Can large amounts of rain in California cause an increase in earthquakes?

There is no pattern in California of large, damaging earthquakes occurring more frequently during periods of heavy precipitation or periods of drought. Therefore, it’s unlikely that seismic hazard is affected by precipitation. This makes sense as rainwater cannot easily percolate several kilometers below the earth's surface to the depths where most earthquakes occur.

However, periods of heavy precipitation or of drought might indirectly affect earthquake-prone faults. For example, patterns of precipitation can affect the weight of near-surface materials and of reservoirs. Also the refilling of subsurface aquifers may cause the Earth’s crust to expand (by a few millimeters) in some locations and contract in others, with the opposite occurring during dry periods. These can cause small changes in the stresses on faults that--in principle--could slightly influence rates of seismicity.

A few studies have found that rates of microearthquakes in California may undergo small variations that are likely related to precipitation patterns. For example, scientists at Stony Brook University studied continuous records of motion from Global Navigation Satellite Systems (GNSS) to detect annual cycles of expansion and contraction of the Earth’s surface, and related them to patterns and intensity of precipitation. Using computer models, they demonstrated that annual variations in stress on California’s faults are larger during years of unusually heavy precipitation or drought. This 2021 study concluded that slightly more earthquakes might be expected in portions of eastern California during winters with heavy precipitation than in winters of drought, and, in contrast, that slightly fewer earthquakes might be expected on the San Andreas Fault during heavy precipitation winters in comparison to normal or drought winters.

Additional studies suggest that a large water load that’s uniformly distributed during the rainy season can suppress seismicity, while surface water that is unevenly distributed (for example water that's concentrated in a valley or on a mountain) has a small potential to increase seismicity. This is an area of ongoing research.