A swarm of thousands of tiny earthquakes that rumbled below Yellowstone National Park in 2017 and 2018 might be the long-awaited aftershocks of a much larger quake — which struck 60 years ago.

In a paper published April 30 in the journal Geophysical Research Letters, researchers examined the seismicity of some 3,345 earthquakes that occurred near Yellowstone’s Maple Creek, in the northwest corner of the park, from June 2017 to March 2018. They found that, for about half of those minor quakes, seismic waves below the park rippled along the same fault line, and in the same exact direction, as the waves behind the so-called Hebgen Lake event — a mammoth, magnitude-7.2 earthquake that struck there in 1959 and killed 28 people.

The team didn’t see any signs that this cluster of quakes was caused by magma movement under the park, leading them to conclude that the rumbles were, in fact, a suite of seismic aftershocks six decades in the making.

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