What’s the difference between geologic hazard and risk?
Geologic hazards are naturally-occurring phenomena capable of causing loss or damage. Risk is the potential that exposure to the hazard will lead to a negative consequence such as loss of life or economic loss. To illustrate the difference, consider the following two examples.
- California has a number of faults that produce earthquakes during any given year. Some of these faults are located in populated regions, putting the people in those areas at risk. Other faults occur in remote areas where few people live and few structures exist. While the hazard may be the same for the two different areas, the regional risk differs because the potential impact is greater in the more populated area.
- Town A and Town B are right next to the same earthquake fault. They have the same earthquake hazard. But they don't have the same risk, because Town A has buildings built to withstand earthquake ground shaking and Town B does not. If you are Town B, you have two things you can do to reduce your risk of being hurt or killed in an earthquake. You can build to withstand earthquake ground shaking. Or you can move.
Bottom line: High earthquake hazard does not mean high risk. Also, while you can’t change the underlying earthquake hazard, you can reduce earthquake risk by managing the built environment.
What are the USGS National Seismic Hazard Maps?
The National Seismic Hazard Maps (NSHM) (and the hazard model from which they are derived) are first and foremost a suite of products aimed at improving earthquake-resilient construction in the United States. Engineers who construct buildings need to know how strongly a particular site might be shaken by earthquakes. The NSHMs address this question by compiling all known earthquake sources (and proxies for unknown sources!), their distance from the site in question, and other seismological and geological information to project potential maximum expected ground motions at a site over a particular period of time (say 50 years). These estimates are computed for hundreds of thousands of evenly distributed sites, and the information is then summarized into a series of maps.
Different maps portray different types of ground shaking information; some maps are designed to inform engineers on the design of small residential structures affected by high frequency ground motion, others are useful for designing tall buildings and long bridges that are more susceptible to longer wavelength ground shaking. The NSHMs (and underlying model) are updated every six years to provide the basis for earthquake provisions in building codes. Regular updates ensure that engineers have access to the most accurate and up-to-date information about potentially damaging earthquakes throughout the United States.
What are other ways in which the National Seismic Hazard Maps are used?
The National Seismic Hazard Maps are used by the insurance industry to set earthquake insurance premiums, by reinsurance companies to evaluate their risk to major disasters, by government officials and land use managers, and by private companies worried about the exposure of facilities and supply chains to natural hazards. Additionally, there are versions of the maps designed specifically for the public, to help people learn more about the hazard from earthquake ground shaking in their area. An example of such a map is discussed below.
What does this map show?
This map shows how often scientists expect damaging earthquake shaking around the U.S. On this map, a “damaging earthquake shaking” is that of Modified Mercalli Intensity (MMI) level VI or higher. See Modified Mercalli Intensity for more information about different MMI levels of earthquake ground shaking and what kind of damage can happen at each level.) Click/tap map to view larger version.
If places on the map have the same color, will they have the same level of damage if an earthquake occurs?
Not necessarily. Areas with the same color on the map should expect a similar number of occurrences of damaging earthquake shaking. However, the level of damage caused by the ground shaking associated with each earthquake could be very different. For example, a smaller earthquake that produces some damage over a smaller area, and a larger earthquake that causes widespread damage, are both counted in occurrences of damaging earthquake shaking.
Why do I need to worry about the earthquakes on this map? 10,000 years seems like a very long time.
This map shows how many times earthquakes could cause damaging ground shaking in 10,000 years and yes, that is a very long time! But that doesn't mean the earthquakes are guaranteed to be far in the future. They could happen at any time, including today. Scientists look at earthquakes over a long time period to get a fuller picture of earthquake hazard.
Is this map a ShakeMap?
No. USGS ShakeMaps show ground shaking from a single earthquake or predicted ground motion from a simulated (scenario) event. The latter are used for planning exercises and response drills. The figure above, however, is a probabilistic map (see below) that shows expectations for damaging shaking from all possible earthquakes over a 10,000 year period.
Do engineers use this map for building codes?
Yes and No. To make buildings that can withstand earthquake shaking, engineers use the same information that is the basis for this map, but in a different way. Engineers can't use this map alone to do their jobs, but this map can help everybody become more aware of earthquake hazards across the country.
What exactly is a probabilistic map?
In a probabilistic map, information from millions of scenario maps are combined to make a forecast for the future. A probabilistic map shows possibilities for the future based on the past. A probabilistic map takes into account a wealth of geologic and seismic information, including:
- The past history of earthquakes on a given fault;
- The past history of small earthquakes;
- How much ground shaking past earthquakes produced;
- The location and distribution of faults in a given region;
- How the Earth and rocks respond to ground shaking ;
- How rapidly the Earth is deforming in response to tectonic forces;
- Where deformation is occurring.
I live in a grey area on the map. Does that mean there is no chance of damaging shaking from an earthquake in my area?
No, that is not what the gray areas mean. There is a chance of damaging shaking anywhere and everywhere in the United States. Most people pay particular attention to areas with the hottest colors. True, these areas will most often have damaging shaking, but don't ignore the cooler shades. Damaging shaking can and will happen in those areas, too, but less often. In fact, damaging shaking is possible in all fifty states. The cooler color areas, like grey, are low hazard but not no hazard.
What are the areas in black outlines?
Notice that in some states there are black outlines. These show areas where the number of occurrences of damaging shaking could go up because recently there have been "induced" earthquakes in these locations. Induced earthquakes are caused by human activity while natural earthquakes are caused by geologic processes. The colors on this map only show the hazard from natural earthquakes. In the areas with induced earthquakes, the number of occurrences of damaging shaking could be much higher than the value shown on this map, and could drop lower again if the human activities that are causing the earthquakes change. The USGS made another map that includes induced earthquakes.
How can I use this map?
This map gives a general overview of how often to expect damaging earthquake shaking to expect around the United States. As is true with any map, there are things you can—and cannot — do with this map. See the chart below to see the types of things you can do with this map.
Map of Frequency of Damaging Earthquake Shaking Around the U.S. - What this map does and does not do
|What this map does||What this map does not do||Important Points|
|This map shows you the general earthquake hazard in your area.||This map cannot show you your earthquake risk.||To understand risk you need to add information about buildings, infrastructure, and/or people.|
|This map shows you the distribution of damaging earthquake shaking across the United States.||This map does not show specific faults or earthquakes, or other hazards besides earthquake.||The USGS Qfaults database shows specific faults. State geological surveys often have maps of other earthquake hazards, such as those from landslides, liquefaction, or tsunamis.|
|This map lets you compare your earthquake shaking hazard with other areas in the United States.||This map does not predict when any earthquake will occur and does not show information for a single earthquake.||No one can predict earthquakes. USGS ShakeMaps shows ground shaking in a single earthquake.|
I want to know what faults are near me; how will this map help?
It won't. This map doesn't show faults. The U.S. fault map will show you faults. Keep in mind that most hazardous areas face danger from more than one fault, and the most dangerous fault is not necessarily the closest.
Could there be a damaging earthquake in my state?
All states have some potential for damaging earthquake shaking. Earthquakes are not just a California or a West Coast problem. Hazard is especially high along the west coast but also in the intermountain west, and in parts of the central and eastern U.S., such as near Memphis, TN, and Charleston, SC. The 16 states with the highest earthquake hazard from natural earthquakes are Alaska, Arkansas, California, Hawaii, Idaho, Illinois, Kentucky, Missouri, Montana, Nevada, Oregon, South Carolina, Tennessee, Utah, Washington, and Wyoming. The states with the lowest ground shaking hazard are Florida, Iowa, Minnesota, North Dakota, and Wisconsin.
What can I do to prepare for an earthquake?
Be aware of the earthquake hazard that could threaten your community—and know what to do in the event of an earthquake. When you are aware and prepared you have a better chance to be safe. Use the Preparedness information to find practical steps to protect yourself and your loved ones.