A tectonically active foundation
The Garzê Tibetan Autonomous Prefecture in Sichuan, China, is one of the most seismically active zones in the country. It sits on the eastern edge of the Tibetan Plateau, a region defined by the immense geological forces of the ongoing collision between the Indian and Eurasian tectonic plates. This constant pressure is accommodated by a series of major fault systems that slice through the crust.
The most significant of these is the Xianshuihe fault system, a 1,400-kilometer-long network of active left-lateral strike-slip faults. This system has been responsible for more than 20 earthquakes of magnitude 6.5 or greater since 1700. The fault moves at a considerable rate, with some segments slipping as much as 10 to 15 millimeters per year. This continuous motion builds up stress in the Earth's crust, which is released in the form of earthquakes. The region naturally experiences frequent seismic events, including earthquake swarms—a series of many small tremors without a distinct mainshock.
The weight of water
In recent decades, extensive hydroelectric development has taken place in this region, leading to the construction of large dams and massive reservoirs. While these projects provide power, they also introduce a new variable into the area's delicate geological balance. The phenomenon is known as reservoir-triggered seismicity (RTS).
There are two primary mechanisms by which a reservoir can trigger earthquakes. First, the immense weight of billions of cubic meters of water adds a significant load to the crust, altering the stress on underlying faults. Second, and more influential, is the effect of increased pore pressure. As the reservoir fills, water is forced deep into the ground, penetrating cracks and fissures in the rock. This increased water pressure can effectively "lubricate" a fault that is already under tectonic stress, allowing it to slip and cause an earthquake.
Seismologists have observed a direct correlation between the filling and water-level fluctuations of reservoirs and the frequency of local earthquakes. For example, after the Zipingpu Dam, located just five kilometers from the epicenter of the 2008 Wenchuan earthquake, was completed in 2006, seismic activity in the area changed. Similarly, after the Three Gorges Dam began filling in 2003, monitors recorded a 30-fold increase in seismic frequency in the surrounding area by the end of 2009. By precisely locating earthquake epicenters and analyzing their timing relative to changes in reservoir water levels, scientists can distinguish these triggered events from the region's natural tectonic activity.