Background seismicity and seismic monitoring in the Lai Chau reservoir area

Reservoir-triggered seismic activity depends not only on the technical characteristics of the future reservoir (filling volume, the height of water column) but also on the seismo-tectonics and the natural seismic processes occurring in the area before construction of an artificial reservoir. Passive seismic monitoring was realised near Lai Chau (Vietnam) before the impoundment started. It allowed exploration of the natural seismicity in the area of the future dam. Locations of seismic events several months prior to the reservoir impoundment were observed with ten stations installed in the reservoir vicinity. Events were mainly located near the dam along the Da river headwaters fault. However, only four stations were available for the entire period before the impoundment. Despite the network limitations, completeness of seismic catalogue and b value were determined and may be used as a baseline for analysis of the seismicity in this area after impoundment. The magnitude completeness level is significantly smaller than in the broader seismogenic zones covering an area of the dam. The b value differs from the results obtained for regional seismicity of Northeastern Vietnam. The capability of the local network for moment tensor inversion was estimated with the use of synthetic data tests. Test results provided the requirements for the station number according to azimuthal coverage of the network to obtain the reliable full moment tensor (MT) solution. Preliminary analysis of the seismic activity after Lai Chau reservoir impoundment indicates some changes in activity related to the impoundment and reservoir exploitation.