The Chance of Having an Earthquake in Nevada

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Over the last 150 years 24 earthquakes of magnitude 6 and larger, and about 5 earthquakes of magnitude 7 and larger have occurred in Nevada. This leads to an average time of 6 years between earthquakes with magnitudes equal to or larger than 6, but actual values have ranged from less than 5 minutes to 33 years. The largest events, those of magnitude 7 and greater, occur in Nevada on average every 30 years. These are relatively high rates of earthquake activity. The map below shows where the most intense shaking in Nevada has occurred between 1857 and 1999.

The Earthquake Hazard Perspective

We can't worry about every one of the several hundred earthquake-generating faults in Nevada every time a building is built. Nor do we need to. To prepare for earthquakes we mainly need to know the intensity of shaking we can anticipate over the expected life of the structure. Even though nobody knows for sure how long a building will last, an estimate of 50 years is commonly used. We estimate the strength of shaking that has a 1 in 50 chance of being exceeded in 50 years and design the building to be strong enough to resist damage from such a quake.

Maps like the one to the upper left are made by considering all the faults and the shaking that each might cause. Every fault needs to be identified and mapped. We study the characteristics needed to estimate the magnitude, primarily the fault length and maximum surface offset during a single earthquake. We also determine the average time between earthquakes or the average slip rate (Faults in Nevada). Finally we need to know how strong, on average, the shaking will be at every place when this fault has an earthquake. Shaking is more intense near the earthquake rupture and gradually decreases as you get farther away. Putting all this together, along with information from historical earthquakes, we make ground-motion maps like the one on the right. This map is similar to the maps that will be used in the International Building Code of 2000 (building codes give guidelines for constructing safe buildings). The map depicts peak ground shaking in percentage of the force of gravity (acceleration). The redder colors represent areas where the shaking is expected to be the strongest.

Using a measure of acceleration is common in earthquake engineering although many other measurements are needed for a complete description of ground motion; one "g" (gravity unit) is the force the Earth exerts on an object that makes it fall toward the Earth. You experience acceleration when you speed up or slow down quickly in a car. Your body is pushed back when you speed up or accelerate and is thrown forward when you slow down or deaccelerate.

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Engineers use computer models and model structures to see how buildings hold up to being accelerated sideways by a seismic wave. The notion of a map like this is very much like rolling the dice and calculating odds. If a rarer, but damaging, earthquake occurs in a part of Nevada that has less frequent earthquakes (yellow to blue colors), the shaking will be every bit as strong as earthquakes we anticipate in western Nevada. If we correlate this shaking potential map with possible damage, all of Nevada could experience damaging shaking. This is why Nevadans need to take messages of earthquake preparedness seriously.