The Influence of Lai Chau Reservoir Impoundment on Seismicity in the Area of Lai Cu (Vietnam)

The accumulation of water in large reservoirs can affect seismic activity due to changes in stress and pore pressure in the bedrock. This paper presents the results of a study on the influence of accumulation of the Lai Chau reservoir in northern Vietnam on local seismic activity. The reservoir was filled in June 2015. It was observed that although seismicity rates and b-values differed markedly in the months immediately following impoundment, the longer-term seismicity around the Lai Chau reservoir before and after its impoundment is broadly similar. The spatial distribution of earthquakes and stress changes induced by water loading and increased pore pressure were investigated. The load-induced stress field was computed from the pre-impoundment bathymetry of the valley and resolved without assuming any orientation of the regional stress field. Pore pressure diffusion was evaluated for three end-member scenarios of fault zone permeability and classified by the actual hypocentral depth of each event, thereby allowing the triggered subset to be bracketed in the absence of in situ hydraulic data. It was found that this influence was limited to the immediate vicinity of the reservoir, where the stress level reached values that allowed for the initiation of an earthquake. The highest seismicity was observed in the southeastern part of the lake, in the area of positive changes in Coulomb stress. The results suggest that hydraulic operations can trigger local seismic events where pre-existing faults close to failure lie within a few kilometers of the shoreline and at shallow depth. However, tectonic processes remain the dominant cause of seismicity in the region.