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Hydrologic loads can stimulate seismicity in the Earth’s crust1. However, evidence for the triggering of large earthquakes remains elusive. The southern San Andreas Fault (SSAF) in Southern California lies next to the Salton Sea2, a remnant of ancient Lake Cahuilla that periodically filled and desiccated over the past millennium3,4,5. 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 Cahuilla5,6. To investigate possible causal relationships, we computed time-dependent Coulomb stress changes7,8 due to variations in the lake level. Using a fully coupled model of a poroelastic crust9,10,11 overlying a viscoelastic mantle12,13, 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 triggering7,8. The destabilizing effects of lake inundation are enhanced by a nonvertical fault dip14,15,16,17, the presence of a fault damage zone18,19 and lateral pore-pressure diffusion20,21. Our model may be applicable to other regions in which hydrologic loading, either natural8,22 or anthropogenic1,23, was associated with substantial seismicity. 

Abstract Stratigraphic evidence for coseismic subsidence has been documented in active-margin estuaries throughout the world. Most of these studies have been conducted in subduction zone or strike-slip settings; however, the stratigraphic response to coseismic subsidence in other tectonic settings would benefit from further study. Here we show evidence of late Holocene coseismic subsidence in a structural estuary in southern California. Below the modern marsh surface, an organic-rich mud containing marsh gastropods, foraminifera, and geochemical signatures indicative of terrestrial influence (mud facies) is sharply overlain by a blue-gray sand containing intertidal and subtidal bivalves and geochemical signatures of marine influence (gray sand facies). We use well-established criteria to interpret this contact as representing an abrupt 1.3 ± 1.1 m rise in relative sea level (RSL) generated by coseismic subsidence with some contribution from sediment compaction and/or erosion. The contact dates to 1.0 ± 0.3 ka and is the only event indicative of rapid RSL rise in the 7 k.y. sedimentary record studied. Consistent with observations made in previous coseismic subsidence studies, an acceleration in tidal-flat sedimentation followed this abrupt increase in accommodation; however, the recovery of the estuary to its pre-subsidence elevations was spatially variable and required 500–900 years, which is longer than the recovery time estimated for estuaries with larger tidal ranges and wetter climates. 

Observations of shallow fault creep reveal increasingly complex time‐dependent slip histories that include quasi‐steady creep and triggered as well as spontaneous accelerated slip events. Here we report a recent slow slip event on the southern San Andreas fault triggered by the 2017M_w8.2 Chiapas (Mexico) earthquake that occurred 3,000 km away. Geodetic and geologic observations indicate that surface slip on the order of 10 mm occurred on a 40‐km‐long section of the southern San Andreas fault between the Mecca Hills and Bombay Beach, starting minutes after the Chiapas earthquake and continuing for more than a year. Both the magnitude and the depth extent of creep vary along strike. We derive a high‐resolution map of surface displacements by combining Sentinel‐1 Interferometric Synthetic Aperture Radar acquisitions from different lines of sight. Interferometric Synthetic Aperture Radar‐derived displacements are in good agreement with the creepmeter data and field mapping of surface offsets. Inversions of surface displacement data using dislocation models indicate that the highest amplitudes of surface slip are associated with shallow (<1 km) transient slip. We performed 2‐D simulations of shallow creep on a strike‐slip fault obeying rate‐and‐state friction to constrain frictional properties of the top few kilometers of the upper crust that can produce the observed behavior.