Evidence for Anthropogenic Surface Loading as Trigger Mechanism of the 2008 Wenchuan Earthquake

TL;DR

This study provides evidence that human-induced surface loading from water reservoir accumulation likely triggered the 2008 Wenchuan earthquake by inducing stress changes on nearby faults, advancing earthquake timing and increasing seismic activity.

Contribution

It demonstrates that static surface loading from the Zipingpu reservoir significantly influenced fault stress and earthquake timing, a novel insight into anthropogenic earthquake triggers.

Findings

Reservoir water loading induced Coulomb failure stresses on the fault.

Stress perturbations correlated with increased seismic activity and fault slip.

Static loading effects equated to up to 60 years of tectonic stress accumulation.

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 <17km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416+/-96km^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 "Coulomb-like" Beichuan fault with rate-and-state dependent 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 micro-seismicity (M>0.5) correlates well with the static stress perturbations, indicating destabilization.