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What's an 'Earthquake Swarm,' And Is One Hitting Taiwan?

Several people have been killed and dozens remain unaccounted for as a series of quakes rattle the region. But are these earthquakes part of a cycle?

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A building tilts to one side after an earthquake on February 7, 2018 in Hualien County, Taiwan.

A magnitude-6.4 earthquake shook northeastern Taiwan Tuesday night, killing at least seven people and injuring more than 250 others. More than 100 aftershocks have been reported to rattle the region, rocking buildings, buckling roads, and cutting off resources to thousands of families. Dozens of people are still unaccounted for.

So what's causing all these quakes? Some are calling the disaster an "earthquake swarm," but others are more hesitant.

What is an 'Earthquake Swarm'?

An earthquake swarm happens when a large number of quakes hit a single area in a relatively short period of time, usually within days or weeks. One earthquake must be identified as the main shock, and the rest are subsequent aftershocks. A swarm hit Reno, Nevada, in mid-January when more than 230 small temblors shook the region over the course of six days.

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The U.S. Geological Survey reports that the earthquake that happened on February 6 was the largest sequence of events in the region over the past several days. A M-4.8 quake hit on February 3, and as of Tuesday, 19 earthquakes in the area were measured at M-4.5 or larger.

Is Taiwan Facing an Earthquake Swarm?

Reviews are mixed. According to a report by Taiwan News, a geology professor at National Central University in Taoyuan City believes the country is entering an earthquake cycle that could last for the next century.

"For the next 10 years in Hualien, the probability of a magnitude 8 earthquake occurring is high. I predict that there will be another earthquake [similar in size] in 2025," says Lee Chyi-tyi, the academic who has been proposing this theory since 2013. "Earthquakes are cyclical and it is about time again."

He says the small area is experiencing massive pressure from tectonic forces, and the earthquake cluster could make it hard to figure out the pattern of the main quake.

Other experts are hesitant to use the loaded phrase.

"I believe that speaking about 'earthquake swarm' may be misused for our purpose," writes Alexandre Canitano in an email. Canitano is a postdoctoral fellow at Academia Sinica's Institute of Earth Sciences in Taipei.

"Long-term earthquake forecasting is a tricky exercise, and there is no evidence supporting the occurrence of a destructive earthquake in a near future," Canitano adds.

Rather, he says Taiwan is being hit by a "preshock-mainshock-aftershock" sequence. It's unclear how the sequence will develop in the coming days and weeks, but it would be interesting to study the relationship between the quakes.

Universiti Brunei Darussalam geology professor Afroz Ahmad Shah, who is also a National Geographic grantee, says these earthquakes seem different from a normal sequence, in which smaller aftershocks follow a larger mainshock. Shah says these quakes seem more like a swarm because they all have similar magnitudes and occurred over a short period of time.

He says we need to look for more accurate magnitude information, which can change based on how clear the data are.