Wasatch Fault

The Wasatch Fault is an earthquake fault located primarily on the western edge of the Wasatch Mountains in the U.S. state of Utah. The fault is 240 miles long and is made up of several segments, each of which can independently produce earthquakes as powerful as local magnitude 7.5. The Wasatch Fault is a normal (vertical motion) fault which forms the eastern boundary of the Basin and Range geologic province which comprises the geographic Great Basin. The Wasatch Mountains have been uplifted and tilted to the east by movement of the fault. The average rate of uplift along the fault over its history is approximately 1 mm per year, although there are indications that more rapid slip has occurred in the past few thousand years. The fault, however, does not continually slowly slip; instead, it remains locked for hundreds to thousands of years until approximately 1 to 4 m of slip occurs rapidly, producing a strong earthquake.

During the past 6,000 years, a strong earthquake (magnitude greater than 6.5) has occurred approximately once every 350 years somewhere along the Wasatch Fault. The last major quake struck around 1600AD. Experts note that the fault is overdue for another major earthquake, and people in the region have become increasingly aware of the threat in recent years.

The urban areas of Salt Lake City, Ogden, and Provo, known collectively as the Wasatch Front, lie on soft lake sediments, a remnant of Lake Bonneville. During strong earthquakes, this sediment will cause Earthquake liquefaction, which acts as quicksand. An earthquake on the Wasatch Fault could severely damage gas, electric, water, communication, and transportation lifelines, crippling the 2,000,000 strong Wasatch Front urban area. [ [http://quake.wr.usgs.gov/prepare/factsheets/Wasatch/ USGS] ]

A recent report released by Bob Carey of Utah's Office of Emergency Services and published by the Deseret Morning News in April 2006 predicts what the possible results of a 7.0 earthquake directly hitting the Salt Lake Valley could be. The report predicts that when the quake strikes it could kill up to 6,200 people, injure at least 90,000, and cause $40 billion USD in economic losses. At least 42% of all buildings along the Wasatch Front could be at least moderately damaged. The earthquake danger was not known when many structures were built in the area, as many hospitals and schools are located atop the faults themselves. About 50% of hospital beds could be eliminated during a quake and the region has about 200,000 unreinforced masonry buildings – buildings particularly vulnerable to shaking – compared to California state's 25,000. Massive landslides are another major threat. [ [http://deseretnews.com/dn/view/0,1249,635199749,00.html Deseret News article] ]

If the event has an intensity of 7.3, large portions of the nearly completely urbanized Salt Lake Valley, home to one million people, are expected to be permanently flooded by the Great Salt Lake from the north and Utah Lake to the south. The worst case scenario would encompass a quake of at least 7.3 striking during a winter blizzard at night, when the lake waters are icy cold, according to many experts.

Scientists have been increasingly aware of the threat since the 1980s and many key structures in the region have recently begun undergoing extensive seismic retrofitting.

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