Earthquake FAQ

Frequently Asked Earthquake Questions


Questions and Answers About Utah Earthquakes


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.
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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.
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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.
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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. Do not attempt to use the stairs or an elevator or run out of the building.
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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.
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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 (usually paper or film) of the earth’s vibrations made by a seismograph. (Also see the glossary for more “seismic” terms.)
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When was the seismograph invented?
In 1880. The earliest seismographs in the U.S. were installed in 1887, in California. (In 132 A.D. a Chinese scholar, Chang Heng, made a mechanical device to detect the first main impulse of ground shaking.)
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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.Magnitude Energy released

(millions of ergs) Energy equivalence

-2 600 100 watt light bulb left on for a week

-1 20000 Smallest earthquake detected at Parkfield, CA

0 600000 Seismic waves from one pound of explosives

1 20000000 A two-ton truck traveling 75 miles per hour

2 600000000

3 20000000000 Smallest earthquakes commonly felt

4 600000000000 Seismic waves from 1,000 tons of explosives

5 20000000000000

6 600000000000000

7 20000000000000000 1989 Loma Prieta ,CA earthquake (magnitude 7.1)

8 600000000000000000 1906 San Francisco earthquake (magnitude 8.3)

9 20000000000000000000 Largest recorded earthquake (magnitude 9.5)

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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.)
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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.
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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.
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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.

“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 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?
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.
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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:

Local Date Magnitude Location

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

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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.
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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.
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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.
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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.)
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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.
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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.
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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.
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