Abstract
At 1:47 a.m. on September 21, 1999, the Mw 7.5 Chi-Chi earthquake struck Taiwan. The purpose of this study is to (1) apply multiple spatiotemporal-frequency analysis to filter the post-seismic change in groundwater head to explore the implicit drawdown associated with excess pore-water pressure release and effective stress relief during post-seismic evolution and (2) establish a stochastic experimental optimization model for identifying the hydrogeological evolution. The approaches used for post-seismic drawdown filtration include multi-rank principal components decomposition, multi-frequent wavelet transforms decomposition, and multi-level wavelet de-noising. This study especially evaluates the following advanced post-seismic evolving parameters: (1) harmonic average leaking/injecting rate, (2) distance between the acting position and monitoring well, (3) storage coefficient under effective stress relief and formation compression, and (4) transmissivity for excess-pore-water pressure release. This study applies the integrated methodology on 179 monitoring wells in the Chou-Shui River alluvial fan. Results show that the overlying principal components PCs and low-level wavelet de-noising can filter additional sources/sinks, in which the extracted drawdown from PC1+PC3 was related to the excess pore-water pressure relaxation process, that from PC2+PC5+PC6+PC7 and high-frequency wavelet de-noised detail cD2 related to the earth tidal fluctuation effect, and that from PC4+PC9 and cD3 related to the barometric effect. According to the Riemann integral and an objective function value duration curve, calculated occurrence probability from the stochastic optimization for SC2, the storage coefficient was reduced from pre-seismic pumping test value 0.00107, post-seismic 27th hour evolving value 0.000826 to post-seismic pumping test value 0.000578 in 2004, and the transmissivity increased from pre-seismic test value 92.4 m2/h, post-seismic 27th hour evolving value 98.6 m2/h to post-seismic test value 147.6 m2/h. The results demonstrate that the SC2 and GH3 zones suffer from crustal compression and the permeability was increased to dissipate excess pore-water pressure and effective stress.
Highlights
The crustal stress caused by earthquakes is a natural driving force, causing the groundwater head to change in a short period in a large area subject to a consistent stressor (Wakita 1975)
The instantaneous rise and fall of groundwater head induced by the earthquake are mainly produced by the fact that the stratum is squeezed by the seismic stress, causing the water pressure of the large area to rise, so that the excess pore-water pressure may be dissipated to the surface
Establishment of the stochastic experimental optimization model This study thoroughly considers all of the possible excess pore-water pressure release-related parameter (Q; r; S; T ) solutions in a specific post-seismic period under different kinds of initial conditions i, and boundary conditions b that best fit the filtered drawdown associated with the relaxation process using ensemble analysis scheme ID number α
Summary
The crustal stress (strain) caused by earthquakes is a natural driving force, causing the groundwater head to change in a short period in a large area subject to a consistent stressor (Wakita 1975). It provides a medium for understanding the hydrogeological evolution of the formation through earthquake-induced changes in groundwater head. The co-seismic step-like change in groundwater head is usually accompanied by a post-seismic recovery phenomenon, which represents the pore-water pressure in the stratum stabilizing This behavior is mainly controlled by the hydrogeological characteristics of the porous medium that can be identified from the dissipation or replenishment of pore-water pressure (Roeloffs 1996)
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