the same focal mechanism sometimes caused different directions of the well water level changes. so far, studies have been little targeted with this phenomenon.In this paper, based on the work of predecessors, we statistically investigated the21co-seismic water level steps of Anxi well-1, Fujian province, China, from2004to July2011, induced by local and distant earthquakes with the magnitude more than4within1000km and more than5within4000km away from the epicenter. The preliminary conclusions are as follows:1) The Anxi well-1water level is sensitive to respond to earthquakes, not only occurring in the Taiwan region and local areas of the borehole, but also in far fields away from the well. The changes are characterized by a rapid step, including both increase steps and decrease steps, which recover to pre-earthquake levels after several days to several weeks according to the step sizes.2) The amplitudes of steps do not fully meet the statistical relationship to the magnitude and the epicenter distance. There are some significant changes following some small earthquakes far from the well, which deviated from the magnitude and epicenter distance constraints.3) The mechanism of earthquake-induced water level changes is very complex. At present, the static strain and dynamic strain changes caused by the earthquake, is often considered as the reason of water level changes. The static strain is related to the focal mechanism solutions. The dynamic strain is due to seismic wave propagation through the aquifer, and affected by the magnitude and the epicentral distance. Conducted to the focal mechanism of the earthquake rupture fault, we discuss the relationship between the morphology of the water level and the distribution in four quadrants and the principal stress orientation distribution based on the focal mechanism solution. The result shows that the correlation between them is not obvious. The spatial discrepancy of co-seismic water level changes can’t fully be explained by the focal mechanism.4) In Previous investigations on earthquake-induced water level changes, earthquake events accompanied large rainfall were often dropped. In this paper, combined with the rainfall data, we found that the co-seismic rise steps induced by earthquakes always occurred in accompaniment of rainfall which put loading up to the well area before earthquakes. The rainfall loading might change the local tectonic stress state within the well area, thus lead to an impact on co-seismic water level changes.5) The response of the well water level to solid tide plays an important role in investigating the aquifer characteristics. BAYTAP-G is used to extract five kinds of tidal wave component from water level observations and theoretical body strain data of earth tide such as the M2wave, the O1waves, K1wave, wave S2, and N2wave, accounting for more than95percent of all the tidal power. The phase difference between the two is calculated for qualitative analysis of the penetration of the medium near the borehole. Considering that M2is less affected by the atmospheric pressure, its amplitude tend to be relatively large and easily identified, and the phase change is relatively stable, we select the M2wave to carry on correlative studies. We found its phase lags change before and after the earthquake. Compared to the earthquakes occurring near the borehole and in Taiwan regions, there are bigger phase lag changes related to the large far-field earthquakes with the magnitude more than8in Chinese Mainland, Japan and Sumatra area. In addition, before and after the rainfall, the phase lag also changes significantly, indicating that rainfall produces not only loading effects, but also seepage to change the nature of the medium near the borehole, thus affects the response of the well water level to earthquakes.