The weight of water
On the Nakai Plateau in central Laos, the Nam Theun 2 hydropower project created a vast artificial lake. Construction of the 39-meter-high dam and its associated facilities began in 2005, with commercial operations starting in 2010. The dam impounds the Nam Theun river, creating the Nakai Reservoir, which at its full supply level covers a surface area of 450 square kilometers. This enormous body of water holds a total storage capacity of 3.9 billion cubic meters.
This is a case of reservoir-induced seismicity (RIS), a phenomenon where the immense mass of water in a new reservoir alters the stresses and pressures in the underlying rock. The simple weight of the water, totaling billions of tonnes, is enough to clamp down on existing underground faults. The water also increases the pore pressure within the rock—forcing its way into microscopic cracks and fissures. This increased pressure can effectively lubricate a fault that has been locked for centuries, allowing it to slip and release energy as an earthquake.
A predictable seismic response
The filling, or impoundment, of the Nam Theun 2 reservoir began in 2008. The seismic activity in the region, which was previously very low, began to increase almost immediately. Scientists monitoring the area with a network of seismometers detected a clear correlation between the rising water level and the frequency of tremors. The seismic events were concentrated in a specific area directly beneath the deepest part of the reservoir.
The activity culminated in a series of noticeable earthquakes. The largest of these events reached a magnitude of 4.6, strong enough to be felt by the local population. This mainshock was part of a swarm of hundreds of smaller seismic events, all clustered at shallow depths of less than 10 kilometers beneath the reservoir. The geological setting of the Nakai Plateau, which consists of Mesozoic sandstones and mudstones, contained pre-existing faults that were reactivated by the new load from the reservoir. The seismic response at Nam Theun 2 shows how rapidly human engineering projects can alter crustal stability.