Two magnitude 3 earthquakes rumble, but these temblors came from closer to Salt Lake City

(Leah Hogsten | Tribune file photo) "I want it insurable," said Larry Hjalmarson, First United Methodist Church member and maintenance overseer, March 19, 2020. "This is really an important building for our congregation, for our community." The church sustained damage from the 5.7 magnitude earthquake on Wednesday. Waves of aftershocks have been felt since that first temblor.

A pair of magnitude 3-plus earthquakes jolted Salt Lake City on Thursday morning, adding to the sequences of hundreds of aftershocks associated with the big March 18 quake, which now appear to be migrating east of the original epicenter near Magna.

According to the University of Utah Seismograph Stations, a magnitude 3.3 struck at 10:12 a.m., followed by magnitude 3 about 14 minutes later. They were two in a burst of eight aftershocks through midafternoon Thursday centered on Salt Lake City’s west side.

“They are more shallow and closer to the surface, so they will feel different,” explained Keith Koper, the U. Seismograph Stations director. “It’s not unexpected for aftershocks to migrate to adjacent places over time.”

As of 4 p.m. Thursday, Utah has experienced 560 aftershocks over eight days since the initial magnitude 5.7 earthquake, according to the U.S. Geological Survey. Most could not be felt, but up to 32 exceeded magnitude 3.

“It is normal for the size of an aftershock zone to expand with time,” the U. Seismograph Stations tweeted. “It may feel different because of the different locations, but it does not mean we are significantly more at risk for a big one.”

Thursday’s burst of seismicity has opened a new pattern in the sequence of aftershocks, each representing a shift in the Earth’s crust in response to new stresses created by all the seismic activity since March 18.

The first aftershocks in Salt Lake City appeared March 20 and grew in frequency and intensity reaching what could be a climax Thursday.

“This cluster moved east from the previous activity [north of Magna],” said U. seismologist Jamie Farrell. “They are a bit shallower.”

Whereas the Magna-centered quakes were about six miles deep, the ones in Salt Lake City occurred three miles beneath the surface. The region’s faults are angled downward toward the west. Farrell wondered if Thursday’s aftershocks occurred on the same fault as the mainshock, just closer to the surface and thus farther to the east.

“The probability of a damaging earthquake is almost the same as it was before this burst of earthquakes [Thursday] morning,” Koper said. “We are going to keep monitoring to see if they track differently over time. This is typical behavior. People shouldn’t be alarmed, but everyone should remember the risk is a little higher now than it was before the 5.7.”