Intermountain Seismic Belt & the Wasatch Fault

Seismic History of the Utah Region

• During the Paleozoic: Utah was at the edge of the continent--a passive margin See Ordovician World Map.
• During the Jurassic: Convergent margin to the west begins to cause folding-continental collision creates thrust faults that culminate in high mountains to the west during the Cretaceous. See Jurassic Paleogeography, Southwestern US, See also Cretaceous Paleogeography, Southwester Geography.
• During the Early Cenozoic: Mountains to the west significantly eroded, uplifts to the east occur--e.g. Uintah mountains, San Rafael Swell--Apparently from heat in the mantle???? See Tertiary Paleogeography, Southwestern US
• During the Late Cenozoic: Rifting of the Great Basin begins. -- normal faulting and the Intermountain Seismic Belt.

About the Intermountain Seismic Belt

• What is it?: A zone of normal faulting that extends from Arizona and Nevada northward to Montana
• See ISB HISTORICAL EARTHQUAKES (1900-1985) for source of this picture, see also ISB Fault Map:
• Why is it there?: Note that there is a piece of the Pacific mid Oceanic Ridge that has been subducted Under North America. Generally this fact is included in any explanation at to what happens:  see:  Tripping Through the Basin and Range country of the Southwestern United States, and

The  Path of the Yellowstone Hot Spot

Note that the Earthquake activity and fault lines are arranged around the advancing Yellowstone Hotspot like the bow wave of a moving ship or the shock wave of a supersonic jet. See Seismicity Map of Idaho.
 

The Wasatch Fault

• The Wasatch Fault is the central fault of the intermountain seismic zone in the populated area of Utah. The Wasatch Fault is found at the base of the Wasatch front on the eastern edge of Ogden, Salt Lake and Provo, as well as a bit south and a bit north.
• It is a normal Fault that defines the Wasatch front. See picture at UUSS.
• When the fault moves the valley drops downward and the mountains are uplifted.
• The fault dips away from the mountain range and runs underneath the valley. See UGS diagram.

Segments of the Fault and Timing of prehistoric Earthquakes

The Wasatch Fault is divided into several segments Listed from North to South. Each of these faults has been trenched to determine the recurrence interval of large, fault-moving quakes:
 

Picture from USGS

• Malad City Segment
• Clarkston Mountain Segment
• Collinston Segment
• Brigham City Segment
• Weber Segment
• Salt Lake City Segment
• Provo Segment
• Nephi Segment
• Levan Segment
• Fayette Segment

See Maps at UUSS for County Maps of the faults
If you look at individual segments, then recurrence intervals are anywhere from 1,200 to 2,600 years, if you go to the northern or southern ends, recurrence intervals are greater than 10,000 years.
If you look at the entire fault, then an earthquake occurs every 350 years (give or take). Some feel that the most recent quake occurred in the Nephi Segment 400 years ago--others say that the most recent was the Provo Segment 600 years ago. The Brigham City segment has gone the longest without a quake, and the Salt Lake City Segment is next Longest. Therefore, The next big quake is most likely to occur at the Brigham City Segment or the Salt Lake City Segment
 

Liquefaction Risks

Liquefaction is a serious risk along the Wasatch Fault because the valley is full of loose, water-laden sedimentary material.

A good coverage of liquefaction risk is provided by the Utah Geological Survey

Historical Earthquakes in the ISB

Map of the major historical earthquakes in the Intermountain Seismic Belt. Click on image to see full sized map at the University of Utah Seismic Station

The map above shows some of the major historical earthquakes that have occured in the intermountain seismic belt. To see a complete listing of these major quakes with data and news articles, go to the UUSS Personalizing the Earthquake Threat, or go right to the data page Utah Earthquakes. Here are some of the highlights:

• August 17 1959 Hebgen Lake Montana--the source of Quake Lake, the largest historical earthquake on the ISB.
• March 12 1934 Hansel Valley Utah (near Kosmo)-- Clearly responsible for one fatality in Utah--the second Utah fatality is a bit of a stretch. Only ground-rupturing event in Utah, and source of the most Earthquake destruction in Utah.
• May 13, 1914 Ogden Utah--a 5.5 or so magnitude earthquake that possibly was located on the wasatch fault.
• August 30, 1962 Cache Valley Utah (near Logan)--A recent significant earthquake.