NOT LETTING UP

Japan has suffered over 600 aftershocks in less than a week

Japan’s Kumamoto prefecture will not stop shaking.

The ground has almost literally not stopped moving in the rural area south of Tokyo since an initial 6.5 magnitude quake hit on Thursday (April 14), followed by a stronger one early on Saturday. As of Tuesday morning, more than 600 tremors have shocked the area, over 180 of them hitting 3.5 or higher on the Japanese seismic intensity scale, according to the Japan Meteorological Agency (pdf in Japanese)—strong enough to be felt by most people.

The quakes have been responsible for dozens of deaths. They’ve also forced tens of thousands of people out of their homes and into evacuation centers. Stable space has become so hard to find that people have been sleeping in their cars to avoid falling roofs and near-constant rain.

“It’s unusual,” said an agency spokesman. “There have been more aftershocks than the Niigata quake in 2004,” the previous record holder that initially registered at a 6.6 magnitude near the country’s west coast.

One factor is that this time the quake’s epicenter was only 11 km (less than 7 miles) from the surface. That’s well within the US Geological Survey (USGS) range of a “shallow earthquake,” which tops out at 70 km down. The Niigata quake epicenter was (barely) deeper at 15.8 km.

The higher the quake is below ground, the more energy reaches to the surface. And the harder the ground shakes, the longer it takes it to resettle.

Meanwhile the 7.8 magnitude quake that shook Ecuador on Saturday was also “shallow,” with an epicenter just 19 km below ground, according to the Ecuador Geophysics Institute. As residents there are also now dealing with hundreds of aftershocks, the death toll has risen past 400, according to media reports.

Kumamoto, with seven quakes registering 6 or higher on the Japanese intensity scale since Thursday, can expect the ground to keep shaking for a while longer.

As the USGS explains, “In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.”

You can follow Ben on Twitter at @bjlefebvre.

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