Loading of the San Andreas fault by flood-induced rupture of faults beneath the Salton Sea

The southern San Andreas fault has not experienced a large earthquake for approximately 300 years, yet the previous five earthquakes occurred at ∼180-year intervals. Large strike-slip faults are often segmented by lateral stepover zones. A Movement on smaller faults within a stepover zone could perturb the main fault segments and potentially trigger a large earthquake. The southern San Andreas fault terminates in an extensional stepover zone beneath the Salton Sea-a lake that has experienced periodic flooding and desiccation since the late Holocene. Here we reconstruct the magnitude and timing of fault activity beneath the Salton Sea over several earthquake cycles. We observe coincident timing between flooding events, stepover fault displacement and ruptures on the San Andreas fault. Using Coulomb stress models, we show that the combined effect of lake loading, stepover fault movement and increased pore pressure could increase stress on the southern San Andreas fault to levels sufficient to induce failure. We conclude that rupture of the stepover faults, caused by periodic flooding of the palaeo-Salton Sea and by tectonic forcing, had the potential to trigger earthquake rupture on the southern San Andreas fault. Extensional stepover zones are highly susceptible to rapid stress loading and thus the Salton Sea may be a nucleation point for large ruptures on the southern San Andreas fault. © 2011 Macmillan Publishers Limited. All rights reserved

Vertical deformation through a complete seismic cycle at Isla Santa María, Chile

Individual great earthquakes are posited to release the elastic strain energy that has accumulated over centuries by the gradual movement of tectonic plates. However, knowledge of plate deformation during a complete seismic cycle—two successive great earthquakes and the intervening interseismic period—remains incomplete. A complete seismic cycle began in south-central Chile in 1835 with an earthquake of about magnitude 8.5 and ended in 2010 with a magnitude 8.8 earthquake. During the first earthquake, an uplift of Isla Santa María by 2.4 to 3 m was documented4,5. In the second earthquake, the island was uplifted by 1.8 m. Here we use nautical surveys made in 1804, after the earthquake in 1835 and in 1886, together with modern echo sounder surveys and GPS measurements made immediately before and after the 2010 earthquake, to quantify vertical deformation through the complete seismic cycle. We find that in the period between the two earthquakes, Isla Santa María subsided by about 1.4 m. We simulate the patterns of vertical deformation with a finite-element model and find that they agree broadly with predictions from elastic rebound theory. However, comparison with geomorphic and geologic records of millennial coastline emergence reveal that 10–20% of the vertical uplift could be permanent