USGS Fact Sheet 131-02: Earthquake Hazard in the Heart of the Homeland

Geologic Hazards Fact Sheet

Earthquake Hazard in the Heart of the Homeland

U.S. Geological Survey FS-131-02 Page 4

How likely is a New Madrid earthquake?

In much the same way as meteorologists forecast rain, earth scientists present forecasts of earthquakes as the chance or “probability” of an earthquake occurring in a specific time interval. Although we are still learning why the New Madrid region experiences large earthquakes, research during the last 15 years in the Central United States has led to new discoveries that, not surprisingly, have led to new estimates of the likelihood of damaging earthquakes. It is generally accepted that earthquakes can be expected in the future as frequently as in the recent past. We determine how often earthquakes reoccur from historical and geological studies (see box “Digging up the past”). Using these data, which were also used to produce the National Seismic Hazard Maps, the USGS and the Center for Earthquake Research and Information of the University of Memphis now estimate that for a 50-year time period:

Probability of a repeat of the 1811 - 1812 earthquakes
(magnitude 7.5 - 8.0) = 7 - 10%

Probability of a magnitude 6.0 or larger = 25 - 40%

Earthquakes in the approximate range of magnitude 7.5 to 8.0 are capable of causing widespread damage over a large region. Magnitude 6.0 earthquakes can cause serious damage in areas close to the earthquake’s location. It is important to remember that earthquake damage mostly results from the shaking associated with the waves emitted by the earthquake. The probability of damaging shaking, or hazard, depends on much more than earthquake size. These other factors, such as where the earthquakes occur and local soil conditions, must also be considered when assessing the hazard posed by earthquakes.

Factors controlling ground shaking

In addition to earthquake recurrence, hazard also depends on how amplitudes of earthquake waves die out as they move away from the earthquake source to the affected site. It is well known that wave energy decreases much more slowly in the Central and Eastern United States than in the West. For the same size earthquake, this leads to greater shaking over larger areas, or higher hazard, in the Central and Eastern United States. Earthquake shaking also may be significantly amplified or damped by the soils immediately beneath a site. This is particularly true for thick sediments that underlie most of the New Madrid seismic zone. Recent research has highlighted the potential amplification of ground shaking by these thick sediments and has begun to shed light on the physical processes that cause this. Because characterization of this amplification

requires detailed mapping of the soils and their properties, amplification characteristics are not included in the National Seismic Hazard Maps. Efforts are underway, however, to produce complementary maps that do show these amplification characteristics on scales appropriate for use in urban planning, earthquake hazard mitigation, and response planning efforts.

Sezmogram showing wave motion over time on top of and below sediments.

When waves of a small earthquake impinge on the boundary between hard rock and overlying sediments from below (lower seismogram), they become amplified as they travel through the sediments to the surface (upper seismogram). A seismogram records the wave motion over time. The amplifying effects of thick sediments above the New Madrid seismic zone results in motions at the surface that are many times larger than below. (Courtesy of C. Langston, The University of Memphis.)

For more information contact:

Joan Gomberg
Eugene Schweig
U.S. Geological Survey
3876 Central Avenue
Suite 2
Memphis, TN 38152-3050
901-678-4858
gomberg@usgs.gov

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