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Seismologist answers common quake questions following continued aftershocks

Posted at 8:29 PM, Apr 16, 2020
and last updated 2020-04-16 22:35:43-04

SALT LAKE CITY – More than a month after a 5.7-magnitude earthquake rocked the Salt Lake Valley, residents are still experiencing large aftershocks. FOX 13 spoke with a seismologist to get answers to some common quake questions.

With two 4.2 magnitude aftershocks in the last two days, including one Thursday morning, many Utahns are wondering – why does this keep happening?

FOX 13 spoke with seismologist Jamie Farrell, from the University of Utah Seismic Center, to get answers to some common quake questions.

How long can we expect to see aftershocks?

“Aftershocks can last for weeks to months, and in some sequences, they can last for years, so this is definitely normal given the way the sequence was going prior to Tuesday night -- having a 4.2 [magnitude] was a little bit unexpected but not out of the ordinary. Now that we had the other 4.2 on Tuesday night could have kind of kick-started the sequence which is maybe why we had the one [Thursday] morning,” said Farrell.

Why do aftershocks happen?

“When you have a relatively large earthquake, like the magnitude 5.7, when that ruptures in the earth it changes the state of stress in the crust and you have that sudden change, the earth wants to get back to some equilibrium like it was before the earthquake and that’s what these little aftershocks are doing, they’re rupturing again and again to try to get that fault patch back to some equilibrium,” Farrell explained. “That’s why these aftershocks typically decrease both in magnitude and rate of occurrence as time goes on, as the earth slowly gets back to this state of equilibrium.”

Why are these considered aftershocks instead of new earthquakes?

“So, the main shock was obviously the 5.7-magnitude event on March 18, anything after that that has a smaller magnitude is considered an aftershock,” said Farrell. “If we had an event after that 5.7 that was bigger, that would then become the mainshock and then everything before that would be considered a foreshock.”

Farrell also said location plays a role in determining an aftershock. If a quake happens somewhere else on the Wasatch front, even if it is smaller, it will be considered a new earthquake.

Can changes in weather cause aftershocks?

“The pressure changes from the weather just aren’t big enough to affect things that are happening at the depths of these earthquakes,” Farrell said.

I’ve lived in Utah my whole life and don’t remember experiencing aftershocks like this, what changed?

“The people that have lived here for that long have probably not experienced a magnitude 5.7 event, especially having occurred in the Salt Lake Valley, a highly populated area, a lot of people are feeling these, this isn’t unusual,” said Farrell.

Why did I feel the aftershock this morning and not on Tuesday night?

“The one [Thursday] morning was definitely more widely felt than the one Tuesday night and there could have been a number of different things, sometimes when you have one that’s felt more than another one it could be shallower or it could be a different type of earthquake,” said Farrell. “These ones are pretty much the same, both about the same magnitude, same depth and the same kind of earthquake located in the exact same spot – so, there’s something else going on that we’re not quite sure of yet, but it’s something we’re looking into to see why the one [Thursday] morning was more widely felt.”

Is this foreshadowing the ‘big one?’

“This event has changed nothing in our expectations of when or where the next big earthquake will happen along the Wasatch fault,” Farrell said. “[We’re] still expecting a big earthquake, we don’t know when it’s going to happen, we can’t predict earthquakes.”

Ironically, Thursday also marked the Great Utah Shakeout – the world’s largest earthquake drill and preparedness day.

“We had an actual shakeout instead of a practice shakeout,” Farrell joked.

If you’re wondering what you can do – seismologists say preparedness is key.

“This is a good wakeup call for people here along the Wasatch front, because the state of Utah, we live in earthquake country and this is the type of thing we should prepare for,” said Farrell. “This is kind of a good practice for us for when the Wasatch front ruptures, the ‘big one,’ the magnitude 7 to 7.5 earthquake that would be much more damaging and much more widely felt than the magnitude 5.7.”

UUSS said residents, especially those in the Magna or Salt Lake areas, can expect to feel more earthquakes as this aftershock sequence continues

Utah Emergency Management said they have not received any reports of damages or injuries following these aftershocks. However, given the magnitude of the quakes they said that would be unlikely.