A magnitude 3.7 earthquake struck the densely populated Bluffdale, Utah, area on Feb. 15, and a magnitude 4.0 hit farther south on Wednesday, according to the University of Utah Seismograph Stations.
People felt shaking, but there was no reported damage, said Mark Hale, a seismologist with the UUSS.
“People inside liken it to a car hitting the house, or the shaking you feel when a big truck moves close by,” he said. “A lot of people heard things rattling in the house.”
The Bluffdale quake south of Salt Lake City was accompanied by more than 100 smaller quakes, including 13 foreshocks, and 88 aftershocks between Feb. 13 and Feb. 21.
Minor earthquakes in Utah are not unusual, Hale said, and the preceding and following shocks are also normal.
But the 3.7 magnitude quake turned a few heads.
The epicenter was estimated at 1.8 miles south of Bluffdale, and about 5.3 miles under the surface.
Its cause is not clear, Hale said. The normal-fault quake may have been associated with the Wasatch Fault to the east or from a minor fault or from upward percolation from localized geothermal activity, which can cause seismic events.
Since 1962, five earthquakes of magnitude 3.0 or greater have occurred within 16 miles of the epicenter of the Bluffdale event.
In the past, earthquakes would go unnoticed, Hale said. But as more development occurs, such as in the rapidly growing Bluffdale area, then more people notice and report them.
The Bluffdale quake generated 8,805 reports from people who reported feeling it in the Salt Lake and Utah Valleys.
By comparison, a 4.0 magnitude quake in the sparsely populated Kanosh, Utah, area on Wednesday generated 78 felt reports. In 1967, a magnitude 5.2 earthquake occurred 23 miles to the southeast of Kanosh, according to UUSS.
Earthquakes typically need to be at least a magnitude 5.0 to cause damage to structures, and larger than magnitude 6.0 for a surface break to occur.
As development grows, more people are experiencing Utah earthquakes, and it has been drumming up more interest in them, Hale said.
“It is a good reminder for people that a lot of injuries from earthquakes are from objects falling down in the house,” he said. “Think about those bowling trophies on the high shelf, or the swords mounted above the bed.”
