Could All the Rain in California Trigger Earthquakes?

California has experienced torrential rainfall in recent days. Parts of the state have seen one of the wettest periods since the 1800s.

And, some of the other regions, such as San Francisco, have received over 12 inches of rain in the past 15 days, the National Weather Service reported.

But, could this heavy rain cause an earthquake?

The state is extremely tectonically active due to the San Andreas Fault—the border section between two massive tectonic plates under the Earth's surface. These plates remain relatively static, meaning large pressures build up over time, sometime leading to highly destructive earthquakes.

And, intense rain has triggered earthquakes in the past, in other parts of the world.

Shimon Wdowinski, a professor at the Department of Earth and Environment at Florida International University, has studied the topic extensively after assessing links between storms and earthquakes in 2010.

This interest was prompted following an incredibly destructive magnitude 7.0 earthquake that devastated Haiti in 2010—about 18 months after torrential rainfall hit the region during a procession of hurricanes and prolonged heavy rain.

A similar situation occurred in Taiwan in 2009, when a magnitude 6.4 earthquake occurred seven months after a typhoon.

Focusing on Taiwan, Wdowinski's research concluded that large earthquakes were five times more likely to happen following severe storms.

A study published in 2021, by scientists at the Chinese Academy of Meteorological Sciences, also found a strong correlation between earthquakes over a magnitude of 6, and rainfall. It found that 74.9 percent of earthquakes in China where accompanied with rainfall.

But Wdowinski told Newsweek, that while "heavy rain can trigger earthquake activity," this only occurs "in certain tectonic environments, not anywhere."

So, while rain has triggered earthquakes elsewhere in the world, could it happen in California?

Will Heavy Rainfall Trigger an Earthquake?

The main risk linked to rainfall and earthquakes is erosion. If rainfall is heavy enough that it begins to erode a fault, these faults could loosen, subsequently causing an earthquake.

Raymond Russo, an Associate Professor at the Department of Geological Sciences at the University of Florida, told Newsweek that the risk of an earthquake due to heavy rains is very small. But there are situations where it could occur.

"A viable scenario is when rainwater infiltrates into a fault zone that is close to failure [i.e., will slip seismically if any triggering event occurs] and decreases the normal stress [think pressure pushing the two sides of the fault together, inhibiting slip] on the fault. A decrease in the normal stress across the fault could lead to failure and seismic slip if the fault is primed and ready to go anyway," Russo said.

If there were hypothetically an increase in the seismicity in California due to these heavy rains, it would only be in certain areas where there is karstified limestone.

"Rain itself has little impact on earthquake genesis, unless it is karstified limestone (which carbonates)," Stephen A. Miller, a professor at the Centre for Hydrogeology and Geothermics at the University in Neuchâtel, Switzerland told Newsweek.

"Heavy rains can fill these networks, resulting in large increases in the hydraulic head—the height of the water column. In this case, the high fluid pressure at the base of the water column can be transmitted to depth and induce earthquakes from increasing fluid pressures."

Filling reservoirs may also be at risk of this, as this leads to additional load on the subsurface, Miller said.

"So my take would be if there is a rain-induced increase in seismicity in California, it will be localized in areas with carbonates, or around reservoirs," Miller said.

But Wdowinski said earthquakes due to rainfall only really occur in convergent tectonic environments—where two tectonic plates move toward one another and cause uplift of the crust, as is the case in Taiwan.

"In this case, heavy rain can lead to significant surface erosion, which can change stresses in the crust and affect timing of earthquake activity. However, this is not the case in California, where the two tectonic plates (Pacific and North America) move horizontally with respect to one another," Wdowinski said.

"In such environment, erosion will not affect much crustal stresses. Therefore, it is unlikely to see a physical connection between the rain and earthquake activity."

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