On the enhanced post-impoundment seismicity in the Three Gorges Reservoir region, China

Since the impoundment of the Three Gorges Reservoir (TGR) in the year 2003 in central China, the region is experiencing enhanced seismic activity. The enhanced seismicity after the TGR impoundment has been reported to be associated with several factors, e.g., Karst collapse, mine collapse, chemical effects due to water percolation, and reservoir-assisted shear failure on the mapped and seismological faults. Here in this article, we explore in detail the role of reservoir impoundment in mobilizing the nearby faults leading to increase in post-impoundment seismicity of the region. For this purpose, reservoir induced stress, pore pressure and their influence on subsurface faults, in terms of fault stability, are calculated to explore the role of TGR in inducing shear failure on the earthquake causative faults. Our analysis suggests that some of the areas of enhanced post-impoundment seismicity can be explained by the shear failure due to the reservoir impoundment. But a large region, despite being under the unfavourable influence of reservoir induced stress, also exhibit enhanced post-impoundment seismicity. Even the pore pressure due to the reservoir impoundment is not enough to mobilize these faults in these unfavourable regions. An extremely high pore pressure or some other mechanism, involving fluid interaction with rock mass due to the reservoir impoundment, is required to explain the enhanced seismicity in such regions. We suggest that dissolution and reduced cohesion in the Karst-Carbonate rocks present in the region also assisted in the enhancement of the post-impoundment seismicity. These post-impoundment earthquakes may be termed as fluid-assisted earthquakes in the TGR region rather than earthquakes linked with reservoir induced shear failure. Further, some of the post-impoundment earthquakes of relatively large magnitude which occurred in the region of pre-impoundment seismicity could be purely tectonic in nature and not influenced by the reservoir impoundment. Thus, we suggest that along with the TGR induced shear failure, various other factors also play significant role in the increase of post-impoundment seismicity.