Out Standing in a Field: What Utah's past earthquakes mean for the future

Tuesday , October 31, 2017 - 5:15 AM

LEIA LARSEN, Standard-Examiner Staff

Utah's Wasatch Front is due for a major, devastating earthquake.

Out Standing in a Field podcasters Benjamin Zack and Leia Larsen visit with a Utah geological hazard mapper and a seismologist to learn about the region's past earthquakes and what they tell us about the future.

They also learned about what it took to make a comprehensive map of Utah’s faults and recorded earthquakes from 1850 to 2016. The impressively detailed map was released by the Utah Department of Natural Resources in September, along with a forecast for the next major earthquake on the Wasatch Front.

There is a 57 percent probability that a magnitude 6.0 or greater earthquake will occur in the next 50 years.

Out Standing in a Field is the Standard-Examiner’s podcast about science and environment. Episodes are available wherever you find your podcasts, including Apple Podcasts, Google Play, SoundCloud, Stitcher and TuneIn.

Follow Out Standing in a Field on Facebook for photos and notes from the field. 

 

A transcript with excerpts from the episode is included below, but trust us — it’s better to listen.

 


 

BENJAMIN ZACK: Welcome, everyone, to the Out Standing in a Field podcast.

LEIA LARSEN: We are the Standard-Examiner podcast on science and the environment.

ZACK: The field we’re heading into this time are those hit by earthquakes, which is really quite a bit of Utah.

LARSEN: More than 36,000 earthquakes have occurred in the Utah region since 1962.

ZACK: You don’t usually think of Utah as an earthquake hub, but we definitely have our fair share here.

The biggest one in modern, human history would be the Hansel Valley earthquake of 1934. It’s up north, kind of in the middle of nowhere by the Great Salt Lake. But you who were around in 1934 felt it all over Northern Utah.

LARSEN: Up to 80 miles away.

ZACK: It was a magnitude 6.6.

LARSEN: You might be wondering, what got us thinking about earthquakes?

ZACK: About a month ago, the Utah Department of Natural Resources issued a map showing all the historical earthquakes and all the earthquake faults in the state.

LARSEN: Right, so we’ve got the geological record on this map and the human record of earthquakes on the map. One thing I noticed was that many of the earthquakes recorded in the state haven’t actually occurred on the Wasatch Fault.

We had so many questions about this map. We decided to reach out to some of the scientists who helped make it.

ADAM MCKEAN: My name’s Adam Mckean, I’m a geologist with the Utah Geological Survey. I’m a geologic mapper with the hazards program.

ZACK: We met with McKean up in Willard where you have some of the most dramatic mountains you find on the Wasatch Front, where you can really see these earthquake histories.

We able to get an up-close look at the Wasatch Fault.

LARSEN: McKean gets to spend weeks, months in the field exploring the nookies crannies of Utah and its geology.

MCKEAN: Here in Willard, I walked up each of these canyons and ridges. It took me about two or three weeks. I’d go up until I hit a cliff then come down. Rock climbing wasn't part of the gig. I didn’t need the data that much.

ZACK: As Northern Utahns probably know, the mountains behind Willard are pretty dramatic. Steep and rugged. All said, McKean clocked 30,000 vertical feet with all that surveying, that walking up and down. That’s enough to climb Mt. Everest.

LARSEN: When you’re looking at a landscape, what are you looking for?

MCKEAN: Here we have Willard peak, Ben Lomond, there’s about 5,000 to 6,000 vertical feet difference between those peaks and the valley floor. As a geologist, I’m always going to ask ‘how do we get that kind of topography, the geomorphology?’ That story of basin and range continues all the way to California, to Reno, Nevada. That’s the first thing, we have a big mountain range and a valley, how do we do that?

Then I start looking for evidence of the most recent surface fault rupturing, the earthquakes. That would be these benches.

While we’re looking here, there’s a rise going from Highway 89 to Holmes Canyon. After the power lines, it takes a step up. You can see a shaded area, that’s the Wasatch Fault. We’re on the Brigham city segment.

LARSEN: How can you tell the Bonneville Shoreline apart from fault?

MCKEAN: Faults don’t care about topography. They cut up against whatever they want. Lake sediment like deposit as nice, level surfaces.

It doesn’t cut and build up as a ramp or slope.

ZACK: That Wasatch Fault Zone has been active for the last 17 million years and it’s still active.

LARSEN: It’s had a lot of time and earthquakes to create those offsets.

Let’s talk about faults. What is a fault?

Faults aren’t actually a line, they are surfaces where slip takes place. These surfaces are often angled, extending many miles underground.

ZACK: As McKean alluded to earlier, this isn’t a single continuous line in the ground, either. They don’t break in the same place every time. The Wasatch Fault, for example, has many segments, including the Provo Segment, the Salt Lake City Segment, the Weber Segment, where we live, and the Brigham City Segment, where we met with McKean in Willard.

MCKEAN: Sometimes fault can just be a single line with what looks like one narrow zone ... or can be hundreds of feets wide or hundreds of yards wide.

LARSEN:So my idea that this big plate is shoving into the Wasatch Front is not the case. It’s little sections.

MCKEAN: Yeah, the Wasatch fault zone is expanding and coming apart. Western Utah is pulling away from eastern Utah. The opposite was true in the Cretaceous, where western Utah, Nevada and California were shoving toward eastern Utah. That was about 120 million years ago.

LARSEN: There are three types of faults — strike-slip , thrust and normal.

ZACK: Strike slip is like what’s happening in California on the San Andreas fault. It’s two surfaces rubbing against each other.

LARSEN: A thrust fault is like the Himalaya Mountains, with two sections of crust ramming into each other.

ZACK: A normal fault is what’s happening in the Basin and Range. Despite the name, they’re not the most “normal” or common type of fault, it’s a leftover term from mining days. It’s when crust is pulling apart.

LARSEN: All the way across what’s know as the Basin and Range province, the crust is pulling apart by an average … of one-third to half an inch. So Salt Lake City and Reno are slowly pulling away from each other.

LARSEN: As you might be putting together, even if an earthquake is centered on the Salt Lake City segment or the Brigham City Segment of the Wasatch Fault, that doesn’t mean we won’t have impacts in Ogden.

MCKEAN: If there’s a large earthquake here, everyone’s going to feel it.

That’s something we need to learn to live with. The more we study and understand them, the better prepared we are.

Now I don’t think about that on a daily basis.

LARSEN: How can you not?

MCKEAN: I’m out here mapping this stuff, looking for the faults, the landslides, describing them in minute detail.

ZACK: That takes us into the second part of this episode. We’ve been talking about what happened here over the past millions of years, but that still leaves questions of what’s happening more recently with earthquakes and also what’s going to happen in the not-too-distant future. We met with the scientists mapping those more recent earthquakes and combining that information with the geological record to try and build some forecasts.

LARSEN: We hopped in the car and traveled south to the University of Utah. We met with one of the scientists working at the Utah Seismograph Stations.

JIM PECHMANN: As far as a title, let’s just call me a seismologist.

LARSEN: What does a seismologist do?

PECHMANN: A seismologist is a type of geophysicist who studies seismic waves that travel through the earth.

ZACK: This group of seismologists based at the U have hundreds of stations around the state of Utah and also do a lot of work in Yellowstone.

The seismographs measure ground motion throughout the state, turn it into an electrical signal that’s transmitted to the recording lab, in their headquarters at the University of Utah. There they have folks combing through that data and analyze what’s happening to the ground.

PECHMANN: We can pick up earthquakes that are way smaller than you can feel.

LARSEN: Pechmann helped create that map we were describing at the beginning of the show. We asked why so many of the earthquakes the mapped aren’t on a fault.

PECHMANN: That’s a really interesting question, I wish I could answer it.

You can’t basically map out the major faults by looking at distribution of small earthquakes. In fact, if anything, the Wasatch Fault notable for its lack of small earthquake activity.

...

You don’t need a very big fault for a magnitude 3 earthquake or a magnitude 2 earthquake, you just need a small fault. They do occur on faults, but these faults are usually unknown or buried faults that don’t come all the way to the surface.

ZACK: Pechmann explained this also plays into hazard in that you can have earthquakes up to a magnitude 6.75 that don’t necessarily occur on known faults but can cause quite a bit of damage. These are called background earthquakes.

PECHMANN: That’s one of the reasons why it’s important to record earthquakes, determine magnitudes, extend that catalog back as far as we can in time. It’s only from that type of information you can predict the rate of these background earthquakes.

ZACK: The first seismograph installed in Utah came in 1907, a year after a big historical earthquake in San Francisco.

The Utah Seismograph Network began in July 1962 with just five stations.

LARSEN: Without a network of seismographs before the 1960s, how did they figure out where and when old earthquakes occurred?

ZACK: Newspapers, maybe?

PECHMANN: Yep, you have to go off felt reports.

Most of the historical earthquake locations and magnitudes are based on felt reports from newspapers and other historical documents.

The area over which it was felt, the area over which it caused damage … that gets quantified into something we call the intensity scale.

ZACK: You take all these newspaper reports and documents from before the seismograph stations went it, compare them with reports right after the seismographs went in and you should have a pretty good idea of what people describe feeling during a magnitude 3 earthquake versus magnitude 6 earthquake, or what kind of damage they document if they’re one mile from the epicenter versus forty miles.

PECHMANN: You map out all those reports, map out the intensity and ideally if you’ve got good geographic coverage, they form a bullseye pattern and earthquake is located in the middle of it.

LARSEN: After gathering all the information from the geological record and the human record, Pechmann and a few other scientist were able to come up with a probability report for when the Wasatch Front will experience the next “big one” — it’s next massive, devastating, destructive earthquake.

PECHMANN: It was 57 percent for mag 6 or greater and 43 percent for 6.75 or greater (in the next 50 years).

ZACK: With these probabilities, is that something fairly constant for any 50-year period, or specifically to 2017 to 2067 Would the numbers be different?

PECHMANN: Good question. They would change a little bit.

Basically any given fault or fault segment, the longer it has been since last earthquake, the closer you are to the next.

It’s not like clockwork, but there is a repeatability to it.

LARSEN: Remember, a probability and a prediction aren’t the same thing. Pechmann isn’t predicting exactly when and where the next earthquake will strike.

PECHMANN: We can say for sure that large earthquakes will occur in the future on most of these faults, if not all of them. It’s not a question of “if” but “when.”

LARSEN: We also saved the best question for last, a question I’m sure you’re all desperately wanting to know.

Could an earthquake cause a tsunami on the Great Salt Lake?

PECHMANN: I get that question a lot. Yes, you could. But basically, the lake level is pretty low right now. It’s not going to cause any damage at these low levels.

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