Influence of the impoundment of the Three Gorges Reservoir on the micro-seismicity and the 2013 M5.1 Badong earthquake (Yangtze, China)

Abstract

On December 16, 2013, right after the Three Gorges Reservoir (TGR) reached its highest annual water level, a powerful M5.1 earthquake occurred in Badong County, China's Hubei Province. The epicenter is 5.5km away from the upstream boundary and 100km from the dam. Was this earthquake triggered by the impoundment of the TGR, and what are itssubsequences? To answer these questions, we constructed a coupled three-dimensional poroelastic finite element model to examine the ground surface deformation, the Coulomb Failure Stress change (∆CFS) due to the variation of elastic stress and pore pressure, and the elastic strain energy potential accumulation in the TGR region upon the occurrence of this event.Our calculated maximum surface deformation values beneath the TGR compare well with GPS observations, which validates our numerical model. At the hypocenter of the earthquake, ∆CFS is around 8.0~11.0kPa, revealing that it may be eventually triggered by the impoundment. We also discovered that the total elastic strain energy potential accumulation due to the impounded water load is around 1.7×10¹²J, merely equivalent to 0.01% of the total energy released by this event, indicating that the this earthquake is predominately controlled by the typical regional 2 tectonic settings as well as the weak fault zones, and the reservoir impoundment might only facilitate its procedure or occurrence. Furthermore, the stress level in this region remains high after this earthquake and the subsequent reservoir-triggered micro-seismicity or even bigger event are highly possible.

Seismic and Geologic Evidence of Water‐Induced Earthquakes in the Three Gorges Reservoir Region of China

“…Using the time delay (35 days) of the earthquake relative to the peak water level date (12 November 2013) and its distance (9 km) to the SNX/LCH river confluence, we estimated the average pore pressure diffusion coefficient along the hydraulic conductive channel to be 27 m 2 /s. It is about 1 to 2 orders of magnitude larger than previously used (Ge et al, 2009;Zhang et al, 2016). Only six small events of magnitudes 1-2 were reported by CENC before the mainshock during the maximum water level period in the region.…”

“…In addition, water in the fault zone promotes failure by reducing material cohesion and friction coefficients through chemical dissolution, particularly in carbonate rocks. Zhang et al (2016) calculated the CFS change at the Badong mainshock hypocenter produced by the reservoir impoundment using a 3-D poroelastic finite element model. They found an increase of CFS from 2003 to the time of the earthquake by 8.0-10.0 kPa, which has been shown to trigger earthquakes in many cases (e.g., Reasenberg & Simpson, 1992;Stein, 1999).…”

“…Seismicity in the TGR region was low before the completion of the TGD in 2003 but increased by 7-8 times in terms of average monthly earthquake counts after the water level of the reservoir was raised to Most earthquakes in the 2013-2014 period occurred in two clusters and seem to be associated with GQF and Xiannvshan Fault, respectively (Figure 1). Given the proximity of these earthquakes to TGR and low background seismicity before TGD, they were generally believed to be reservoir induced (e.g., Chen & Su, 2004;Chen et al, 2014;Ma et al, 2010;Yi et al, 2012;Zhang et al, 2016). Possible mechanisms include the collapse of karstic caves under the reservoir load, slip on certain interlayer interfaces weakened by reservoir water, and tectonic earthquakes triggered by stress changes due to the impoundment.…”

Variations in seismic parameters for the earthquakes during loading and unloading periods in the Three Gorges Reservoir area

“…Moreover, multiple sets of joint fissures are developed in these areas, which have direct hydraulic relationships with reservoir water. A karst conduit system is developing in the Badong area, and Triassic weak detachment layers are developing in the area, which are conducive to the occurrence of reservoir-induced earthquakes 26 .…”

“…When the Badong M5.1 earthquake occurred, the coulomb stress at the source area was greater than 0.1 MPa and indicated a high risk at this time. During the high-water-level operation, pore pressure diffusion under drainage effect especially along the fault and large porosity rock mass resulted in the occurrences of moderate earthquakes 26 .…”