Nevada Seismological Laboratory

Living with Earthquakes in Nevada: A Nevadan's guide to preparing for, surviving, and recovering from an earthquake

Faults in Nevada

Photo 
by Craig dePoloIn Nevada, faults occur along many of the range fronts, within ranges, and within valleys. Normal-slip faults commonly appear as steps in the landscape related to the vertical offset, whereas strike-slip faults usually are expressed by linear features, such as small valleys, and alignments of features, such as springs. Historical earthquakes have ruptured both kinds of faults in Nevada. The Earth's crust in Nevada has countless faults formed through the geologic ages. The youngest faults are the most likely to cause future earthquakes, even though older faults can fail sometimes. Scientists studying earthquakes commonly limit their studies to Quaternary-age faults (those that have moved in the last 1.6 million years). Nevada has thousands of Quaternary faults, hundreds of which are major faults. Thus, there is no shortage of potential earthquake sources. Understanding the fault setting is essential to understanding the earthquake hazard. Normal-slip faults occur throughout the state, whereas strike-slip faults are most notable in western, central, and southern Nevada.
Photo above: View to the south of the White Mountains which are bounded by a major nomral-slip and normal-oblique-slip fault zone (located at the base of the mountains).

The outer portion of the Earth is made up of plates that move in different directions over the hotter interior. These plates can slide sideways, pull apart, or collide with one another. Huge systems of faults develop to accommodate the motions between the plates. When these faults move abruptly in the cooler, brittle, outermost part of the Earth's crust, called the seismogenic zone, earthquakes are produced. Most earthquakes occur along plate boundaries.

Quaternary Faults
Estimating the Size of Earthquakes

To predict the potential size of an earthquake that might occur along a fault, we estimate the length of potential earthquake rupture and how much the ground might be offset during an event. We compare these lengths and offset measurements with those from historical earthquakes to estimate the magnitude of a possible future earthquake. In general, the greater the length and/or the offset during an event, the greater the earthquake magnitude. Based on fault studies, the largest earthquakes we expect in Nevada are in the magnitude 7-8 range.


Estimating How Often Earthquakes Occur

The best way for scientists to tell how often earth-quakes occur along a fault is to dig a trench across a fault, identify prior earthquakes (which might appear as offsets in soil layers), and find some material related to these earthquakes for which an age can be determined. From this, scientists compile a history of earthquakes along a fault. After determining when earthquakes occurred in the past, one can determine the average time between earthquakes along a fault, and ultimately what the chances are that an earthquake might occur. Unfortunately, getting this information takes a lot of resources and effort for each fault, and there are hundreds of faults. For faults for which we lack a detailed history of fault activity, an estimated rate of movement (slip rate) is commonly used. The faster a fault moves, the shorter a period of time it takes a fault to store up the energy for an earthquake. As might be expected from Mother Nature, earthquakes don't occur at regular time intervals, but occur at variable time intervals and can occur in groups.

The time between large earthquakes along individual faults in Nevada is typically from thousands of years to tens of thousands of years. This is a long time between earthquakes, and if there was only one of these faults in Nevada, perhaps we wouldn't worry so much. But there are hundreds of faults in Nevada that can produce earthquakes. Even though there are long periods of time between earthquakes on an individual fault, we expect large earthquakes every few decades because of this large number of faults.

How do we know a fault is active?

  • If a large earthquake has bro ken the fault since we began keeping records.
  • If earthquakes on the fault have left surface evidence, such as fault scarps (surface ruptures made by earth quakes).
  • If earthquakes have left geologic evidence (such as young units that are broken).
  • If it is a fault that produces small earthquakes that we re cord with the seismographic network.
  • If geodetic deformation shows fault movement.
  • If geophysical data indicate recent fault offsets.

Some Major Faults in Nevada
*Scientists usually use metric values, particularly millimeters per year, for slip rates of faults. To convert to inches per year, multiply by 0.039.
**Because we lack detailed studies in many cases these values are approximations that cover wide ranges of potential values.
Fault Potential Earthquake Magnitude Length in Miles (km) Slip Rate Millimeters Per Year* Average Time Between Earthquakes (years)**
Genoa fault 7.4 47 (75) 1 - 3 1,500 - 4,000
Pyramid Lake fault zone 7.3 47 (75) 0.4 - 1.1 1,500 - 4,000
Toiyabe Range fault zone 7.3 69 (110) 0.1 - 0.8 2,000 - 15,000
Steptoe Valley fault zone 7.2 87 (139) 0.04 - 0.1 18,000 - 45,000
Ruby Mountains fault zone 7.2 62 (99) 0.05 - 0.3 10,000 - 100,000
Mt. Rose fault zone 7.1 25 (40) 0.2 - 0.4 2,000 - 10,000
Dixie Valley fault zone 7.1 60 (96) 0.3 - 0.6 6,000 - 12,000
Carson City fault 6.8 9 (14) 0.4 - 1 1,500 - 8,000
Frenchman Mountain fault zone 6.8 16 (26) 0.02 - 0.2 5,000 - 50,000
Black Hills fault 6.8 17 (27) 0.05 - 0.2 5,000 - 20,000

Unknown Faults

Nevada has been through hundreds of millions of years of geological processes that have pulled it apart and pushed it together, creating a severely faulted crust. There are many faults throughout the state that are unknown because of a lack of geological study or because the surface effects are minor, buried, or absent. Geophysical techniques might detect some of the larger faults, but minor faults escape detection unless tunneled into.

Nevada has been through hundreds of millions of years of geological processes that have pulled it apart and pushed it together, creating a severely faulted crust. There are many faults throughout the state that are unknown because of a lack of geological study or because the surface effects are minor, buried, or absent. Geophysical techniques might detect some of the larger faults, but minor faults escape detection unless tunneled into.

Further Reading
McCalpin (1996) Paleoseismology (technical)
Keller and Pinter (1996) Active Tectonics, Earthquakes, Uplift, and Landscape (technical)
Reiter (1990) Earthquake Hazard Analysis (technical)
Kramer (1996) Geotechnical Earthquake Engineering (technical)

Shacking causes other kinds of hazards to occur in certain settings

  • Landslides -- Block of soil and rock loosened by shaking may slide a short distance or, on steep slopes, may slide onto shallower slopes below.
  • Rockfall/boulders falling -- Rocks and boulders dislodged from steep, barren slopes with rocks exposed. Rocks and boulders may slide, roll, or bounce down the slope for considerable distances
  • Ground liquefaction -- Temporary transformation of water-saturated sandy deposits from a solid state to a liquid state due to shaking.

How do we study faults?

Scientists study faults by examining aerial photographs, making detailed geologic maps, and looking at different layers of geologic units in trenches across a fault. They are interested in exactly where the fault is located, how large the fault is, the history earthquakes along the fault, and how much the fault moved during recent earthquakes This information can be quite difficult to obtain without detailed studies.

Surface features that have been broken and offset by the movement of faults are used to determine how fast the faults move and thus how often earthquakes are likely to occur. For example, we might determine that sedimentary deposits have been offset feet (91m) across a fault, and that these deposits are 75,000 years old. Thus, this fault would be moving at an average speed of 1.2 mm (0.048 inches) per year. We might further determine by measurements at the surface or made in trenches dug across the fault that the last earthquake offset the ground 3 feet (0.9 m). If we assume that all the earthquakes along this fault offset the ground 3 feet well, then we will have earthquakes on average every 750 years (36 inches by 0.048 inches per year equals 750 years) This does not mean earthquakes will along this fault exactly 750 years apart. The well-studied San Andreas Fault in California has an average of 130 years between events, but actual times between earthquakes ranged from 45 years to 300 years.

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 2004 Nevada Seismological Laboratory