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Could heavy rains really trigger earthquakes?

Gives new meaning to the term “earthquake weather”

Plumes of smoke on the horizon during the 1906 earthquake.
Public Domain

On top of everything else, California’s recent slew of huge rainstorms may have primed the state’s faults for a major earthquake, or so claims an article by science journalism site the Conversation.

Here’s how it supposedly works:

Water pressure in the fault zone is important in controlling when a geological fault slips. Fault zones invariably contain groundwater, and if the pressure of this water increases, the fault may become “unclamped.” The two sides are then free to slip past each other, causing an earthquake.

The article cites a one-two punch of storms and earthquakes in Germany and Switzerland in 2002.

But readers are also reminded that there are only “possible connections” between the phenomena, that the “effect is likely to be subtle,” and such findings would be “controversial.”

So, is there really any chance that recent floods have in effect lubricated Bay Area fault lines?

No one at the US Geological Survey was immediately available for comment. [Update: Art McGarr, a geophysicists in Menlo Park, told Curbed SF that he’s “very skeptical that heavy rainfall has influenced earthquake activity.”

McGarr acknowledges that quake researchers often analyze seemingly unlikely phenomena as possible tremor enablers. “In recent years we’ve considered whether the sun and the moon can cause deformations in the earth,” he says by way of example.

“But rainfall is a different story. It’s measured very routinely almost everywhere and it has been for hundreds of years. I cannot rule out the possibility that tiny quakes below our detection threshold could be influenced,” he adds, but beyond that he’s unpersuaded.]

Seismologists long ago debunked the popular myth of “earthquake weather,” but the conventional belief was always that arid days are prime for tremors.

The Cyprus structure in 1989.

“The common misconception that earthquakes occur during hot and dry weather dates to the ancient Greeks,” says the California Department of Conservancy. “Earthquakes take place miles underground, and can happen at any time in any weather.”

Still, the Conversation notes that quakes triggered by surface activity, such as digging and hydraulic boring, are a well-established phenomena.

In 2008, the magazine New Scientist cited studies that seem to take it for granted that rain may lead to quakes, only haggling about the scale of the effect:

It was already known that rainfall could cause tremors, but the amount of water needed is much more than previously thought, says Steve Miller, a geologist at the University of Bonn, Germany.

[...] Some experts have suggested that although the rainfall was heavy, the fact that rain could trigger an earthquake at all suggests that it takes extremely little to produce a tremor. They concluded that the Earth’s crust in a delicate balance, teetering on the edge of a slight shake-up at any moment.

In 2011, the University of Miami concurred in the pages of National Geographic:

Analysis revealed that Taiwan's large earthquakes [...] were five times more likely to occur within four years after storms than if the storms had had no effect. The weight of the water itself does not trigger the earthquake—rather, it's the ensuing erosion from landslides.

California researchers found a similar phenomena in the Himalayas. The data was presented at a meeting of the American Geophysical Union here in San Francisco.

But potentially triggering an earthquake is not the same thing as causing one.

Earthquakes happen for the same reason they always have—mounting pressure from the movements of the earth’s crust. Heavy rainfall isn’t causing that, as seismologists noted when Earth magazine reported on this same research.

Rather, it could, if anything, speed it up a bit. Then again, maybe not. USGS’s Dr. Malcolm Johnston maintains:

Very large low-pressure changes associated with major storm systems (typhoons, hurricanes, etc) are known to trigger episodes of fault slip (slow earthquakes) in the Earth’s crust and may also play a role in triggering some damaging earthquakes. However, the numbers are small and are not statistically significant.

The survey notes elsewhere that “every region of the world has a story about earthquake weather,” but it changes depending on the context.

For example, just over two years ago the peer-reviewed scientific journal Nature published research suggesting that the drought might induce quakes.

It’s possible both phenomena could be true, of course. Or neither. In cases like this, the public should do what the scientists do: Don’t draw a conclusion without more research.