Utah’s mini-wave of minor earthquakes doesn’t portend that the big one is coming our way anytime soon — but it doesn’t lessen the chances either, according to James C. Pechmann at the University of Utah Seismograph Stations.
“I don’t think they’re related,” he said of quakes centered near Bluffdale in southern Salt Lake County and near Kanosh in Millard County. “They’re too far away from each other. And they’re not big enough to cause what we call remote triggering.”
Kanosh is about 120 miles from Bluffdale.
On Friday morning, two earthquakes — one a 3.2 magnitude, the other 3.7 — centered south of Bluffdale shook southern Salt Lake and northern Utah counties. On Saturday, there was a 2.4 aftershock in that same area; a quake measuring 3.3 near Kanosh; and a small quake in Yellowstone National Park, which is also monitored by the U.
“Usually when your [alarms] go off three times in a day," Pechmann said, “it’s all earthquakes in the same place.”
A second, magnitude 4 quake hit just after midnight early Wednesday morning, south of Kanosh. A 2.6 magnitude earthquake shook Bluffdale just after 8 p.m. There have been no reports of significant damage in either Bluffdale or Kanosh.
None of the quakes is unusual “over the long term," Pechmann said. "Over the short term, yes, it’s a little unusual to have earthquake activity going on in three different parts of our region at the same time. But, again, I don’t really think they’re related.”
The seismologist said quakes over magnitude 7 “have been demonstrated to trigger earthquakes even hundreds of miles away. But it's not been shown statistically or scientifically to occur for earthquakes as small as these.”
He did, however, debunk the notion that minor quakes lessen the chances of a big one.
“Small earthquakes do not act as a safety valve. They don’t relieve pressure,” Pechmann said. “In fact, the occurrence of small earthquakes means that the chance of a larger one is higher, not lower.”
Every time there is an earthquake, the probability of more earthquakes increases. That’s not a prediction, Pechmann stressed, just an analysis of statistics.
When an earthquake occurs, there is “roughly a 1-in-20 chance” of a larger aftershock. “Now, by larger, I mean anything bigger than the one that just occurred,” he said — so maybe a 4 would be followed by a 4.05.
“The probability of it being a foreshock to a 6 or larger is much lower than that. Probably a hundred times lower," Pechmann added, “but still much higher than sort of a background probability.”
Aftershocks in the 2 range have continued in the Bluffdale area since Friday, although probably only those directly on top of the quakes are aware of them.
“If it’s right underneath you, you can feel earthquakes as small as around 2,” Pechmann. “So I would imagine at least some of those Bluffdale aftershocks are being felt. They just don’t call us about it anymore because they know what it is.”