A sizable aftershock rumbled across the Salt Lake Valley early Thursday, 29 days after a powerful earthquake struck the region and on the very day Utahns participated in drills to prepare for a major quake.
Thursday’s temblor began at 7:41 a.m. and registered magnitude 4.2, according to the U.S. Geological Survey. The epicenter was about 2.5 miles northeast of Magna, near the site of the mainshock, a magnitude 5.7 quake that struck March 18.
This is the second substantial aftershock this week after a similar 4.17 magnitude quake hit late Tuesday.
Since that March 18 quake, the USGS has measured 70 aftershocks of magnitude 2.5 or more, clustered in that same area of Magna.
Experts at the University of Utah Seismograph Stations said Thursday that the movement residents have experienced since the March 18 quake is within the normal range of seismic activity.
“Sometimes earthquakes barely have any aftershocks, and sometimes they have more than average,” according to the stations’ Twitter feed. “This feels abnormal to us because this is the first time in most of our lives that we’ve lived near an earthquake sequence like this. But for the earth, this is business as usual.”
Thursday’s aftershock hit less than two hours before a statewide preparedness exercise was scheduled to start. The annual Great Utah ShakeOut, organized by state and federal agencies, had some 650,000 participants registered to take part in earthquake drills, according to the event’s website.
“We’ve joked for years: ‘What if there’s a quake on the day of the ShakeOut?,'” said Joe Dougherty, spokesman for the Utah Division of Emergency Management, one of the agencies leading the event. The aftershock, he said, was "just one more reminder that Utah is earthquake country, that we live in a place where the earth shakes.”
For the ShakeOut, participants are asked to imagine a massive earthquake striking at 10:15 a.m. and to react as if it were real.
“We don’t have to imagine it today,” Dougherty said.
Because of the coronavirus pandemic, Dougherty said, Thursday’s ShakeOut was different than in past years.
“Normally, schools are in session, and that’s our largest participation group,” Dougherty said, adding that state agencies encouraged teachers and parents to conduct virtual earthquake drills as part of their online lesson plans. Groups also can perform drills later in the year, once the stay-home orders are lifted, Dougherty said, and they will be counted for this year’s ShakeOut.
Dougherty recommends Utahns look around their houses, schools and workplaces for items that might fall on them during a strong quake — such as books on a shelf over a bed. He also reminds people not to run outside when a quake hits, because that’s when they could get hit by, for example, brickwork falling off the house.
“The best thing to do is to make sure you are staying in place,” Dougherty said. “Large earthquakes are known to throw people to the ground.”
According to a survey by the state’s Historic Preservation Office, 145 historic structures in Salt Lake City and Magna suffered some visible damage from the March 18 quake. Most were brick buildings, without wood frames to bolster the masonry when the shaking started.
Dougherty said the state has received no reports of building damage or injury from Thursday’s aftershock.
Seismologists are still studying which fault running through the Salt Lake Valley was the one that shifted in the March 18 quake. But, Dougherty said, "a fault isn’t just the line that you see on a map. It’s really a big cross section of earth that we are walking around on all the time.”