The ‘Big One’ still likely because Magna quake didn’t relieve much stress on Wasatch fault lines

The magnitude 5.7 earthquake that struck Magna in March didn’t relieve much stress on the fault lines running along the Wasatch Front and a major temblor — the Big One — is still expected in the future, a seismologist told a state committee on Thursday.

“There’s still a lot of strain and there’s still a lot of energy waiting there on the fault that can be released,” said Keith Koper, director of the University of Utah’s Seismograph Stations and a professor of geology. “And so people shouldn’t really let down their guard.”

Koper told the Utah Seismic Safety Commission that while the earthquake that hit March was big and the largest on the Wasatch Front ever recorded, “in the grand scheme of things” it was a moderate quake.

A magnitude 7 temblor would release 90 times more energy than the Magna quake, Koper told the commission. Such a large earthquake historically occurs in the Salt Lake Valley every 1,300 to 1,500 years, and the last one of that size happened 1,400 years ago.

“You know, [we] shouldn't scare people and say the risk is way higher — it's about the same,” he added. “But we still have this major hazard here in the Salt Lake City region and really along the Wasatch fault.”

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The state commission was hearing Thursday from various parties involved in analyzing the earthquake's genesis, its impact and the response.

While the March 18 quake was felt all over the Salt Lake Valley, there were no reports of serious injuries or major structural damage. Unreinforced masonry buildings near the epicenter of the quake were affected, the commission was told, and some mobile homes in West Valley City were shifted off their foundations, displacing residents. The Angel Moroni on top of the Salt Lake Temple lost its trumpet.

But earthquakes like the Magna one happen all over the world all the time, Koper told the commission.

“So it's really common,” Koper said. “It's just that it happened right next to us.”

Koper said the Magna earthquake did release some pressure on the fault line there but “there’s still a lot of strain that is loaded on the Salt Lake City segment. So as you go south and you go towards the Point of the Mountain, there’s still a lot of potential strain energy there.”

The Salt Lake City segment of the fault line is “still capable of producing a magnitude 7 earthquake,” Koper said. “So the amount of stress relieved by the Magna event was not really big enough to change that.”