The Southwest US megadrought as a possible cause of the current seismic quiescence at Salton Trough, Southern California

Previous studies link the seismicity of the Salton Trough area over the past 2,000 years to the repeated flooding of Lake Cahuilla, whose modern successor is the Salton Sea. They suggest an active role of water processes for earthquake triggering through pore pressure propagation to faults. In this paper, earthquake data since 1900 are statistically analyzed to test two hypotheses: triggering by the lake filling and triggering by meteoric groundwater recharge. Binomial logistic regression and contingency analysis are used to assess the time correlation between the occurrence of M_W≥5.7 earthquakes, Salton Sea level fluctuations, and subsurface water recharge, using the Palmer Drought Severity Index (PDSI) as a proxy for the latter. It is found that only PDSI correlates with seismicity, indicating that groundwater recharge should be preferred over Salton Sea level rise as a possible triggering factor. In particular, the seismic quiescence of recent decades (just one earthquake in 38 years compared to 14 in the previous 88 years, averaging one every 6.3 years) may be related to the strict sequence of arid periods in the Southwest US, including the six-year drought of 1987-1992 (one of the two longest in the 20th century) and the megadrought of 2000-2021, the most severe in the last 1200 years. A similar correlation applies to the rest of Southern California, leading to the postulation of a large-scale effect of groundwater that acts beyond strictly local climate and geological conditions. The statistical results outlined here are not sufficient to demonstrate a physical connection between ongoing climate change and deep crustal processes, but they encourage further research in this direction.