Tidal response variation and recovery following the Wenchuan earthquake from water level data of multiple wells in the nearfield

Abstract

An important dataset to emerge from the Wenchuan earthquake Fault Scientific Drilling project is direct measurement of the permeability evolution of a fault zone. In order to provide context for this new observation, we examined the evolution of tidal responses in the nearfield region (within~1.5 fault lengths) at the time of the mainshock. Previous work has shown that seismic waves can increase permeability in the farfield, but their effects in the nearfield are more difficult to discern. Close to an earthquake, hydrogeological responses are generally a combination of static and dynamic stresses. In this work, we examine the well water level data in the region of the large M w 7.9 Wenchuan earthquake and use the phase shift of tidal responses as a proxy for the permeability variations over time. We then compare the results with the coseismic water level pattern in order to separate out the dynamic and static effects. The coseismic water level pattern for observed steps coincident with the Wenchuan mainshock mainly tracks the expected static stress field. However, most of the wells that have resolvable tidal responses show permeability enhancement after this large earthquake regardless of whether the coseismic response for the well water level is increasing or decreasing, indicating permeability enhancement is a distinct process from static poroelastic strain.

Comparison of Regression Methods to Compute Atmospheric Pressure and Earth Tidal Coefficients in Water Level Associated with Wenchuan Earthquake of 12 May 2008

“…5b). Figure 5c shows co-seismic response of water level (step change in water level) which is similar to the observations made by several others SHI et al 2012;LEE et al 2012;LAI et al 2014). After the earthquake, water level is slowly increasing (Fig.…”

“…Earth tidal effect on the well water level (BREDEHOEFT 1967;HEATON 1975;KUMAR et al 2012;LAI et al 2013LAI et al , 2014DONG et al 2015) refers the micro-dynamic state of the well water level, which is induced by the deformation of aquifer through the variation of the gravitational attraction of Sun and Moon and other celestial bodies. When the distance is close, the gravitational attraction is large and the earth is relatively expanded; when the distance is far from Earth, the gravitational attraction is small and the earth is relatively compressed.…”

“…The atmospheric pressure effect of the well water level (WEEKS 1979;LAI et al 2013LAI et al , 2014DONG et al 2015) means the micro-dynamics of water level in the well, which is caused by the fluctuation of atmospheric pressure. When the atmospheric pressure increases, the well water declines (the observed value of water level becomes bigger); when the atmospheric pressure decreases, the well water rises (the observed value of water level becomes smaller).…”

“…in boreholes as well numerous physical and geophysical parameters on the earth crust to find out reliable earthquake precursors (SUN and LIU 2012;HE et al 2012;LIU et al 2013;LAI et al 2014;YAN et al 2014;SHI and WANG 2015;YE et al 2015). Most of the parameters are being monitored since last several decades and efforts have been made to deploy modern instrumentations with automatic data recorders to collect good quality of data.…”

Mechanism of co-seismic water level change following four great earthquakes – insights from co-seismic responses throughout the Chinese mainland

“…These include: (1) poroelastic response to coseismic static strain (Ge and Stover, 2000;Jónsson et al, 2003;Shi et al, 2013b); (2) undrained consolidation of sediments (Wang and Chia, 2008); (3) clogging or unclogging pore and fractures by oscillatory flows produced by passing seismic waves Elkhoury et al, 2006;Lai et al, 2014;; (4) co-seismic gas bubble nucleation and growth (Crews and Cooper, 2014;Linde et al, 1994); (5) shaking-induced compaction or dilatation (Bower and Heaton, 1978;Zhang et al, 2015).…”