Sunday, December 05, 2021

Geology in the Wild - The Thistle Landslide

Landslides are unfortunately a common problem around the globe, especially in areas of growth where humans alter the landscape. But landslides can happen even when there isn't human activity to artificially steepen slopes or reduce the cohesiveness of the land surface. One such landslide was the Thistle Landslide, that ended up destroying the town of Thistle, Utah.

The picture above is overlooking the landslide itself, which came down the valley in the center of the picture. The landslide filled the Spanish Fork River valley, which is located down below the train tracks as seen here. Fall of 1982 and winter/spring of 1983 had been extra wet, and a late snowfall in April along with a quick thaw caused 15 million cubic meters of earth to start sliding down the slope. The valley was populated with eroded debris from the North Horn and Ankareh Formations (a mixture of shales, silstones, and sandstones) that were nestled in a "trough-shaped depression", AKA a paleovalley.   


The slide started on April 13th, causing the Denver and Rio Grande Western Railroad tracks to start shifting over a few inches (the railroad was previously located further downslope than it is located today). By the 15th, the railroad had been shut down and the surface of the road, Highway 6, (also located along the valley floor) started to buckle. By the 17th, despite attempts at dredging the river, it was deemed impossible and the town of Thistle, located around the mountain towards the left of the photo, was abandoned. The landslide drowned the houses of Thistle and the entire town was left as a ghost town. At top speed the landslide reached speeds of 3.5 feet per hour and was 1000 feet wide, 200 feet thick, and over a mile long. The part of the landslide that dammed the Spanish Fork River edned up being about 220 feet high. 

View of Thistle Lake after the landslide. Image from the USGS

After the landslide, the railroad and river had to be rerouted through the mountain as seen in the image above and the highway was rerouted to just behind where I took the picture. In total, direct damage cost over $200 million (in 1983 dollars) and remains one of the costliest landslide in US history. 

References

Wednesday, December 01, 2021

Geology in Pop Culture - Salt Lake City Trax Station

 I love little nods to geology in everyday life and one of the Salt Lake City Trax stations has some geological themed plaques that I had been wanting to get pictures of for years. Well, since I was planning on moving I figured it was now or never and I ended up getting the pictures about a month before I left (in March of 2021). This is actually nearby to another Trax station with other geological themed materials I had taken pictures of previously.  

The Trax Station, which is known as Gallivan Plaza, has a series of "X" shaped plaques embedded within the floor of the station with various facts about Salt Lake City and Utah in general.

There are 8 X's in total (at least the ones I found) and I am including all of them here. Even though some of the X's pictured at the bottom here don't have geologically related content, they do have interesting information. 

...the Great Salt Lake usually measures about 2100 square miles; Lake Michigan is about 24,400 square miles...
...the pronghorn antelope is Utah's fastest mammal and Antelope Island was named for it...
The Great Salt Lake is one of the most notable geological features in the region, being an end basin (meaning the water does not flow out to any ocean and ends up in the lake) and is the largest lake in the United States outside of the Great Lakes (of which only Lake Michigan is completely in the United States). Since it is an endbasin, all of the dissolved minerals like salt build up over time and eventually make the Great Salt Lake the salty landmark it is today.

...salinity in the Great Salt Lake is about 8 times that of the ocean and companies harvest the salt for more than 1,000 uses...
...the first successful artificial heart and kidney were transplanted here...
As mentioned above, the salts in the Great Salt Lake have built up over many 10's of thousands to 100's of thousands of years as the water eroded the surrounding landscape and deposited the salts in the lake like a pan of water set to boil too long. Oceans typically have salinity between 34 and 36 ppt (parts per thousand), meaning that if you take 1,000 grams of oceanic water and evaporate all of the water away, you would be left with 34 to 36 grams of salt. While the Great Salt Lake varies depending on location within the lake, the range of salinity goes from 50 ppt in the Southern Arm to 240 ppt in the Northern Arm. Eight times the value of oceanic salinity at 35 ppt is indeed around the highest levels of salinity in the Great Salt Lake (~280 ppt). 

...about 350 tons of fossil were excavated near Dinosaur National Monument in the early 1900's...
...a mountain once stood where the world's largest open pit mine now sits in nearby Binghan (sp) Canyon...
Although proclaimed a national monument in 1915, the main dinosaur quarry was still actively excavated until 1924. The excavation was led by Earl Douglass, working for the Carnegie Museum of Pittsburgh, and continued until funding eventually ran out after Andrew Carnegie died. Douglass left the quarry in 1924, and since then no bones have been removed. From discover in 1909 until 1924 Douglass ended up removing nearly 350 tons of fossils, mostly dinosaurs, including fossils of Apatosaurus, Camarasaurus, Diplodocus, Barosaurus, Allosaurus, Ceratosaurus, Torvosaurus, Camptosaurus, Dryosaurus, and Stegosaurus. The quarry sits within the Jurassic age Morrison Formation and I go much more into the geology of Dinosaur National Monument HERE

Located in the Oquirrh Mountains on the west side of the Salt Lake Valley, the Bingham (Spelled "Binghan" in the plaque) Canyon mine was indeed a former mountain that had been slowly whittled away over time. Per the Utah Geological Survey, the Bingham Canyon Mine:
"...  is one of the largest and most efficient mines in the world. It has produced more copper than any other district in the U.S., accounting for over 16% of total U.S. copper production. In addition to copper, the mine produces gold, molybdenum, and silver. KUC’s combined annual value of these metals peaked in 2011 at $2.9 billion."
Currently the Bingham Copper Mine is the 2nd most active copper producing mine in the US and one of the top gold producers in the US as well. I go much more into the geology of the mine HERE as well. 

...11 national parks are located less than a day's drive from here...
...the usual humidity here is 15% or less...
I assume the 11 national parks here are just the parks designated as "National Parks" including Yellowstone, Grand Tetons, Great Basin, Arches, Bryce, Zion, Capitol Reef, Canyonlands, Rocky Mountain, Death Valley, and the Grand Canyon. That doesn't even include the numerous parks run by the National Park Service within and surrounding Utah.  

Although Salt Lake City itself doesn't qualify as a desert, the western part of the state easily does. A desert, by definition, is any place that gets less than 10 inches of precipitation a year. That means that there is typically not much water in the air, and hence a very low amount of humidity. Salt Lake City itself averages 19.5 inches a year of rainfall, and much of that is due to the mountains inducing rain to the east of the city.

..."Wasatch" means "mountains of many waters"... 
...several major and minor fault lines are located along the Wasatch Range...
The Wasatch mountains are the mountain range found along the east side of the Salt Lake Valley. Although the the translation on the plaque says Wasatch mean "mountains of many waters" the most common translation that I can find is that Wasatch is a Ute word meaning "low place in high mountains", of which I can see the correlation between the two. 

And as for the faults:

As this map from the Utah Geological Survey can attest to, the Wasatch Front is indeed riddled faults. The Wasatch Fault itself is not just one continuous line but a series of short faults that all interconnect with each other. That is why it is more accurate to say the "Wasatch Fault Zone" since there are so many smaller fault segments. 

The following plaques have some of the less geologically related facts.
...a recipe for storable pemmican cake: dried grasshoppers and crickets mixed with berries...
...no fish live in the Great Salt Lake but other lake life include tiny brine shrimp and larvae of small brine flies...
Random fact: the brine shrimp of the Great Salt Lake have been more commonly mass marketed as "sea monkeys". 

...the Great Salt Lake Valley has been continuously inhabited for over 10,000 years...
...Utah is named for the Ute Indians and "ute" means "a high place"...

I love the Native American facts that these plaques provide as well. Especially since Utah has such a rich Native history with tribes indeed occupying the lands now known as Utah for many centuries before any of the European descended explorers entered the picture.  

...the first recorded knowledge of the Great Salt Lake was by Silvestre de Escalante in 1776 but he never actually saw the lake...
...the world's largest genealogical library is located here and records date back to the 1500s...
And the final plaque. We actually visited the genealogical library run by the The Church of Jesus Christ of Latter-day Saints (you don't need to be a member to use the library), which has a ton of resources for those seeking their family history, not just from Utah but from around the globe.

Wednesday, November 24, 2021

Geology of the Salt Lake Valley

One of the things I wanted to do before I moved from Utah was to get a nice panoramic picture of the Salt Lake Valley from above Salt Lake City. Here is a shot of the Salt Lake Valley looking south (so east is left and west is right). In the picture the Wasatch Mountains are on the left side of the valley and the Oquirrh Mountains are on the right side of the photo.  

The reason for this, besides just being a beautiful valley, was the geological setting of the valley. The Salt Lake Valley holds an important place, geologically speaking, in the landscape of the United States. The valley is the eastern most extension of the region known as the Basin and Range. 

Coverage of the Great Basin. Image courtesy of the NPS.

The Basin and Range extends from the Wasatch Mountains in the east to the Sierra Nevada Mountains in the west. It encompasses the western half of Utah and pretty much all of Nevada, as well as parts of Idaho, Oregon, California, and Arizona.


Illustration of the plate tectonics of the west coast of North America with the Farallon Plate subducting beneath North America. Image courtesy of the NPS

The formation of the Basin and Range Province began long ago when a plate known as the Farallon Plate was being subducted under the west coast of North America. This occurred along the Californian coast and south into Mexico. 

Evolution of the western coast of North America going from a subduction zone to a transform plate boundary causing the formation of the extensional Basin & Range Province. Images courtesy of the NPS.

The subduction, where one plate goes beneath another, produced volcanoes in California and other places in the American west, and it also squeezed North America. Around 40 million years ago, most of the Farallon Plate was completely subducted beneath North America. What that did was a few things:

  1. It released the pressure that was squeezing North America, like someone releasing a squeezed sponge. 
  2. The subduction zone was no longer. In it's place a new plate boundary formed, a transform plate boundary known as the San Andreas Fault.  

As the pressure was released, the plate started to expand. This expansion produced a series of north-south running blocks, which were rotated from the pressure release. These rotated block produced the north-south running mountains we see across the region today, as well as the north-south trending faults that border all of the mountain ranges.  

As the mountain ranges were rotated upwards, they were eroded. The eroded material washed down into the gaps between the mountains, producing the sediment laden valleys we know so well today, including the Salt Lake Valley. This is the reason that earthquakes can also be particularly dangerous in this region, because all of the sediment has a tendency to shake like Jell-O in an earthquake. 

The eastern valley produced from these rotational blocks was the Salt Lake Valley, with the Wasatch Fault, the fault at the base of the Wasatch Mountains, representing the eastern most extension of the region. It is also one of the most active faults of the whole region since it is a boundary fault. 


This can also be seen in the fa├žade I designed for the University of Utah parking garage seen here with some of the various Basin and Range mountain ranges as well as their fault lines. 

Saturday, November 20, 2021

Geological Christmas Gift - Coal Bags

 Last year I thought it would be fun to put together some coal bags for those of us who really enjoy getting coal for Christmas (or any of the season's holidays). I have the bag available up on Etsy right now for anyone interested.

While living in Utah, we lived close enough to a prolific coal seam that jutted out along a road cut on a highway, so I collected a few samples to be used in the gift bags. 

Coal along the Helper, UT roadcut. The coal belongs to a geological formation known as the Blackhawk Formation. 

Stratigraphic column of the Blackhawk Formation and neighboring formations from the Geological Survey publication on the rock units.

The Blackhawk Formation, specifically the coal bearing member, is Late Cretaceous in age. Specifically it is from the Campanian, ~80 million years ago. Coal forms in swamps. Living plant matter dies over time in the swamp, but there is so much decaying plant matter in a swamp that there's not enough oxygen in the water to break it all down. So the plant material builds up over time, and eventually gets buried by mud, sand, silt, or other sediment. This preserves the plant matter that slowly gets buried and cooked over time. The cooking process removes all of the other materials in the plants than the carbon, which is what the coal is.

For the gift bags, I wanted to include as much of that information as possible. 
Text on the inside of the tag:

Type of rock: Bituminous Coal

Rock Source: Blackhawk Formation (coal bearing member)

Age: Late Cretaceous (Campanian, ~80 million years old)

Environment: The coal you hold was once plant matter in a swamp. There was so much life in the swamp, that when the plants died there wasn’t enough oxygen in the water to break them down, so the plant matter built up. Eventually this plant matter was buried by sand and mud, and over time the plant matter was slowly “cooked” below the surface of the earth until all that was left was the carbon from the plants, creating coal.

I also wanted to include two varieties of gift tags that you can choose from.  
The traditional "Naughty" version, where the person getting the coal was bad:

Someone was naughty,

And made Santa mad.

So, this coal is your gift,

Cause let’s face it; you were bad!


Or the geology version, where the person getting it just likes rocks. 

Not everyone who’s been bad gets coal.

Sometimes that may be one’s very goal.

So here is a gift to stuff your sock.

As someone who really love rocks.


Each gift bag available here comes with:
- 2 to 4 pieces of coal that total about 6 ounces in weight.
- A red felt gift bag with draw string
- A naughty or nice tag, complete with geological information about the coal on the inside.

So if you are interested, or know someone who is interested, please pass the word along :-). 

Thursday, November 18, 2021

What are ... Archaeologists vs Paleontologists vs Anthropologists

 For the next entry into my What are ...? series, I am looking at something that is often confused, the difference between an Archaeologist and a Paleontologist. This confusing was brought up recently when I was reading the question cards in my daughter's game of Dino Math Tracks:

Within the game, there are cards where the person must solve the math word problem in order to figure out how many spaces they can move. This is the card in question:


Archeologists must ship 24 tons of dinosaur bones to the museum. If each truck can hold 2 tons, how many trucks do they need to ship the bones? Move forward that number of spaces.

This brings up a common mistake in everyday life: mistaking what an archaeologist is versus a paleontologist. 

Archaeologist: According to Webster's Dictionary archaeology is the study of past cultures through their surviving relics

This means that an archaeologist studies the remains of civilizations that people leave behind. This can include some things like arrowheads to the pyramids, bowls to clothing, eating utensils to cave dwellings, and everything in between. Archaeologists study the changing human behavior but not the changes in our physical selves. This article puts it perfectly:

... archaeology is the reconstruction of ancient behavior from the things people left behind.

Paleontologists: Paleontology on the other hand is the study of life in past geologic periods, according to Webster's Dictionary.  

This means that paleontologists are focused on the life that once lived and often how it connects to the life that is still around. They can look at the physical remains of the animals and plants to reconstruct what they looked like and how they moved and interacted. They can also look at the behavioral remains left behind like footprints and bite traces to determine how the animals lived. All of this combined will often give paleontologists a fairly good idea of what past environments even looked like. Paleontologists are most often thought of as studying dinosaur bones, but there are many types of life that a paleontologist can study and if there was once something living on this planet that we know about, you can be sure that there was, or is, someone that has studied it. 

The difference here is that paleontologists can study any animal remains, but they don't study the behavioral creations of humans. The question then arises, that is humans are animals, then wouldn't a paleontologist then technically also study humans. Here is where lines start to get blurry and people's definitions may vary but there is a cross specialty that encompasses both worlds, and that is anthropology.

Anthropologists: Anthropology is like the blurry boundary between paleontology and archaeology. Webster's describes anthropology as the science of the origin, early development, and culture of humankind

Anthropologists study the human remains to discover how our species has evolved over time, physically, behaviorally, and culturally. Archaeology, on the other hand, is typically seen as a sub-category of anthropology focusing only on the physical items humans created and left behind, while human paleontology (a.k.a. paleoanthropology) could be seen as another name for the study of the human evolution and remains typically seen as one of the sub-disciplines within the overarching body of  anthropology. 

As this website puts it: The goal of anthropology is to better understand the different cultures of human history and to look for solutions in human problems. Anthropologists use social, biological, and physical science to get a better grasp on human cultures.

Monday, November 15, 2021

Geological Destination - Escalante Petrified Forest State Park

 Just outside of the town of Escalante in Utah is the Escalante State Park, also known as the Escalante Petrified Forest State Park. The park sits on the shores of the Wide Hollow Reservoir, which is a great little reservoir to swim in or boat and fish. But the geological destination for this park is the Nature Trail, also known as the Petrified Forest Trail. Along the trail, large petrified trees are easily visible. Although not as densely packed with trees as Petrified Forest National Park, this is still a fantastic view of the logs in an unexpected location. 

Many of the logs are easily visible from the trail. The park itself preserves about 5.5 million tons of petrified wood across the 1,400 acres. These logs had been petrified, which is a type of fossilization specifically referring to trees. The wood molecules had been slowly replaced over time by molecules of minerals, in this case silica, also known as quartz. Over time all of the wood molecules would have been replaced with the silica creating a tree shaped rock that preserves many of the intricate details from the tree itself. These details include the tree rings and bark. 

The state park sits right in the middle of several National Parks including Grand Staircase-Escalante National Monument to the south and Capitol Reef National Park to the north. 

The logs sit within the Brushy Basin Member of the Morrison Formation from the Late Jurassic (~130-140 million years old).  The wood is thought to have eroded out of the overlying conglomeratic part of the Brushy Basin and tumbled down the slope to the lower mudstone, where it sits now. The mudstone then erodes much more easily than the harder, and heavier, quartz logs, so the logs remain behind as the other material is washed away/eroded over time.

The petrified wood layer within the Brushy Basin Member is one of the youngest layers of the Morrison Formation. This stratigraphic section is from the Utah Geological Association Publication of the Escalante State Park Geologic Trail Guide

During the hike you get a pretty good overview of the campground and the reservoir as well as the various layers of the Brushy Basin from the caprock of the area, the conglomeratic unit, to the underlying softer mudstone. 

The trees were deposited within a braided stream system, much like the above braided stream system from Banff National Park in Canada. These streams are often slow moving stream systems with periodic large amounts of water, so they are able to move a large array of sediment sizes from sand to gravels, and even large pebbles and boulders. These sediments are often more varied and larger than are frequently found in more meandering streams like the Mississippi River. The trees in this park would have likely grown up on the banks of the river, when they were uprooted and rolled along the river until they came to rest in the gravel and sand channel.

View of one of the logs highlighting the rings. The trees in the park are not of the best preservation to be able to identify them very well. However, some of the trees had been identified as conifers, however the level of preservation is not fine enough to be able to identify which species of conifer.

Saturday, November 13, 2021

Dinos in Pop Culture - LEGO Dinosaurs

 When LEGO decided to release the Dinosaur Fossils set it became a "must have" for me. These are far from the only dinosaurs LEGO has ever produced as well but besides being specifically the skeletons of the dinosaurs (and Pteranodon, which is not a dinosaur) they are set up as if they were museum displays. This makes the paleontologist heart in me happy. 


The line of LEGOs comes from their LEGO Ideas sets, which are LEGO sets that had been submitted by  users as potential builds. These ideas/suggestions are then reworked by LEGO professional builders to make them work in a mass market possibilities. This set was originally designed by Jonathan Brunn, who is a French graphic designer and reworked by the LEGO Designer Niels Milan Pedersen. 


The set itself comes with the three main fossil skeletons, Tyrannosaurus rex, Triceratops horridus, and Pteranodon longiceps. On top of that it comes with a little paleontologist, box of paleontological tools and books, as well as a "LEGO sapiens" model (i.e. the bones of a LEGO figure).  

The models are set up as skeletons in a 1:32 scale with the skeletons on a display pad complete with identification plaque. Most of each figure is adjustable as well, so that you can get the models in the pose that you really want. 

Even though they are obviously LEGO figures, the models themselves are actually quite awesome. They are not a 1-to-1 recreation of a real life skeleton, but they are as close to what would happen if a real life skeleton got put through a LEGOification machine. 

And I love the variety that you get. You have the two stalwarts of dinosaur popularity representing the carnivores (T. rex) and herbivores (Triceratops), and you also have the non-dinosaurian flying Pteranodon complete with a stand so you can place it in "flying mode". 

Your crate of goodies also comes with some bones, a magnifying glass, an egg, a cup, a rock hammer, a book about fossils, and some unidentifiable tools.  

Overall, this set earns an A+ from me.

Thursday, November 11, 2021

Utah's Pandemic Earthquake of March 2020

Continuing on through my pile of photos I come across the start of the lockdown in March of 2020 and I am reminded that at the very start of the lockdown, Utah was hit with the largest earthquake it has seen in recent memory and the largest earthquake I personally have ever been a part of. 


On the morning of March 18th, 2020 at 7:09 am local (MST) time, western Salt Lake County was hit with a 5.7 magnitude earthquake. At the time we lived in the valley to the west, not 13 miles from the epicenter of the earthquake. The epicenter is the spot on the surface directly above where the fault moved (known as the focus). Earthquakes occur due to energy that is released after a slip occurs along a fault. This energy produces shaking in the ground, both up and down, side to side, and back and forth. This motion of the ground is what we feel on the surface. 


Upon the start of the earthquake we were getting ready for our day. The wife was getting ready for work, the child, having off of school that week for spring break, was playing in her room, and I was still in bed. Upon the start of shaking I jumped out of bed and grabbed her from her room into the doorframe of my bedroom since our bedrooms were directly across the hall from each other. 

The doorframe is one of the safest locations in a home during an earthquake because structurally it is one of the strongest spots. Lights, parts of ceiling, pictures, and stuff on shelves are less likely to fall on you in a door frame. Walls are also less likely to collapse there. Under a desk or table is also another safe location for the same reason, the structure will provide protection from falling objects. Running outside is actually a terrible idea because the initial shaking of an earthquake only lasts a couple of minutes at most, and in that time you won't get very far and around the outside of a building is the most dangerous place to be since things like bricks can fall off a building and glass can break from windows. So, stay put in a safe location, if possible.   

Above is the initial shake map from the earthquake from the USGS. Earthquake.usgs.gov is my favorite website for quickly identifying earthquakes and their sizes and locations. The website is updated within about 5 minutes of an earthquake. This is also helpful during the numerous aftershocks we felt to wonder if they were an actual aftershock or just a passing truck. The shake map tells you the location and the size of the earthquake, as well as how far away it was likely felt. This information will get refined over the coming hours and days as more information is obtained.

One of the things about an earthquake that I never realized is that I was constantly feeling aftershocks for months afterwards, regardless if we actually had them or not. Having never been in an earthquake, I thought it would be super cool as a geologist to be part of one. I was wrong. It was terrifying. And still is, because there is no warning and no way to really get mentally prepared except to have everything you need in the event that one happens and your home or place of work to be structurally prepared. 


Within a few hours the area was inundated with aftershocks including four of a magnitude 4.0 or greater. The above map represents the shake map of the 4.6 aftershock that hit in the afternoon of March 18th. 

Above is the most up to date shake map of the main, 5.7 Magnitude earthquake. This is what is called a Modified Mercalli Map, which uses people's real life experiences to help shape the impact of the event. If something fell of a shelf in your house you would have felt a different different level of intensity than if your entire house fell down. Each of those dots represent points of data where someone contributed to the map. If you are a part of an earthquake, this is a great way for everyday citizens to participate in citizen science at USGS's Did You Feel It

As identified by the University of Utah Seismographic Station, there were over 2,500 earthquakes that took place in the same area from March 18th through the following year. 

UUSS's seismic graph from earthquakes.utah.gov.

Of all those 2,500+ earthquakes, all but one was identified as an aftershock, the one being the main event earthquake. An aftershock is an earthquake along the same fault line that occurs after the main earthquake and is always lower in intensity. If for some reason we had an earthquake that was a 6.0 after the 5.7, the 6.0 would not be considered an aftershock. At that point the 5.7 would be renamed a foreshock (taking place before the main earthquake) and the 6.0 would become the main earthquake. So in actuality, aftershocks are only identifiable as such as long as they are smaller than the main earthquake and to understand the entire picture of an earthquake, some time has to progress until we see all of the impacted fault movement and earthquakes.  

The Basin and Range Province. Image courtesy of Miracosta.edu.

Geologically speaking, Utah is located along the eastern edge of what is called the Basin and Range Province. This is an area that is actively undergoing extension. Meaning the the plate is essentially being stretched apart here. The result of which is what we see, a series of parallel, north-south running mountain ranges with valleys in between. Intermixed with all of this is a series of volcanic eruptions occurring all over the region due to the thinning crust. I had covered a bunch of Utah's volcanic legacy in a previous post HERE.

Extensional activity in the Basin and Range Province. Image courtesy of Miracosta.edu.

The types of faults most commonly associated with this type of extensional activity is known as a Normal Fault. 

A Normal Fault is where the overriding block (the hanging wall or head wall) moves downward in relation to the under-riding block (the footwall). 
Location of the Wasatch Fault in Salt Lake City.

Along the eastern edge of the Salt Lake Valley is the Wasatch Fault, the largest fault in the region, and also a Normal Fault. Generally it is considered a "fault zone" because of all of the offshoots and short segments of the fault, especially since scientists are not 100% sure about where exactly the fault is located in all locations. 

Wasatch Fault Scarp in Salt Lake City

Where the fault does hit the surface in a fairly well identified area is in the eastern edge of Salt Lake City, where a fault scarp can plainly be seen. A fault scarp is the cliff or surface disruption along the surface rupture of the fault where the two blocks can easily be identified, as in the picture above, because the one block juts up compared to the other block. But a fault isn't likely to cause an earthquake just where it surfaces. The fault also extends many miles below ground. And as can be seen in the diagram above and below, the Wasatch Fault curves towards the west, below the Salt Lake Valley.

A great graphical representation of the Wasatch Fault below ground from earthquakes.utah.gov.

And that is where the earthquake of March 18th occurred on. The movement occurred on the Wasatch Fault, but it occurred 6 miles (10 km)  below the surface. This is the reason why although the fault lies along the eastern part of the Salt Lake Valley on the surface, the earthquake occurred almost on the western edge of the valley on the same fault. 

Right now it is estimated that an earthquake of a 7.0 to a 7.6 will occur sometime in the future along the Wasatch Fault Zone. This is based on historical estimates of the previous earthquakes along the fault and size estimates based on the size of the fault. Earthquakes are only able to generate energy based on the size of the fault. The larger the fault, the larger the potential earthquake. So, based on the size of the Wasatch Fault, a 7.6 would be the maximum size that an earthquake could be. However, that would be detrimental to the region because many buildings and other infrastructure are not built to withstand an earthquake of that magnitude.

The odds of a 7.0 earthquake occurring within our lifetimes is generally on the low side, though. So, although there is no reason to constantly live in fear, there is reason to build our community with earthquake resiliency in mind. Especially since our buildings will hopefully far outlast us.