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 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.
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” |
|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”|
Earthquake Risk no the Wasatch Fault
|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 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
Salt Lake Earthquake Loss Estimations
Life threatening injuries
Individuals needing temporary shelter
Lifeline/essential utility loss
Total short-term economic loss
Hospitals, at least moderately damaged
15 out of 32
Available hospital beds after earthquake
Bridges, moderately damaged or worse
595 out of 1,805
Tons of debris generated
Truckloads to remove (25 tons per truck)