SALT LAKE CITY — A magnitude 7.3 earthquake which shook Yellowstone National Park in August 1959 may have seen continuing aftershocks as late as 2018, according to a study published by University of Utah geoscientists.
The original earthquake, known as the Hebgen Lake earthquake, jarred Yellowstone for around 30 seconds, according to a University of Utah press release. The dining room fireplace in the Old Faithful Inn toppled, the ground dropped 20 feet in some places and 28 people died. In 2017, the researchers said the Hebgen Lake quake rippled through Yellowstone once again.
More than 3,000 small earthquakes swarmed through the national park between June 2017 and March 2018, localized in the Maple Creek area, according to the study.
“These kinds of earthquakes in Yellowstone are very common,” Keith Koper, director of the University of Utah Seismograph Stations, said in the press release. “These swarms happen very frequently. This one was a little bit longer and had more events than normal.”
The researchers divided the 3,345 earthquakes that occurred into two major subgroups, according to the study. They identified a northern cluster in which quakes fell along the same fault line and were oriented the same way as the Hebgen Lake event and a southern cluster that likely was caused by other influences.
The researchers could not find evidence that the northern cluster was caused by magma or other activity beneath the ground, and they added that it’s not unheard of to see aftershocks from an earthquake even decades after the first shake, according to the press release.
“There are formulas to predict how many aftershocks you should see,” Koper told UNews. “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 southern cluster also lined up with the Hebgen Lake fault, but the lineup rotated an estimated 30 degrees and the tremors were 0.6 miles shallower than the quakes in the northern cluster were, according to the press release. Researchers believe that section was likely caused by magma moving beneath the surface, though there was probably some influence from the northern cluster.
Koper pointed out that earthquakes are different from other hazards in that they don’t end when the initial event ends. Aftershocks may continue on for years and even decades.