The increase in seismic activity caused by the filling and emptying of large reservoirs is called reservoir-induced seismicity (RIS). Early reports of RIS were associated with the Quedd Fodda Dam in Algeria where, in 1932, a magnitude 3 earthquake was reported at a very shallow depth of only 300 meters, and in Greece at the Marathon Dam several quakes were reported up to 5.7 in magnitude in the years following impoundment.
The two causes of RIS are the load of the reservoir's weight and water injection into existing fault zones. Both of these actions affect the pore pressure in rocks in the fault zone. Pore pressure is the pressure of groundwater held within soil or rock, in the gaps or pores between grains. Increasing the pore pressure in a fault zone is thought to have the effect of lessening the friction between particles or grains and allowing a fault to slip, or having the effect of lubricating the fault.
As a reservoir fills it can compact the more brittle stone beneath, as the stone compacts the pore spaces get smaller and the pore pressure increases. This report suggests compaction by the reservoir load of Lake Kariba, on the Zambezi river, the largest man made reservoir by volume, increased stresses on a previously stressed fault by only a few percent causing some 50 tremors located 15 to 65 kilometers down stream from the dam, too far to have been triggered by fluid intrusion pressures.
The second way pore pressure can be increased by filling a reservoir is for the water from the dam to intrude into fissures or small cracks in the bedrock until it penetrates to levels where the fault stresses exist. The resulting increase in pore pressure can trigger earthquakes in previously stressed faults. The above mentioned report on Lake Kariba suggests that on the west side of the lake some 200 tremors were reported that may have been caused by intrusion of water into the fault zone.
A more recent report concerning the huge Three Gorges Dam on the Yangtze River shows a 30 fold increase in seismic activity since the dam reservoir started being filled in 2003. At least one quake reached a magnitude of 4.1. RIS from the Zipingpu dam in Sichuan province of China has also been suggested as the trigger for the 2008 earthquake that killed almost 90,000.
RIS is a topic avoided by dam operators due to liability concerns, since added earthquake resistance to buildings in the area may be needed. The desire for cheap hydroelectric power and fresh water for agriculture causes many third world countries to overlook or play down the risks of human induced seismic activity.