Southern Taiwan was hit by thousands of landslides after Typhoon Morakot in 2009.
When the deadliest typhoon to hit Taiwan in modern history struck the island in 2009, it dumped 3 meters of rain in as many days, triggering thousands of landslides. But what surprised scientists was the slew of earthquakes that followed Typhoon Morakot’s landfall. Now, a new study suggests why they occurred: The landslides scraped away so much soil and rock that Earth’s crust, newly lightened, buckled in new ways.
“In previous cases most people only considered that earthquakes could trigger landslides,” says Zhigang Peng, a seismologist at the Georgia Institute of Technology who was not involved in the research.
Roughly 10,000 landslides struck Taiwan in the days following Typhoon Morakot, heaving 1.2 cubic kilometers of soil and rocks down hillsides. Over time, rivers carried much of the debris away. That permanently flushed the landscape of hundreds of millions of tons of sediment—equivalent to scraping 3 centimeters off the entire surface of Taiwan, says Philippe Steer, a geoscientist at the University of Rennes 1 in France.
That tectonic shift might have affected seismic activity, Steer and his colleagues hypothesized, so they mined a catalog of more than 340,000 earthquakes that occurred in Taiwan between 1995 and 2015. In areas affected by Typhoon Morakot–related landslides, the researchers found a threefold increase in the earthquake rate immediately following the storm, they report today in Scientific Reports. They also found an uptick in the proportion of small earthquakes after the storm. Seismic activity in those areas remained higher than normal for roughly 2.5 years, the team noted.
They say the increase can be explained by the removal of hundreds of millions of tons of sediment, which likely caused Earth’s crust near Taiwan to flex slightly upward. That motion would have altered stresses within the crust, potentially unclamping faults that were already near the point of rupturing, the researchers surmised.
That makes sense, Peng says. “It’s not the landslide itself. The subsequent erosion process generates stress.”
This isn’t the first time researchers have linked erosion to earthquake behavior. In 2010, scientists suggested the tendency for large earthquakes to occur in the New Madrid region in the central United States is due to thousands of years of erosion caused by the Mississippi River. No other modern cases of erosion-induced earthquakes have been documented, but Haiti and Puerto Rico would be good places to look, Peng says, because these nations are often slammed by hurricanes and see lots of landslides.
“It’s very rare to be able to observe these kinds of changes,” Steer says. But because climate change could lead to more frequent extreme events, there could be more erosion—and therefore earthquakes—in our future.
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