Analysis of Groundwater Variation Induced by Earthquake in the Jianan Plain.

In 2010, Jia-Sian earthquake hit Taiwan, causing casualties, house collapses, and landslides, also caused many liquefaction cases in the alluvial plain of southern Taiwan. Groundwater level changes during earthquakes were recorded widely. The groundwater monitoring network established by the Water Resources Agency to the Jianan Plain recorded the groundwater level changes during the Jia-Sian earthquake. In this research, records of six wells were chosen for simulation from Jianan Plain study area. They are Ju-Wei, Guei-Lin, Ping-Shi, Huan-Ya, Shia-Ying and Tzung-Ye. A simplified stratigraphic column profile was used for the water-rich strata in the earthquake simulation. Results were compared with the data of groundwater monitoring network for validation of water level changes using different modes of pore water excitation for the seismic analysis. In order to understand the seismic behavior of water-rich strata, this research uses SHAKE program for deconvolution of ground motion record as the base motion of groundwater monitoring wells. Then FLAC was used with the simplified stratigraphic profile with input acceleration history for simulating the groundwater level changes caused by earthquakes. Conceptual model of pore water pressure seismic response is constructed using the finite difference program, FLAC where both Finn and Biot modes of pore water pressure generation were used for analysis. Analysis showed that simulation of Finn mode is better than Biot mode. In addition, There are two types of groundwater level changes, step-wise and oscillating changes in response. The simulated water pressure curve from the Finn mode shows reactions of step-wise changes, and Biot mode exhibits oscillating changes. Finn mode triggered a large amount of the pore water pressure, while in Biot mode the pore pressure change is limit to some range. Comparing numerical results with the observed data, the results of input primary-direction ground acceleration is better than input E-W and N-S ground acceleration. This is related to the fault dislocation and seismic wave transmission direction. Therefore, the simulation results with Primary ground acceleration is the best. Generally speaking, simulation results of Finn mode of Aquifer 1 and 2 are fairly good. There are differences between the simulation results in Finn mode and the recorded groundwater level changes. The variations of simulated results with the peak ground acceleration are positively correlated, but irrelevant with the epicenter distance. Biot mode in ground water changes results in a small range, and only Aquifer 1can obtain good simulation results, when the depth increasing, the simulation of pore pressure change has not changed much. Finally, by using the parameters of the simulation results to calculate the coefficient of consolidation in the observation well, the variations of simulated results with the coefficient of consolidation are found good correlation. In addition, the change of coefficient of consolidation also correlated well with the peak ground acceleration.