How strong was Wednesday’s main earthquake?
Striking at 7:09 a.m., it was recorded at magnitude 5.7, which is considered moderate. Each unit of magnitude represents a thirtyfold jump in the amount of energy released. This quake was Utah’s largest since 1992, when a magnitude 5.8 jolted St. George, according to the University of Utah Seismograph Stations. By contrast, the Loma Prieta earthquake that struck the Bay Area in 1989 was a magnitude 6.9, causing $12 billion in damage in today’s dollars.
Where were the quake and its aftershocks centered?
About two miles northeast of Magna, or 10 miles west of downtown Salt Lake City. The main earthquake occurred six miles under the Earth’s surface. Because it was so shallow, the ground shook with great intensity near Magna, registering a level VIII, considered “severe,” on the Mercalli Intensity Scale.
Which fault moved?
It appears to be a shallow fault that is part of the Wasatch fault zone, stretching 200 miles along the Wasatch Front from Idaho to Nephi. University of Utah seismologist Keith Koper likened it to a “branch of a tree” connected to the Wasatch fault, or the tree’s “trunk,” running through Salt Lake City. It is expected to release a major quake in the coming decades.
How many aftershocks will there be?
Hundreds, Koper said, if not thousands.
“We have had several strong moderate-sized aftershocks. The largest one was a magnitude 4.6. We do expect felt aftershocks to continue probably for weeks.”
As of 6:15 p.m. Wednesday, 43 aftershocks greater than magnitude 2 were recorded, according to the U. Seismograph Stations, which Koper directs.
Could a larger quake be coming?
There is a slight chance that the first quake was a "foreshock" to a larger shake.
“It is not imminent that there’s another, larger earthquake to come,” Koper said. “There’s a very small probability, 4% to 5% that we could have an earthquake larger than the 5.7. As time goes on, that probability is decreasing.”
The U.S. Geological Survey issued an aftershock forecast, indicating a magnitude 7 is a 1 in 300 chance, while a 6 is a 3% likelihood. A magnitude 5 or higher is a 17% chance.
Will these earthquakes reduce the risk of catastrophic earthquake in the future?
Not likely, according to Koper, because they were too small to relieve much strain on the Wasatch fault.
“Very close to where it happened, it will reduce stress," he said, “but there’s still more or less the same probability, maybe increased a little bit for other decent-sized earthquakes to happen on the Wasatch fault system.”
Did experts foresee this one coming?
Yes, in a way, but experts cannot predict precisely when quakes will strike. According to a 2016 report from the Working Group on Utah Earthquake Probabilities, there was a 93% chance of an earthquake in the Wasatch fault zone exceeding magnitude 5 in the next 50 years. It also predicted a 43% probability of a large earthquake — magnitude 6.75 or greater, the kind that would damage structures and cause significant disruption. Even so, a magnitude 5 quake can cause “considerable” damage — as northern Utahns witnessed firsthand Wednesday.
When have earthquakes this large struck Utah in the past?
Here’s a complete list of earthquakes exceeding magnitude 5 over the past century, according to the U. Seismograph Stations:
1992 — St. George, 5.8.
1989 — South Wasatch Plateau, 5.4.
1962 — Cache Valley, 5.7.
1962 — Magna, 5.2.
1961 — Ephraim, 5.
1949 — Salt Lake City, 5.
1921 — Elsinore, 6.
1910 — Elsinore, 5.
1909 — Hansel Valley, 6.