In the past week or so, some 400 earthquakes have added to the already precarious land at Yellowstone National Park. Although the area is the largest supervolcano in North America, the rumbling is a bit more than normal.

Like bees, when earthquakes occur in great amounts in one area, it’s called a swarm, reports NPR. But this swarm is bigger than the usual that would happen. The swarm has a magnitude of 3.9 on the Richter scale, and the quakes have happened at greater frequency than the norm.

And all of the earthquakes have occurred under Yellowstone Lake. The last time something like this happened was 20 years ago, reports Wyoming’s Local News 8. Said one geologist on the station: “We think it’s where more magma heat and steam escaped through cracks in the crust. That’s probably what’s causing the earthquakes.”

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No one knows for sure why this is happening, though. But no worries to the people living around Yellowstone – the last huge volcano eruption was about 640,000 years ago.

Earlier in the year, around April, the same thing happened in Oregon, except it was 600 earthquakes in 10 days. That one, though, was different – it didn’t occur around any places where tectonic plates meet, the vast land areas that form the Earth’s crust. Three of those quakes had a magnitude of 5.0 or higher. Most, however, happened out at sea, and were barely felt on land.

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Casualties of global warming? Who knows. There’s not much anyone can really do about these, except wait them out.

Photo Credit: moonjazz at Flickr under a Creative Commons License