Wasatch Fault Earthquake Scenario

 This scenario literally hits home for me. I live along Utah's Wasatch Fault and while I don't think the epicenter of the next Big One will be too close to where I live, the effects of this big earthquake will seriously affect my life and the lives of my wife, children, and many of my extended family.

Most people have only "seen" the effects of a big earthquake from the media, either news or movies. It's different to personally feel a big earthquake and see the aftermath. For lack of a better descriptor, it possible effects of a Big One become real.

Because a Big One one the Wasatch Fault is more personal, I've done more research on it. And, interestingly, information on a Wasatch Fault earthquake scenario is easier to find. Maybe that's just my bias. It's not that you can't find earthquake threat information for other parts of the country, it's finding actual scenario "what ifs" and loss estimations that are based in scientific data.

In any case, even if you don't live along the Wasatch Fault many of the hazards and risks a big earthquake are similar regardless of where the big one occurs. That said, each risk area does have unique challenges. For example, a large earthquake along a coastal region may cause a tsunami. That's not a threat along the Wasatch Fault, although seiches would be for those near large bodies of water. Similarly, those living along the foothills might experience landslides or rockfalls, and those in the valleys, particularly near the waterways of the Provo or Jordan Rivers, or near Utah Lake or Great Salt Lake are more vulnerable to liquefaction.

Before getting into the earthquake scenario, it's good to know a bit of background about the area. Sometimes Wasatch Fault and Wasatch Front seem to get mixed up. Although they are geographically similar, the Wasatch Fault is the series of fault segments (more on that later) and the Wasatch Front is the area along the mountain range where most of Utah's population lives.

The Wasatch Front

The Wasatch Fault runs along the Wasatch Front, which is a section of the Wasatch mountain range.

The Wasatch Front spans a four-county area of northern Utah, from Brigham City in the north to Levan in the south. The population of this metropolitan area is nearing 3 million, and more than 75 percent of Utah’s economy is found in this region. Utah’s capital, Salt Lake City, is near the center of that stretch.

The population of those four counties is like Cleveland and Pittsburgh, and well above that of New Orleans and Nashville metropolitan areas. While Utah isn’t known for earthquakes, like California, there are more people living along a major fault line in Utah than in any other state, other than the Golden State (The Biggest Reason to Worry about Recent Utah Earthquakes, and How to Prepare). 

However, Utah has a few unique geographic risks that could cause even more havoc than in other high-risk areas. The Great Salt Lake in Salt Lake County and Utah Lake in Utah County could both pose threats to nearby cities in the event of a major earthquake. Seiches could wash ashore, damaging communities along the shorelines of the lakes. Tectonic subsidence could lower or tilt the valley floor, causing water to rush onto what was once higher ground. Soil liquefaction could cause structures to sink, particularly in areas near the lakes where the water table is higher, and the soil composition is more susceptible to this hazard.

Unlike Southern California and even parts of northern California, northern Utah—where the Wasatch Front is located—experiences winter weather. From November to March, freezing temperatures are common, with single digit lows likely, particularly in late December through January. Since the probability of an earthquake is the same throughout the year, there’s just as good of a chance it could strike during the cold winter months.

It’s practically a guarantee the power will be out after an earthquake. The question is, how long? Other utilities, including potable water, will also be out or severely crippled.

Earthquake scarps—tall ledges—can be found all along the Wasatch Front from historical quakes. These scarps form when sections of the land along the fault drop or rise up (depending on your view) during an earthquake. The resulting scarps are, on average, three to nine feet high. Twenty-foot scarps are not uncommon. Some ancient scarps are about 130 feet high, which most experts believe were caused from repeated earthquakes. Many of the smaller ancient scarps have eroded, or modern construction has obscured their presence.

A magnitude 7.0 earthquake would certainly create new scarps across the valley. A possible thirty-mile scarp could form along the eastern side of the Salt Lake Valley, and a similar one is possible on the western side. One new scarp, and definitely two, would severely impact utilities and transportation. Other scarps from the various faults in the valley could cripple sections of the city, complicating immediate rescue efforts and hampering recovery.

Think of a scarp like a kink in a hose, only worse. A scarp across a freeway would at best cause a minor issue with the road and cripple the flow of traffic for a few days. At worse, a large scarp would rupture the transportation corridor and completely prevent traffic from moving from one side to the other. Alternates would need to be established until a bypass over (or through) the scarp were created. Of course the likelihood of a scarp affecting only one road is unlikely.

Similarly the rise or drop of land would likely damage or destroy other infrastructure that passes through the area. The free flow of water, electric, gas, and sewer could all be affected.

Here's one of the biggest concerns. Unlike California, too many of Utah’s buildings are not earthquake ready. There are 150,000 buildings lacking any kind of earthquake retrofit. Especially concerning are more than 500 brick schools in the Salt Lake Valley that need reinforcement.

A Deeper Look at Utah's Wasatch Fault

The Wasatch Fault is a normal fault, where the Wasatch range mountain block of crust slips mostly vertical, relative to the western, valley side that has a downward movement. It is the “most studied…normal fault in the world” (Earthquake Probabilities for the Wasatch Front Region in Utah, Idaho, and Wyoming). 

The fault runs for about 240 miles and has ten segments. The north end reaches towards Malad City, Idaho, and the southern-most segment extends to Fayette, Utah. Each segment is thought to rupture independent of the others. Because of the multiple segments, the term Wasatch Fault Zone is often used interchangeably with Wasatch Fault.

Wasatch Fault segments, central segments are in red
Segments of the Wasatch Fault. Central segments are in red. Other northern Utah faults are shown in dark gray. Image from the Utah Geological Survey document “Earthquake Probabilities for the Wasatch Front Region in Utah, Idaho, and Wyoming” 

The Wasatch Fault is the most active fault of the seismically active area known as the Intermountain Seismic Belt, which extends from south from Montana to Arizona and west to Nevada. 

The five central segments of the Wasatch Fault have had at least 22 surface-faulting earthquakes during the last 6,000 years. These central segments extend from Brigham City in the north to Nephi in the south. The Wasatch Front urban corridor, which includes the cities from Ogden to Salt Lake City to Provo, are in these segments. Another large earthquake hit the Levan segment, south of Nephi, about 1,300 years ago.

Collectively, the average recurrence interval (long-term average) of these large earthquakes is about once every 300 years. The last definitively large earthquake happened on the Provo segment about 600 years ago. 

There are some discrepancies among sources about a large quake on the Nephi segment (just south of the Provo segment) that possibly occurred as recent as 400 years ago. Some carbon dating indicates it struck the area about 1,200 years ago. However, research in the last few years has once again revised that estimate to about 200 to 300 years ago. If anything, this just proves how much earthquake research is changing and revising what we think we know.

Historic major earthquakes on the five central segments of the Wasatch Fault
Past major earthquakes on the five central segments of the Wasatch Fault. Image from the Utah Geological Survey document “Earthquake Probabilities for the Wasatch Front Region in Utah, Idaho, and Wyoming” 

Individually, each central segment has its own earthquake recurrence interval ranging from 700 to 2,700 years. The outlying, or distal, segments have recurrence intervals of about 10,000 years.

The largest earthquake potential for any of the segments along the Wasatch Fault is estimated to be a magnitude 7.5.

Based on the limited 6,000-year earthquake record, the next Big One is past due. It’s been over 600 years since the rupture on the Provo segment, and possibly less than that since the last Nephi event. However, using the 300-year average, the area is past due for its next Big One. Where it will happen is unknown. Of the five central faults, the Brigham City segment has gone longest since any major activity, so it could be the most susceptible. However, if segment ruptures are regular, then the Salt Lake segment will be the next one hit.

Earthquake Risk no the Wasatch Fault

Previous estimates put the likelihood of a large earthquake at 13 percent in the next 50 years, and 25 percent in the next 100 years. However, the Working Group of Utah Earthquake Probabilities (WGUEP) conducted a thorough analysis of the probability of large earthquakes along the Wasatch Front. They then produced a report in 2016, and the results were surprising.

The analysis factored in the various fault zones in the area. Previously, evidence of only 18 surface-rupturing quakes had been found along the central segments. Additional research identified at least 22 large earthquakes in the last 6,000 years in the segments from Brigham City to Nephi.

Another change based on the 2016 report was the estimated recurrence interval. Previously, the average interval between large earthquakes was estimated to be about every 350 years. The new interval is every 300 years. The average rupture intervals for individual central segments were also changed, ranging from 700 to 2,700 years. Earlier studies had recurrence intervals ranging from 1,200 to 2,600 years.

The study confirmed that both the Brigham City and Salt Lake segments have gone the longest without major earthquakes, which places the highest risk of large quakes on those segments. 

During the past 6,000 years, large earthquakes on the Salt Lake segment have occurred about every 1,300 years. Over a 14,000-year period, large ruptures happen on average about every 1,500 years. The last Big One on the segment happened 1,400 years ago.

Overall, the Wasatch Front region has a 43 percent probability of experiencing a magnitude 6.75 or larger earthquake in the next 50 years (this is probability is from 2014 to 2063, and, as of this writing we're already 8 years into that). The chances of a magnitude 6.0 or larger are 57 percent. The Wasatch Fault itself is projected to have a probability of 18 percent for large earthquakes of magnitudes 6.0 and 6.75 or larger.

It should be noted that the probabilities are projecting one or more earthquakes of at least these magnitudes. That means there is a chance that there could be more than one Big One. So, even if a Big One occurs, don’t get complacent thinking another can’t happen in your lifetime.

It should also be noted that the percentages for individual fault zones are specifically for those zones. So, if one fault zone has a major rupture, the others still have their own probabilities of Big Ones.

Probabilities of one or more earthquakes of magnitude 6.0 or greater from 2014 to 2063
Probabilities of one or more earthquakes of magnitude 6.0 and greater from 2014 to 2063. Image from the Utah Geological Survey document “Earthquake Probabilities for the Wasatch Front Region in Utah, Idaho, and Wyoming” 

Probabilities of one or more earthquakes of magnitude 6.75 or larger in the next 50 years
Probabilities of one or more earthquakes magnitude 6.75 or larger in the next 50 years. Image from the Utah Geological Survey document “Earthquake Probabilities for the Wasatch Front Region in Utah, Idaho, and Wyoming” 

The Salt Lake City Earthquake Scenario

Although Federal Emergency Management Agency (FEMA) HAZUS earthquake scenarios have been created for both the Provo and Salt Lake segments, we’ll focus on the Salt Lake segment. Salt Lake City is the capital of Utah and the Salt Lake Valley has the greater population. Additionally, the Salt Lake Segment is more likely to have a large earthquake first.

A 2008 FEMA study ranked Utah as 6th in the nation for relative earthquake risk. Salt Lake City was ranked 11th out of 43 metro areas. FEMA’s Salt Lake HAZUS report was created in 2012 for a magnitude 7.0 earthquake. This ShakeMap scenario, as it is called, is the basis for the earthquake scenario analysis calculating hazards and losses.

In 2015, the Utah Chapter of the Earthquake Engineering Research Institute developed a “Scenario for a Magnitude 7.0 Earthquake on the Wasatch Fault—Salt Lake City Segment Hazards and Loss Estimates.” This scenario incorporated the Salt Lake HAZUS data into its analysis. 

Magnitude estimations of past earthquakes on the Salt Lake segment have projected normal magnitude large earthquakes to be 7.1, plus or minus 0.2. So, a 7.0 earthquake scenario is not a worst-case event.

The 2013 Real Gross Domestic Product (Real GDP) for Utah was valued at $131 billion. The urban corridor from Ogden to Salt Lake City to Provo accounted for 85 percent of Utah’s Real GDP, and the Salt Lake City metro area was 54 percent of that.

A large earthquake is probably the single greatest threat to Utah’s people and economy. As mentioned, the earthquake scenario is for a magnitude 7.0 rupture on the Salt Lake segment. The scenario is only for the main shock event and does not factor in aftershocks

Using aftershock models, in the 30 days following the main shock, the area could reasonably expect three aftershocks of magnitude 6.0 or larger, thirteen aftershocks greater than 5.0 magnitude, and seventy-seven quakes at 4.0 or higher. The additional structural damage, injuries, and economic impact from the aftershocks are not calculated in the scenario. Additionally, these aftershocks will affect rescue and recovery efforts and further cause panic. 

With a magnitude 7.0 earthquake on the Salt Lake segment, the entire valley and surrounding vicinities will experience strong to severe shaking, which corresponds to intensity levels ranging from VI to IX on the Mercalli Scale

The Big One will certainly cause surface fault ruptures, liquefaction, landslides, and ground subsidence. Wet soil conditions would increase the number of landslides as well as the effects of liquefaction.

Salt Lake Earthquake Loss Estimations

The loss estimations for the event are considered severe. The following loss estimations are from the 2015 scenario, so the the valuation in today's dollars would likely be much higher.

A short-term economic loss of over $33 billion, which includes $24.9 billion in losses in structural, non-structural, content, and inventory in direct building-related losses. Income-related losses would exceed $6.9 billion. Lifeline-related losses—which include damage to electric systems, water, gas and oil pipelines, as well as major transportation corridors like highways, bridges, airports, and railroads—would top $1.4 billion.

The event would displace 84,000 households, and 53,000 individuals would need shelter.

Like other scenarios, the time of day would impact the number of injuries and deaths. Fatalities are estimated to range from 2,000 to 2,500. The injured and those needing hospital care could range from 7,400 to 9,300. Unfortunately, the number of available hospital beds will likely be reduced from 4,790 to 3,200.

One challenge would be the need to evaluate the safety of over 300,000 structures in the following thirty days.

Remember earlier when it was mentioned that many of Utah's buildings are not earthquake ready?Unreinforced masonry (URM) buildings will receive the brunt of damage, with 61 percent (90,200 buildings) of those being moderately damaged to totally destroyed in the main shock. Many more will likely be further damaged or destroyed with strong aftershocks.

Another challenge would be in removing over 820,000 truckloads of debris. At 25 tons per truck that's a lot of destruction. In total, 21 million tons of debris are likely.

The following table is a summary of some of the loss estimations.

Loss

Estimate

Life threatening injuries

7,400–9,300

Fatalities

2,000–2,500

Displaced households

84,400

Displaced individuals

Over 263,300

Individuals needing temporary shelter

52,700

Building-related loss

$24.9 billion

Income loss

$6.9 billion

Lifeline/essential utility loss

$1.4 billion

Total short-term economic loss

$33.2 billion

Hospitals, at least moderately damaged

15 out of 32

Available hospital beds after earthquake

3,200

Bridges, moderately damaged or worse

595 out of 1,805

Tons of debris generated

21,000,000

Truckloads to remove (25 tons per truck)

821,600


Essential utility lifelines—like water, electricity, sewer, and gas—could be disrupted for days to months, with the areas hardest hit experiencing even longer outages. Being aware of this potential should really open one’s eyes to preparedness needs.

Here's the estimate of number of households that will be without potable (drinkable) water and/or electricty in the earthquake's aftermath.

Households Without…

Day 1

Day 3

Day 7

Day 30 

Day 90

Potable water 

483,600 

466,100 

442,800 

362,900 

332,800 

Electricity 

444,600 

251,200 

105,900 

27,300 

800 



With most electrical distribution systems being above ground, the power grid should be quickly restored.

However, of note is the number of households that are expected to not have potable water—even after three months. Immediately after the main shock, nearly a half-million households (this isn't the number of people, which will be much higher) will be without water. Even after three months, the number of households without potable water is likely to still exceed 300,000. 

Even if your home survives with minimal impact, what are your plans if you don’t have water for more than three months? In addition, while running water may be restored, it is likely to remain contaminated for many months due to cracks and leaks in the distribution system.

Infrastructure Impact

The impact on infrastructure will make life difficult. Electrical and communication systems—including lines, distribution stations, and towers—as well as the natural gas lines, sewer pipes and treatment systems, and the water supply system will all be affected. It will be many months before any resemblance to “normal” is restored, although parts of the area may take years to fully recover.

Transportation infrastructure--airports, bridges, highways, and local roads--will be affected to varying degrees. 

The Salt Lake City International Airport could face issues with liquefaction as well as structural damage from the shaking. While there are municipal airports in the area, most in the Salt Lake Valley will likely be unusable for a time. The regional airport in Provo would the next largest airport, and in the aftermath of a large Salt Lake earthquake even the Provo airport will likely be out of service for a short time.

The Salt Lake valley is a major intersection of interstates 80 (east-west) and 15 (north-south). Due to the western desert of Utah, the Great Salt Lake, and the Wasatch Mountains, severe damage to one or both of these interstates would not just affect the local economy but also disrupt regional transportation. Here's a bit about each interstate.

Interstate 80 is a major east-west freeway. In this part of the country it runs from Chicago, through Denver, then Salt Lake City, and west to San Francisco. The freeway runs straight through the middle of the valley (and the Salt Lake segment of the Wasatch Fault) and it would likely be affected by a large earthquake. Besides road and overpass damage, I-80 runs through the Wasatch mountains to the east where earthquake-induced landslides could cripple passage. Cross-country transports would have to re-route north, possible to I-84, or south to I-70 or Highway 50. 

Interstate 15 is a major transportation artery running from Los Angeles through Las Vegas and up along the Wasatch Mountains and Wasatch Front--where it parallels the Wasatch Fault--and then it continues north into Idaho and Montana. Like I-80, significant damage to I-15 would result in significant delays on transportation running through the Salt Lake Valley. 

From a strategic view, and due to the geography of the west, the Salt Lake City area is a major transportation hub of the western United States. If a foreign power were to ever invade the United States from the west, which seems unlikely but is an interesting "What if" scenario, the Salt Lake valley would be one of the few key east-west spots to control.

So while Utah may be 6th in relative earthquake risk, and Salt Lake City is ranked 11 of 43 metropolitan areas, a large earthquake event centered in the Salt Lake segment of the Wasatch Fault would have severe repercussions. The effects would certainly be worse for the local area, but economic effects would cascade throughout the region and, I think, over much of the nation.




Much of this information, as well as many of the posts in this blog are in my book, Earthquake! What, Where, and How to Prepare.

Comments

Popular Posts

What is the Mercalli Intensity Scale?

Earthquake Hazards: Surface Fault Rupture