What is a seismic zone, or seismic hazard zone? Where can I find information on seismic zones 0, 1, 2, 3, or 4? What seismic zone is location X in?
A seismic zone could be one of three things:
- A region on a map in which a common level of seismic design is required. This concept is obsolete.
- An area of seismicity probably sharing a common cause. Example: "The New Madrid Seismic Zone."
- A region on a map for which a common areal rate of seismicity is assumed for the purpose of calculating probabilistic ground motions.
Building code maps using numbered zones, 0, 1, 2, 3, 4, are practically obsolete. 1969 was the last year such a map was put out by this staff. The 1997 Uniform Building Code (UBC) (published in California) is the only building code that still uses such zones. Generally, over the past two decades, building codes have replaced maps having numbered zones with maps showing contours of design ground motion. These maps in turn have been derived from probabilistic ground motion maps. Probabilistic ground motion maps have been included in the seismic provisions of the most recent U.S. model building codes, such as the new "International Building code," and in national standards such as "Minimum Design Loads for Buildings and Other Structures," prepared by the American Society of Civil Engineers.
Zone maps numbered 0, 1, 2, 3, etc., are no longer used for several reasons:
- A single map cannot properly display hazard for all probabilities or for all types of buildings. Probabilities: For very small probabilities of exceedance, probabilistic ground motion hazard maps show less contrast from one part of the country to another than do maps for large probabilities of exceedance. Buildings: Short stiff buildings are more vulnerable to close moderate-magnitude events than are tall, flexible buildings. The latter, in turn, are more vulnerable to distant large-magnitude events than are short, stiff buildings. Thus, the contrast in hazard for short buildings from one part of the country to another will be different from the contrast in hazard for tall buildings.
- Building codes adapt zone boundaries in order to accommodate the desire for individual states to provide greater safety, less contrast from one part of the state to another, or to tailor zones more closely to natural tectonic features. Because of these zone boundary changes, the zones do not have a deeper seismological meaning and render the maps meaningless for applications other than building codes. An example of such tailoring is given by the evolution of the UBC since its adaptation of a pair of 1976 contour maps. First, the UBC took one of those two maps and converted it into zones. Then, through the years, the UBC has allowed revision of zone boundaries by petition from various western states, e.g., elimination of zone 2 in central California, removal of zone 1 in eastern Washington and Oregon, addition of a zone 3 in western Washington and Oregon, addition of a zone 2 in southern Arizona, and trimming of a zone in central Idaho.
Older (1994, 1997) versions of the UBC code may be available at a local or university library. A redrafted version of the UBC 1994 map can be found as one of the illustrations in a paper on the relationship between USGS maps and building code maps.
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What is the likelihood of a large earthquake at location X? Is it safe to go to X since they've been having a lot of earthquakes lately?
New seismic hazard and risk assessments can help at-risk communities prepare for future earthquake disasters
New Audiences, New Products for the National Seismic Hazard Maps
Friday's magnitude-5.2 earthquake in southern Illinois is a reminder that earthquakes are a national hazard.
A new geologic map of surficial deposits in the nine-county San Francisco Bay region that can be used to evaluate earthquake hazards has been released in digital form by the U.S. Geological Survey in Menlo Park.
USGS scientists conduct passive seismic study in the Washita Reach 1 study area.
This map depicts the difference in USGS estimates of earthquake hazards across the nation in 2002 and 2014. The new FEMA earthquake loss report includes the 2014 data, while the previous 2008 report includes the 2002 data. The negative values represent a decrease in estimated hazard since 2002 and the positive values represent an increase.