Major southern San Andreas earthquakes modulated by lake-filling events.

Hydrologic loads can stimulate seismicity in the Earth's crust 1 . However, evidence for the triggering of large earthquakes remains elusive. The southern San Andreas Fault (SSAF) in Southern California lies next to the Salton Sea 2 , a remnant of ancient Lake Cahuilla that periodically filled and desiccated over the past millennium 3-5 . Here we use new geologic and palaeoseismic data to demonstrate that the past six major earthquakes on the SSAF probably occurred during highstands of Lake Cahuilla 5,6 . To investigate possible causal relationships, we computed time-dependent Coulomb stress changes 7,8 due to variations in the lake level. Using a fully coupled model of a poroelastic crust 9-11 overlying a viscoelastic mantle 12,13 , we find that hydrologic loads increased Coulomb stress on the SSAF by several hundred kilopascals and fault-stressing rates by more than a factor of 2, which is probably sufficient for earthquake triggering 7,8 . The destabilizing effects of lake inundation are enhanced by a nonvertical fault dip 14-17 , the presence of a fault damage zone 18,19 and lateral pore-pressure diffusion 20,21 . Our model may be applicable to other regions in which hydrologic loading, either natural 8,22 or anthropogenic 1,23 , was associated with substantial seismicity.