SALT LAKE CITY — The most powerful earthquake ever to shake Yellowstone National Park left 28 people dead and dropped the ground a full 20-feet in some areas after it struck on August 17, 1959. Groundwater swelled and receded in wells as far away as Hawaii from the stunning 7.2 magnitude Hebgen Lake Earthquake. Nearly 60 years later, scientists say effects from that very same earthquake rattled Yellowstone again — this time in the form of 3,000 small temblors in the Maple Creek area of the park.
Researchers at the University of Utah said this swarm of smaller quakes are at least partially aftershocks of the Hebgen Lake Earthquake, and were recorded between June 2017 and March 2018.
“These kinds of earthquakes in Yellowstone are very common,” explains study co-author Keith Koper, director of the University of Utah Seismograph Stations, in a statement. “These swarms happen very frequently. This one was a little bit longer and had more events than normal.”
The researchers determined that many of the quakes in the cluster were Hebgen Lake aftershocks by observing that the quakes occurred along the same fault line and were oriented in the same way. They note that it’s not necessarily out of the ordinary for aftershocks to crop up again months, years, or even decades after the initial quake.
“Earthquakes don’t happen as a single discrete event in time,” says Koper. “There are formulas to predict how many aftershocks you should see. For Hebgen Lake, there looked like a deficit in the number of aftershocks. Now that we’ve had these, it has evened things out back up to the original expectations.”
The study is published in the journal Geophysical Research Letters.