FAQ

 

When and where do large earthquakes occur in Utah?

Large earthquakes (magnitude 6.5 to 7.5) can occur on any of several active segments of the Wasatch fault between Brigham City and Levan. Such earthquakes can also occur on many other recognized active faults in Utah. During the past 6,000 years, large earthquakes have occurred on the Wasatch fault on the average of once every 400 years, somewhere along the fault's central active portion between Brigham City and Levan. The chance of a large earthquake in the Wasatch Front region during the next 50 years is about 1 in 4.

 

What would happen if a magnitude 7.5 earthquake occurs along the Wasatch fault?

Future large earthquakes will break segments of the fault about 20 - 40 miles long and produce displacements at the surface of up to 10 - 20 feet. Strong ground shaking could produce considerable damage up to nearly 50 miles from the earthquake. The strong ground shaking may be amplified by factors up to 10 or more on valley fill compared to hard rock. Also possible are soil liquefaction, landslides, rock falls, and broad permanent tilting of valley floors possibly causing the Great Salt Lake or Utah Lake to inundate parts of Salt Lake City or Provo.

 

How much damage would be caused by a large earthquake on the Wasatch Front?

If the earthquake were to occur on a central part of the Wasatch fault, Utah should expect damage to buildings to exceed $4.5 billion in Davis, Salt Lake, Utah and Weber counties. This may only represent 20% of the total economic loss. Unreinforced masonry buildings (for example, brick homes built before 1960) are particularly vulnerable to ground shaking and are expected to account for 75% of the building losses. Surface faulting and ground failures due to shaking during a large earthquake will cause major disruption of lifelines (utilities, water, sewer), transportation systems (highways, bridges, airports, railways), and communication systems.

 

Do we need to worry only about large earthquakes causing damage?

No. A moderate-sized earthquake that occurs under an urbanized area can cause major damage. Magnitude 5.5 - 6.5 earthquakes occur somewhere in Utah on the average of once every 7 years. Estimates of damage from a "direct hit" to one of the Wasatch Front's major metropolitan areas reach $2.3 billion for a magnitude 6.5 earthquake, and more than $830 million for a magnitude 5.5 earthquake. Since 1850, at least 15 independent earthquakes of magnitude 5.5 and larger have occurred in the Utah region. Recent magnitude 5.0 and larger earthquakes in the Utah region include:

  • Jan. 29, 1989 5.4 16 miles SE of Salina
  • Aug. 14, 1988 5.3 Central Emery County
  • Mar. 27, 1975 6.0 Pocatello Valley (Utah - Idaho border)
  • Oct. 14, 1967 5.2 Marysvale
  • Aug. 16, 1966 5.6 Utah-Nevada Border
  • Sep. 5, 1962 5.2 Salt Lake Valley
  • Aug. 30, 1962 5.7 Cache Valley

 

When were the largest historical earthquakes in Utah?

Since settlement in 1847, Utah's largest earthquakes were the 1934 Hansel Valley earthquake, north of the Great Salt Lake, magnitude 6.6, and the 1901 earthquake near the town of Richfield, estimated magnitude 6.5.

 

How often do earthquakes occur in Utah?

About 700 earthquakes (including aftershocks) are located in the Utah region each year. Approximately 2% of the earthquakes are felt. An average of about 13 earthquakes of magnitude 3.0 or larger occur in the region every year. Earthquakes can occur anywhere in the state of Utah.

 

How many earthquakes occur in the Wasatch Front region?

About 500 earthquakes are located in the Wasatch Front region each year. About 60% of the earthquakes of magnitude 3.0 and larger in Utah occur in the Wasatch Front region.

 

When was the last earthquake?

Worldwide: In the last minute, somewhere in the world. Utah: Within the past 24 hours, somewhere in the state. (The last large earthquake in Utah occurred on the Wasatch fault north of Nephi about 400 years ago.)

 

When were seismographs first installed in Utah?

In 1907, by James Talmage at the University of Utah. A skeletal statewide network began in 1962. Modern seismographic surveillance in the Wasatch Front began in 1974. Computerized recording of earthquake data began in 1981.

 

Do earthquakes occur only on visible faults?

No. Many of the active faults in Utah are deep below the earth's surface, and are not visible to us.

 

Is the Wasatch fault the same type of fault as the San Andreas fault in California?

No. The San Andreas fault slips horizontally with little vertical movement. This is called a strike-slip fault. The Wasatch fault slips in a primarily vertical direction, with the mountains rising relative to the valley floor. The Wasatch fault is a so-called normal fault. All earthquakes produce both vertical and horizontal ground shaking. Usually the horizontal shaking is more energetic and more damaging because structures generally resist vertical loads, like gravity, more easily.