# 9 - Utah Earthquake Risk

 Easily ranked in the top 10 states with the highest earthquake risk, Utah's primarily risk lies among the Wasatch Fault Zone.

The following image is of a building damaged in the February 2011 Christchurch earthquake in New Zealand. There have been several comparative studies done between New Zealand buildings damaged by earthquakes and similar structures in Utah.

Earthquake damage on unreinforced masonry building

Utah

The Wasatch Fault Zone stretches about 240 miles from southern Idaho into northern Utah. While it is the longest fault zone in the area, it’s not the only one. Other large fault zones in the area include the Oquirrh-Great Salt Lake, West Cache, Stansbury, and Eastern Bear Lake fault zones. 

The Wasatch Fault is at the base of the mountain range that runs through the state. Along the urban corridor, this section of the mountains is referred to as the Wasatch Front. With many communities built along the foothills, a major seismic rupture could not only cause a lot of liquefaction damage in the lowlands but hundreds of rockslides that could cascade into neighborhoods.

Running north-south along the Wasatch Fault is the main transportation artery, Interstate-15. Interstate-80 runs east-west through the Salt Lake Valley. A large earthquake would virtually shut down most transportation into and through the area. 

Throughout Utah, there are more than 200 active faults, and many of these can produce ruptures of magnitude 6.5 to 7.5. While the Wasatch Fault tends to receive the most attention, and it has the highest risk, Utah’s high earthquake risk area extends from the Idaho border on the north down to the Arizona and Nevada borders. Over 95 percent of Utah’s population live within this high-risk zone (Utah's Earthquake Risk?).

With nearly 80 percent of Utah’s population living and working within fifteen miles of the Wasatch Fault and about 75 percent of the state’s economy also in the same area, combined with a large number of buildings susceptible to earthquake damage, there is a very high risk of economic and property loss in the event of a large earthquake.

A 2016 report estimated the probability of a magnitude 6.75 or higher earthquake in the next 50 years at 43 percent, which is much higher than earthquake experts had expected. The Wasatch Fault itself has an 18 percent probability, with other fault zones adding the extra 25 percent risk (Working Group on Utah Earthquake Probabilities, 2016).

In the event of a 7.0-magnitude tremor, it’s estimated that as many as 1 in 3 homes, about 76,000 single-family houses, in Salt Lake County are at risk of major damage or collapse. But residences aren’t the only buildings at risk. In some of the older, historic districts, as many as 9 in 10 brick-and-mortar buildings are built with unreinforced masonry (URM) and are high risk (1 in 3 Salt Lake County homes at greater risk of crumbling in earthquake). Throughout Utah, over 200,000 buildings are constructed with URM, and about 25 percent of the population work, go to school, or live in these buildings. As a comparison, California has about 25,000 URM buildings. These URMs pose a great hazard, and it’s estimated that up to 90 percent of the deaths and injuries in Utah’s next major earthquake will be from damaged or collapsed URMs (My Risk? Every Utahn should know).

In recent recorded history, the two largest earthquakes in Utah were a magnitude 6.6 in Hansel Valley in 1934, and a 6.5-magnitude quake in the town of Richfield in 1901. Two other quakes, although not as large, have occurred since then: a magnitude 5.7 earthquake hit Richmond in 1962, and a 5.8 shook St. George in 1992. These last two smaller-magnitude quakes actually caused more damage (Earthquakes & Utah).

Check out this post for more information on the Wasatch Fault Earthquake Risk

Comments

Post a Comment

Popular Posts

Wasatch Fault Earthquake Scenario

Image
 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....
Read more

Earthquake Hazards: Surface Fault Rupture

Image
 A fault rupture is a slip and resulting displacement along a fault. When that rupture extends to the earth’s surface, where we can see it, it’s called a surface rupture. These surface fault ruptures can be vertical or horizontal, and can happen in a small location or extend over a larger area. In many earthquake hazards, surface fault ruptures are identified as ground displacement. Not all earthquakes will produce surface ruptures. The biggest problem with surface ruptures is when a building or other structure is over the rupture. California passed the Alquist-Priolo Earthquake Fault Zoning Act in 1972 to mitigate surface ruptures. The Act prevents the construction of human-occupied buildings on the surface of active faults.  Outside of California, many other earthquake hazard locations don’t have statutes regulating development on faults with surface rupture potential. The main reason for the lack of regulations is usually a lack of historical evidence of earthquakes rupturi...
Read more

San Francisco Earthquake Scenario

Image
An earthquake scenario for the San Francisco area is one of the scenarios where finding more recent information is difficult. For an actual scenario, the best I could find was from a simulation in 2006. There was also some information from a 2008 ShakeOut Scenario. There are more recent earthquake hazard and risk calculations available, but I have yet to find these risks and hazards incorporated into a full scenario, where costs and losses are calculated for the earthquake. While the details may be a little different, the whole purpose of this, and other scenarios, is to help you start to grasp the magnitude of damage and loss that a large earthquake can cause. And will very likely cause. Remember that California has generally had better earthquake building codes for a longer time than most of the rest of the United States. An earthquake of a much lesser magnitude can have just as much (if not more) damage and losses in an area where the buildings and infrastructure are not as prepared...
Read more