Utah’s Earthquake Risk?

The Utah Geological Survey made the following statement on Utah’s earthquake risk:

Utah is seismically active and at risk from large, damaging earthquakes. Since pioneer settlement in 1847, Utah has experienced 17 damaging earthquakes greater than magnitude 5.5. Furthermore, Utah has more than 200 active faults, many of which could generate earthquakes up to magnitude 6.5 to 7.5. The Wasatch fault, which extends along the Wasatch Front, is Utah’s most active and hazardous fault. Geologic studies show that over the last 7000 years, a magnitude 6.5 to 7.5 earthquake has occurred somewhere along the central, most active part of the Wasatch fault about every 270 years. The most recent large earthquake on the fault occurred about 300 years ago near Nephi.  The remaining segments of the Wasatch fault and other large faults in Utah are less understood but together add significantly to Utah’s earthquake risk.  Utah’s highest-risk zone extends in a broad band southward from the Cache Valley-Wasatch Front area to the St. George area and includes about 95% of Utah’s population.  Large-magnitude earthquakes have the potential to break the ground surface and cause widespread strong ground shaking and damage, especially to poorly built structures such as unreinforced masonry (generally brick, block, or stone) buildings. Earthquakes generally occur without warning. We don’t know when or where the next large earthquake in Utah will occur, but it could be soon and Utah citizens should act now to protect themselves and their families.

 

Major collapse of a building, Yalova, Turkey, 1999; British Geol Survey ok to use
Yalova, Turkey, 1999, photo from British Geological Survey

Some facts:

  • Over 2.5 million people in Utah live within 30 miles of a major earthquake-producing fault.
  • Utah’s highest risk area is a broad zone that sweeps south from the Idaho border through the western half of the state to the Arizona and Nevada borders. Over 95% of Utah’s population lives within this high-risk zone.
  • The largest and highest risk fault is the Wasatch fault, which extends from southernmost Idaho, through the Wasatch Front, to south of Nephi. The Wasatch fault, especially the central part, has been the subject of dozens of detailed scientific studies that demonstrate its high risk.
  • Studies indicate that Utah is due or overdue for a devastating earthquake (Utah Geological Survey).  These studies show that the central most active part of the Wasatch fault has averaged one very large (magnitude 6.5-7.5) earthquake about every 270 years. The last major earthquake on the central Wasatch fault was on the Nephi segment about 300 years ago.  However, the actual spacing between past major earthquakes on the central Wasatch fault varied greatly from a few years to over 500 years, decreasing our ability to predict earthquakes.
  • Most people have heard of the Wasatch fault, but few are aware that Utah has up to 200 other large faults that could also produce devastating earthquakes; most in the western half of the state. Very few studies have been done on these other faults, such that we know very little about how often they rupture or how big the earthquake is likely to be.  While we don’t know as much about these faults, they significantly increase the frequency and risk of potentially devastating large earthquakes in Utah.

    Utah_Fault_Map GW editorial Feb 26 15 med res
    Map showing about 100 of Utah’s 200 known or suspected active faults that could produce earthquakes. These faults are in Utah’s highest risk zone, but other parts of the state also have potentially active faults and every part of the state has some earthquake risk. Faults from Utah Geological Survey and U.S. Geological Survey data. World Terrain Base from ESRI Resource Center.
  • The largest estimated earthquake will be very severe—as high as magnitude 7.5.
  • Violent shaking will be the most widespread and devastating hazard from any major earthquake in Utah and will affect a large area around the earthquake epicenter; for example, an earthquake on the Salt Lake City segment of the Wasatch fault will likely cause violent shaking in the entire Salt Lake Valley and severe shaking from Ogden to Provo.
  • Even if your house, apartment, business, or school is not built directly on a fault, you will still experience severe to violent shaking from a nearby earthquake.  Many people incorrectly assume that they are “OK” if their building is not built directly on a fault.
  • Shaking may be particularly severe in Utah earthquakes compared to many other parts of the world because major faults extend beneath our valleys—earthquakes on these faults will occur directly beneath large populations and buildings.  Also, the shape of our valleys and the composition of the sediment will increase shaking for many people.
  • While we can’t predict when an earthquake will happen, we can estimate the stress that has accumulated on given faults; several faults in Utah have high accumulated stress.
  • Nearby modern examples of the type of earthquake likely to occur on major Utah faults are the Borah Peak earthquake in Idaho (magnitude 6.9 in 1983), the Hebgen Lake earthquake in Montana (magnitude 7.3 in 1959), and three earthquakes in Nevada (7.1 in 1915, 7.2 in 1932, and 7.1 in 1954) (USGS data). Two of these earthquakes caused deaths even though they all occurred in very rural areas.

Leave a Reply

Please log in using one of these methods to post your comment:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s