SALT LAKE CITY — The earthquake that rattled the Salt Lake Valley and beyond Wednesday morning does not mean that a bigger one is on the way, a University of Utah seismologist said.
Keith Koper, a U. geology and geophysics professor, said the 5.7 magnitude quake that hit the Wasatch Front was “moderate” in size, but considered big for Utah.
“It is not imminent that there is another large earthquake to come,” he said. “Do not expect a much larger earthquake. Do not expect a magnitude 7.”
Koper said there is small — 4% to 5% — probability for a bigger earthquake to follow, but historically most earthquake sequences don’t unfold that way. The epicenter was north of Magna, between the city and Antelope Island near the edge of the Great Salt Lake, according to the U.S. Geological Survey.
“We know it’s possible that there’s this few percent chance that it could happen and that’s just based on previous observations,” he said. “But the fact is, we can’t and we don’t predict earthquakes.”
State agencies were also quelling speculation on social media that a magnitude 9.0 earthquake would hit the state because of Wednesday’s event.
“Although it is possible that a larger earthquake in this area could be in the magnitude range of 7.0 to 7.5, the U.S. Geological Survey estimates that the chance of an earthquake of this size is about 1 in 300. Earthquakes of magnitude larger than 7.5 are unlikely to occur in Utah. A magnitude 9.0 earthquake is not possible in Utah,” according to the U. Health Department.
And from the Utah Division of Emergency Management on Wednesday morning: “We’re here to stop rumors. People have been saying that officials are predicting an earthquake in the next 30 minutes. This is not true. Earthquakes cannot be predicted. We are 95% certain that this was the main shock.”
Utahns, however, can expect “hundreds to thousands” of aftershocks in the coming days and weeks before tapering off, and would be more strongly felt at the epicenter near Magna, Koper said. The largest aftershock Wednesday was 4.6 magnitude at 1:12 p.m., in what he said was a “robust” aftershock sequence.
“The greater Magna area is probably not going to get to sleep a lot in the next few days because of this,” said Bob Carey, response and recovery bureau chief for the Utah Division of Emergency Management.
Steven Bowman, Utah Geological Survey geologic hazards program manager, echoed Koper, saying, “As we go forward, we expect that we’re going to continue to have some of these smaller aftershocks.”
Bowman said in addition to helping building inspectors, crews of geologists were out in the field checking for geological disruptions like landslides or liquefaction, a phenomenon that occurs after earthquakes where sand “boils” up from the ground. Bowman told reporters the damage was minimal.
“So far, we’ve only seen very small geologic evidence of damage,” he said at a news conference. “Some of these older buildings that are brick have had a lot more damage.”
Koper described the quake as “relatively shallow,” with a depth of about 6 miles, and because of that it would be felt more strongly near the epicenter.
Jess Anderson, Utah Department of Public Safety commissioner, said the earthquake was felt as far away as southern Idaho and Wyoming, across the Uinta Basin and into Millard County.
“We have every person that has always planned and prepared for this at the table and fully functional at this time,” Anderson said.
Sometimes the magnitude of an earthquake can change with more data, and the Magna quake could go up to 5.8 magnitude, Koper said.
“It’s not going to get very much bigger than that,” he said.
Still, having had at 5.7 magnitude earthquake doesn’t reduce the risk of having future earthquakes on the Wasatch Fault system.
“This was not the big one. This is sort of a wake-up call,” Koper said, adding the state should maybe install an earthquake early warning system like the one that recently went live on the West Coast.
Carey said Utahns should use Wednesday’s “designer” earthquake as an opportunity to “calmly” review their family emergency preparedness plans.
“This is the earthquake I always wanted to have happen. I know that sounds nutty. But it puts us now to understand that we can actually have this. It’s not that we’ve been barking at the moon for all these years because it kind of feels like that,” he said.
“And this is just a little one. Just think what our big one is going to look like.”
Wednesday’s earthquake happened on the Wasatch Fault system, Koper said. The fault parallels the western base of the Wasatch Mountains for 220 miles from near Fayette, Sanpete County, in the south to near Malad, Idaho, in the north.
Koper compared the fault to a tree trunk with many branches, and said the earthquake occurred a little west of the main fault line.
“One of the reasons we’re having all these aftershocks is because of this main shock that happened. In general, these are all on the Wasatch Front system, so we’re not giving it a new name, a different name,” he said.
Utah is built on an ancient lakebed. Part of the reason Wednesday’s shaking was relatively strong is that there’s a lot of sediment or weak rock in the basin, he said.
“When you have an earthquake, the ground motion will get amplified. The energy in a sense will get trapped in the basin and it will act sort of like a bowl of jelly,” he said.
That’s why people in tall buildings downtown felt significant swaying and shaking and people on the bench not as much, Koper said.
Carey noted that some people in close proximity to each other had different experiences. One woman felt the shaking outside, but a man in the garage did not, he said.
“People feel it different ways. The soil responds differently,” he said.
Koper described the main 5.7 magnitude shock as a “normal faulting” earthquake, meaning the earth’s crust was stretched sideways. One side of the fault slipped downward on the other side of the fault, he said.
“This is the type of earthquake that is very typical for this area,” he said. “It is different than the type of earthquakes that normally happen in California, which tend to be more strike-slip or the earthquakes that happen in Alaska, which tend to be crust earthquakes.”
Koper said a magnitude 7.0 earthquake would release 1,000 times more energy than a magnitude 6.0.
“It’s a totally differently animal, a magnitude 7 versus a magnitude 5.7. It would have been horrific if a magnitude 7 happened where this 5.7 happened this morning,” he said.
“You wouldn’t be here. You wouldn’t be able to get here. That would be the difference. You would have significant infrastructure issues. Your biggest problem would be to try to figure out if whether your loved ones had made it out of that,” Carey told reporters at a news conference.
“This is one that’s going to catch your attention, and I think we can use this to help better prepare ourselves, but it’s a walk in the park.”
Contributing: Kyle Dunphey