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How rain and snow play a role in Noto’s ‘seismic swarm’

Japan sits on top of four major tectonic plates and is one of the countries most at risk of earthquakes.

That fact was tragically and dramatically demonstrated on January 1, when a magnitude 7.6 earthquake struck the Noto Peninsula in Ishikawa Prefecture, directly and indirectly killing as many as 275 people, knocking out major infrastructure, thousands of people were displaced and dozens of people were seriously damaged or destroyed. of thousands of homes.

The Noto Peninsula has been experiencing a persistent ‘seismic swarm’ since late 2020 – many earthquakes in a relatively small area that do not fit into a ‘mainshock-aftershock’ pattern – producing earthquakes ten times greater than the average. regional rate. No one knew why, but now an important clue has emerged.

A new study from scientists at the Massachusetts Institute of Technology shows that heavy snow and rain can affect how and when earthquakes occur. When searching for the cause of an earthquake, the search usually takes place internally and starts underground. But the study, published May 8 in Science Advances, shows that earthquakes can also have external, above-ground triggers.

The scientists analyzed a series of earthquakes in the Noto region that occurred between 2012 and 2023, finding that the region’s seismic activity is synchronized with changes in subsurface pressure, which is influenced by seasonal patterns of precipitation. According to them, heavy snowfall has more influence than heavy rain.

For Qingyu Wang, lead author and seismologist, it is important to analyze the interaction between the internal and external processes.

“Currently, most people only look at the environmental consequences (such as the weather) or … at the internal process, such as long-term tectonic forces. For me, I am very interested in combining both the internal and the external,” she says.

Joint Staff Office of the Ministry of Defense of Japan / via REUTERS

The climate crisis also has an influence. “Sea level changes in the Sea of ​​Japan play a role,” Wang said, with sea levels rising by about 1 meter in summer and winter.

And the effect of climate change could become greater, the researchers predict.

“If we move to a climate that is changing with more extreme precipitation events, and we expect a redistribution of water in the atmosphere, oceans and continents, that will change the way the Earth’s crust is stressed,” said William Frank, one of the authors. of the study and an assistant professor in MIT’s Department of Earth, Atmospheric, and Planetary Sciences in a press release.

Still, Wang emphasizes that heavy precipitation in itself is not the direct cause of earthquakes. The main cause is sudden movements along fault lines on tectonic plates; Japan currently has more than 2,000 active fault lines. However, this trigger alone was not enough to explain why the Noto Peninsula experienced so many earthquakes.

Water clogged

Inland earthquakes in Japan mainly occur at relatively shallow depths of 10 kilometers or less. The earthquakes on the Noto Peninsula started at a depth of about 15 km, suggesting there was an underlying force. And the distinct way in which these earthquakes migrated upward and decreased in speed suggested that this underlying force was the presence of deep fluids, resulting from precipitation.

“The earthquake swarm is then caused by deep fluid migration caused by intense seasonal changes in the excess pore pressure caused by intense seasonal precipitation,” says Wang.

Just as the pores in our faces are small openings that can become clogged by oil and sweat from our glands, the pores in the Earth’s crust on the Noto Peninsula became clogged with water by seasonal patterns of snow and rain. When these pores in Noto became clogged, it slowed the speed of seismic waves, causing the deep fluids to shift into the shallow, pre-existing fault zones where the earthquake swarm occurred.

Japanese scientists had previously suggested that there might be a build-up of water behind the seismic swarm.

Although this study only looked at earthquakes in Japan, the researchers note that heavy precipitation could also play a role in earthquakes elsewhere.

The New Year’s Day earthquake, meanwhile, occurred in the same swarm source that Wang’s research analyzed. But without further research, the MIT scientists say it’s unclear whether heavy precipitation played a role or not.

“Once there will be more high-resolution spatio-temporal analyzes (of the New Year earthquake), a more accurate conclusion will be made about the possible link between (it) and the earthquake swarm,” Wang said.

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