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Earthquakes result from strain build-up from without and weakening from within faults. A generic co-seismic condition is presented that includes just three angles representing, respectively, fault geometry, fault strength, and the ratio of fault coupling to lithostatic loading. Correspondingly, gravity fluctuations, bridging effects, and granular material production/distribution form an earthquake triad. As a dynamic constituent of the gravity field, groundwater fluctuation is the nexus between the triad components. It is pivotal in regulating major seismic irregularity, by reducing natural (dry, or purely tectonic, stationary seismicity) inter-seismic periods and by lowering magnitudes. Specifically, to exert stress on the fault, groundwater does not need to reside deep in proximity to the locked fault interface, as it can work remotely. It can act mechanically-direct (MD), by a differential de-loading and superimposing a seismogenetic lateral stress field, thereby aiding plate-coupling, from without, or mechanically-indirect (MI) by enhancing fault fatigue, and hence weakening the fault, from within. To verify this hypothesis, gravity measurements, and a numerical model, are used. The remote action hypothesis is globally applicable. Detailed results are presented for the Himalayan and New Zealand regions. The gravity recovery and Climate experiment (GRACE measurements) reveals that major earthquakes (Mw 5 and above) always occur in the dry stage, indicating drought and associated groundwater extraction is an important trigger for major earthquakes. By exploring 73 historical records successfully reproduced by the model, it is found that for collisional (e.g., the peri-Tibetan Plateau) and strike-slip (e.g., the San Andreas Fault) systems, the MD mechanism dominates, because the orographically induced spatially highly variable precipitation is channeled into greater depth by through-cut faults. Droughts elsewhere also are seismogenetic, but likely through MI effects. In a warming future climate, mechanisms identified here play a greater role in increasing the recurrence frequency of major earthquakes, but also in slightly reducing their severity.

How Droughts Influence Earthquakes