Abstract
Two and a half years prior to China’s M7.9 Wenchuan earthquake of May 2008, at least 300 million metric tons of water accumulated with additional seasonal water level changes in the Minjiang River Valley at the eastern margin of the Longmen Shan. This article shows that static surface loading in the Zipingpu water reservoir induced Coulomb failure stresses on the nearby Beichuan thrust fault system at <17 km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416 ± 96 km^{2}. These stress perturbations, in turn, likely advanced the clock of the mainshock and directed the initial rupture propagation upward towards the reservoir on the "Coulomblike" Beichuan fault with rate and statedependent frictional behavior. Static triggering perturbations produced up to 60 years (0.6%) of equivalent tectonic loading, and show strong correlations to the coseismic slip. Moreover, correlations between clock advancement and coseismic slip, observed during the mainshock beneath the reservoir, are strongest for a longer seismic cycle (10kyr) of M > 7 earthquakes. Finally, the daily event rate of the microseismicity (M ≥ 0.5) correlates well with the static stress perturbations, indicating destabilization.
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Acknowledgments
The author is grateful to Think Geohazards for its generous financial support. He also thanks the five anonymous reviewers for their constructive critiques and C.H. Scholz and L. Seeber from LamontDoherty Earth Observatory for their suggestions and comments to improve this manuscript.
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Appendix
Appendix
Everyday, the moon and sun cause tidal elongations on earth (Bartels 1985). These daily pull/push effects, in turn, induce stabilizing and destabilizing stresses on preexisting fault zones in the earth’s crust and are independent from any geological forces on earth, including endogenous forces (e.g., volcanism, tectonics) and exogenous forces (e.g., erosion, sedimentation). Moreover, it has been shown that tidal stress changes have weak effects on triggering medium to largesize earthquakes (Beeler and Lockner 2003a). Thus, it can be anticipated that any triggering stress perturbation on the earth’s crust must exceed at least stress levels resulting from the tidal elongation of the earth. Analytically, it can be shown how high these tidal stress changes are.
Let’s assume, F _{0} is the earth’s gravitational potential, which results from both attraction force and centrifugal force of the earth and moon/sun. Tidal forces V, however, deform F _{0}:
This results in a vertical surface displacement ξ with respect to the average value of the gravitation acceleration on earth g = 9.798 ms^{−1}.
Tidal forces change with geocentric zenith distance θ from the moon/sun (Bartels 1985), whereas the main term of the tidal potential is
with the lunar tidal constant G _{ l } = 2.6206 m^{2} s^{−2} and the solar tidal constant G _{ s } = 1.2068 m^{2} s^{−2}, the radius of the earth r _{ E } and the mean radius of the earth \(\overline r_{\rm E} = 6371.221\) km. Assuming the earth is a sphere \((r_{\rm E} = \overline r_{\rm E})\), the general form of vertical surface displacement is
The displacement due to the moon and sun is
Thus, the ξ_{ l } and ξ_{ s } are amplified at the zenith (θ = 0°) by 0.356 m and 0.164 m. On the other hand, they are depressed at the nadir (θ = 90°) by 0.178 m and 0.082 m. The peaktrough difference for the moon is 0.534 m and 0.246 m for the sun.
Both vertical displacements induce maximal shear stresses τ and normal stresses σ_{ n } on preexisting faults (e.g., dipping 45°) in the earth’s crust with an average shear modulus of about G = 30 GPa and friction angle of, let’s assume, ϕ of 29°:
Thus, maximum induced stresses that need to be exceeded by any additional triggering stress (e.g., due to surface loading) are:

by the moon τ_{ l } = 5.03 kPa and σ_{ n,l } = 9.07 kPa,

by the sun τ_{ s } = 2.32 kPa and σ_{ n,s } = 4.18 kPa.
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Klose, C.D. Evidence for anthropogenic surface loading as trigger mechanism of the 2008 Wenchuan earthquake. Environ Earth Sci 66, 1439–1447 (2012). https://doi.org/10.1007/s1266501113557
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Keywords
 Earthquake
 Geomechanics
 Geoengineering
 Triggered Earthquakes
 Water Reservoir
 Tides
 Sun Moon
 Gravitation
 Seismology