dc.description.abstract | Fluctuations of Water level in wells often are in response to changes of volumetric strains caused by tectonic deformation or by passage of seismic waves. Continuous monitoring of water level fluctuations in two wells, coded HLC-05 and HLC-03, in Hualien area of eastern Taiwan has been made since 2002 by digital recorders with high sampling rates at 1- and 6-second intervals, respectively. The data thus far show that fluctuations of water level are often induced by earthquakes. The results can be summarized as follows: 1) Observations of water level fluctuations can be induced by earthquakes of magnitude M³0.43+2.39log10D, where D is the hypocenter distance from the well; 2) The peak water level fluctuation (PWL) is linearly proportional to either the peak ground velocity (PGV) or peak ground displacement (PGD) on logarithmic scales, and the correlation coefficients of the former one are larger than those of the latter one. On the contrary, no clear trends with the peak ground acceleration (PGA) are found.
A well, coded HLC-05, with high sampling rate of 1 Hz for monitoring the water level changes has been installed since May 2002 in Hualien, eastern Taiwan. The changes of tectonic strains induced by earthquakes have led to changes of water level in the well, including the static (steps) and dynamic behaviors (fluctuations). Static water level changes, with which water level raises to a step persistently for about 30 seconds and then decreases gradually to its background trend which is in response to the long period effects (i.e., earth tides and rainfalls), induced by three events (ML4.5, 5.8, and 6.5) are well recorded so far. Based on a dislocation model of a finite fault plane, we try to describe the correlation between volumetric strain changes and water level steps. Due to lack of pumping tests, an one-dimensional diffusion equation (Crank, 1975; Brodsky et al., 2003) applied to a blockage system can provide a good estimate of stratum parameter (i.e., hydraulic diffusivity) from the continuous drops of water level as function of lapse time. From the results of observation from the three events, we have determined three values of hydraulic diffusivity at 0.21, 0.17, and 0.17 m2/sec, correspondently, with an average value of 0.18 m2/sec. The results can be verified by the future events or by pumping tests in the well. | en_US |