Frequently Asked Earthquake Questions
- What is an earthquake?
- How long do earthquakes last?
- Is there an "earthquake season" or "earthquake weather"?
- Where is the safest place to be in an earthquake?
- Will the ground open up during an earthquake?
- What is a seismometer, seismograph, and a seismogram?
- When was the seismograph invented?
- What is the Richter Scale?
- Do many small earthquakes prevent larger earthquakes?
- Can we predict earthquakes?
- What is liquefaction?
Questions and Answers About Utah Earthquakes
- When and where do large earthquakes occur in Utah?
- What is the difference between UUSS and USGS?
- What would happen if a magnitude 7.5 earthquake occurs along the Wasatch fault?
- How much damage would be caused by a large earthquake on the Wasatch Front?
- Do we need to worry only about large earthquakes causing damage?
- When were the largest historical earthquakes in Utah?
- How often do earthquakes occur in Utah?
- How many earthquakes occur in the Wasatch Front region?
- When was the last earthquake?
- When were seismographs first installed in Utah?
- Do earthquakes occur only on visible faults?
- Is the Wasatch fault the same type of fault as the San Andreas fault in California?
What is an earthquake?
A trembling or shaking of the ground caused by the sudden release of energy stored in the rocks below the surface, radiating from a fault along which movement has just taken place.
How long do earthquakes last?
Generally, only seconds. Strong ground shaking during a moderate to large earthquake typically lasts about 10 to 30 seconds. Readjustments in the earth cause more earthquakes (aftershocks) that can occur intermittently for weeks or months.
Is there an "earthquake season" or "earthquake weather"?
No. Earthquakes can occur at any time of the year and at any time of the day or night. Earthquakes occur under all weather conditions, sunny, wet, hot, or cold–without special tendency.
Where is the safest place to be in an earthquake?
In an open field, where nothing can fall on you. Earthquakes do not injure or kill people; buildings and falling objects do. If you are indoors, when you feel the ground start to shake, take cover immediately under a table or sturdy piece of furniture, placing a barrier between falling objects and yourself (Drop, Cover, Hold On). Do not attempt to use the stairs or an elevator or run out of the building.
Will the ground open up during an earthquake?
The ground does not open up and swallow people (a commonly feared myth). Open ground cracks may form during an earthquake–related, for example, to landsliding or ground slumping. But such fissures are open gaps (they don’t “swallow”) that a person could stand in.
What is a seismometer, seismograph, and a seismogram?
A seismometer is a sensor placed in the ground to detect vibrations of the earth. A seismograph is an instrument that records these vibrations. A seismogram is the recording of the earth’s vibrations made by a seismograph. Seismograms used to be recorded on paper or film, but now are recorded digitally. (Also see the glossary for more “seismic” terms.)
When was the seismograph invented?
This first known instance of an apparatus similar to a seismograph was in 132 A.D.; Chinese scholar Chang Heng made a mechanical device to detect the first main impulse of ground shaking. Modern seismographs were invented in 1880. The earliest seismographs in the U.S. were installed in 1887, in California.
What is the Richter Scale?
A scale for determining the size of an earthquake from the recording of earthquake waves made on a seismograph. The maximum height of the visible recording is adjusted for the distance from the instrument to the earthquake. This is not a physical scale (in other words, one cannot look at or hold the “Richter Scale”). Each 1-unit increase in the Richter Scale roughly corresponds to a 30-fold increase in energy release and a 10-fold increase in ground motion at any site. The Richter magnitude is the number generally reported in the press, and in principle the value should be the same at all recording locations (though natural variations and the use of diverse scales may lead to reported numbers that slightly differ). Due to the earth’s physical limitations, the largest earthquakes have Richter magnitudes in the upper 8 range; however, it is considered mostly effective for magnitudes up to 5. For magnitude 5 and above, the moment magnitude scale (denoted Mw) is a more accurate of earthquake size.
Graph modified from EarthScope Consortium (iris.edu) and earthquakes.utah.gov
Do many small earthquakes prevent larger earthquakes?
No. Observed numbers of small earthquakes are too few to equal the amount of energy released in one large earthquake. It would take roughly 24 million earthquakes of magnitude 2 to release the same energy as one earthquake of magnitude 7.
Can we predict earthquakes?
No. We cannot predict the precise time, location, and size of earthquakes in the U.S. (except in special study areas, such as Parkfield, CA). In order to predict earthquakes there has to be an adequate history of repeated earthquake cycles and/or extraordinary instrumental observations. Long-term forecasts (on scales of years or decades) are becoming common for well-studied earthquake zones. The Chinese have correctly predicted some earthquakes, evacuated cities and saved lives. They have also had large earthquakes occur with no predictions and have predicted earthquakes that never occurred.
What is liquefaction?
Water-saturated sands, silts, and other very loosely compacted soils, when subjected to earthquake motion, may be rearranged, thereby losing their supporting strength. When this occurs, buildings may partly sink into the ground and sand and silts may come to the surface to form sand flows. In effect, the soils behave as dense fluids when liquified.
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, such as the Oquirrh-Great Salt Lake fault zone and West Cache Valley fault zone in the Wasatch Front region, and the Hurricane fault system in the southwestern portion of the state. 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 (Magnitude 6.75 or greater) in the Wasatch Front region during the next 50 years was estimated to be 43% by a 2016 study.
"It is useless to ask when this [earthquake] disaster will occur. Our occupation of the country has been too brief for us to learn how fast the Wasatch grows; and, indeed, it is only by such disasters that we can learn. By the time experience has taught us this, Salt Lake City will have been shaken down…” -G. K. Gilbert, 1883
“Whatever the earthquake danger may be, it is a thing to be dealt with on the ground by skillful engineering, not avoided by flight….” -G. K. Gilbert, ca. 1906
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What is the difference between UUSS and USGS?
The University of Utah Seismograph Stations (UUSS) and the U.S. Geological Survey (USGS) are partner agencies. All seismic data from the Utah region are collected and initially processed by UUSS. The resulting earthquake locations, magnitudes, and ShakeMaps are submitted to the USGS, which serves the information on a USGS website. USGS personnel often provide additional information on their website related to how the earthquake ruptured, how widely the earthquake was felt, the potential economic and life impacts, and the chances of aftershocks and landslides. As members of the Advanced National Seismic System (ANSS), UUSS and USGS work together to provide the most accurate and complete information about earthquakes in the Utah region.
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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.
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How much damage would be caused by a large earthquake on the Wasatch Front?
In 2015, the Utah Chapter of the Earthquake Engineering Research Institute published a report estimating damages for a M7 earthquake on the Salt Lake City segment of the Wasatch fault: “The estimated short-term economic loss is over $33 billion. This includes (1) direct building-related capital losses (including structural, non-structural, content, and inventory) of $24.9 billion, (2) income losses of $6.9 billion, and (3) life-line-related losses of $1.4 billion.” Adjusting for inflation to June 2024, this becomes $44 billion in short-term economic loss, and $33 billion in building damages! The report estimates more than 84,000 households would be displaced.
Unreinforced masonry buildings (URMs; for example, brick homes built before 1960) and non-ductile concrete building collapses are expected to be one of the most significant sources of damage, as they are more vulnerable to ground shaking. The 2015 report damages are based on an estimate that about 60% of the building stock consists of URM buildings.
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.
Total damage estimates for the M5.7 Magna, UT earthquake in 2020 were over $150 million (Pankow and others, 2020). Most of the damage occurred on buildings with unreinforced masonry. West Lake Junior High suffered substantial damage and had to be completely rebuilt; As of 2023 the rebuild estimate was $55 million, with an expected reopening in summer 2024. Fortunately, there were no students in the building during the earthquake.
Since 1850, at least 14 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:
Local Date Magnitude Location
Mar. 18, 2020 5.7 near Magna
Sept. 2, 2017 5.4 near Soda Springs, ID
Sept. 2, 1992 5.9 near St George
When were the largest historical earthquakes in Utah?
Since settlement in 1847, Utah’s largest earthquakes were the 1934 Hansel Valley magnitude 6.6 earthquake, north of the Great Salt Lake, and the 1901 earthquake near the town of Richfield, estimated magnitude 6.5.
How often do earthquakes occur in Utah?
About 1,500 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 (M ~7) in Utah occurred on the Wasatch fault north of Nephi about 200 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.
The San Andreas fault is also a plate boundary fault (interplate); the contact between the North American and Pacific plates. The Wasatch fault is located within the North American plate (intraplate) and does not extend fully through the crust.