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Seismologist Explains The Science Behind Utah's 5.7 Magnitude Earthquake

A map of the segments
U.S. Geological Survey
The Wasatch fault system runs from southern Idaho to central Utah and is divided into segments. Wednesday's 5.7 earthquake took place in the Salt Lake City segment.

On Wednesday, a 5.7 magnitude earthquake shook the Salt Lake Valley. To help explain the science behind this event, KUER’s Caroline Ballard turned to Dr. Jamie Farrell, a seismologist and research professor at the University of Utah. 

This interview has been edited for length and clarity.

Caroline Ballard: What kind of an earthquake was this? 

Jamie Farrell: This is what we call a “normal earthquake.” This is a common type of earthquake we have here in Utah and throughout the Intermountain West. This is where the crust of the earth has kind of been stretched or pulled apart. And these types of earthquakes accommodate that. These are different than the earthquakes that you would typically see in California. Those are “strike-slip” earthquakes, where you have two parts of the earth sliding horizontally next to each other. 

This one here is where you would typically see the valley block dropping down mostly, and then the mountain block kind of coming up just a little bit. Right now, we're saying that this earthquake occurred on what we're calling the Wasatch fault system. Not sure if it's on the Wasatch fault itself yet.

CB: What is the Wasatch fault system and how big is it?

JF: The Wasatch fault is what we call the normal fault. It accommodates extension. The western U.S., at least where we live, is being stretched from east to west. It goes from central Utah up into southern Idaho. It’s broken up into multiple segments. We’re on the Salt Lake City segment.

But when you have these large faults, you also have a lot of related faults. This may have occurred on one of those. It may have occurred on the Wasatch fault. 

CB: Tell me a little bit about the geologic makeup of the valley and how its composition might affect an earthquake.

JF: We have two main things. We have the mountains which are mainly bedrock — hard, hard rock. And then you have the valley here which is filled with a lot of lake sediments from old Lake Bonneville.

And when you have those types of sediments in these deep valleys, those tend to amplify the shaking. Since this earthquake occurred out towards Magna, a lot of amplified shaking is occurring out there. In addition, this earthquake was relatively shallow so that tends to cause a lot of higher energy ground shaking. 

When you have taller buildings, the higher you are in a building, the more amplified the ground shaking is. If there was anybody in some of the taller buildings in Salt Lake City, they probably felt a little more shaking than somebody on the ground floor.

CB: Scientists have predicted the possibility of a large earthquake here in northern Utah along this Wasatch fault system. Was this it?

JF: No. This is what we call a moderate earthquake. We believe the Wasatch fault is capable of producing magnitudes of 7 to 7.5. This was a magnitude 5.7. So this really doesn't do anything to change when we’re going to get one of these larger 7 to 7.5 earthquakes. But this definitely does act as a reminder that we live in earthquake country and that earthquakes are going to occur here. They've occurred many times in the past and they're going to occur in the future.

CB: This definitely felt a little bit like a wake up call happening [just after] 7 a.m. What should people do with that information now that they have experienced a more moderate earthquake?

JF: Well, it's a perfect time to go over your earthquake preparedness, what your response would be. Grade yourself on how you responded to this one. See how you're gonna do better the next time when possibly the earthquake is larger. 

Right now, there's not a lot of damage. Travel right now is still happening at least on the roadways.

Of course, if we have one of these larger ones, then it'll be just much, much, much more. And the more that we can prepare, the more that we know what to do when you feel an earthquake, the better we’ll be. When you feel ground shaking — you may feel a lot more of this ground shaking in the next few days because we have quite a few aftershocks — you don't run outside. Drop, cover and hold until the ground shaking stops and then you take inventory of what's going on.

CB: What can scientists and researchers learn from today's earthquake?

JF: We'll take a hard look at this earthquake. We'll look at where it happened [and] figure out what fault system it happened on. We'll look at the aftershock sequence. The aftershock sequence right now looks quite intense.

The aftershock sequences tend to map out what part of the fault ruptured, so we can look at that. We can look at what type of earthquake it was, how deep it was, what level of ground shaking was felt throughout the valley. And we can use that to predict what level of ground shaking would be felt in a larger earthquake, so we can better model what type of damage we can expect in some of these large earthquakes.

Caroline Ballard hosts All Things Considered at KUER. Follow her on Twitter @cballardnews

Caroline is the Assistant News Director
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