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December 11, 2019

USGS experts recently released a study that incorporates the latest earthquake science findings into an update of the National Seismic Hazard Model for the “lower-48” United States.  Read the full report here:

Research from government, academia, and industry is used to produce such models, which consider where future earthquakes might occur, how often the earthquakes happen, how big the earthquakes might be, and how strongly the ground could shake. Earthquake hazards are an important consideration in planning efforts across the U.S., and during the past 6 years strongly felt earthquakes (>M 4) occurred in 21 states in the U.S. (Alaska, Arizona, California, Colorado, Delaware, Hawaii, Idaho, Kansas, Michigan, Montana, Nebraska, Nevada, New Mexico, Oklahoma, Oregon, South Carolina, Tennessee, Texas, Utah, Washington, and Wyoming).  As new earthquakes occur, the National Seismic Hazard Model Project Team considers how to incorporate those scientific observations into improved hazard models across the country. For example, the 2019 Ridgecrest earthquakes (M 6.4 and M 7.1) showed that surface ruptures can be very complex (Figure 1) and can cause complicated ground shaking that should be accounted for in updated models.

A woman and two men inspect and measure a large crack in the earth's surface
Geologists with USGS, the California Geological Survey (CGS) and Naval Air Weapons Station China Lake (NAWS) worked together in response to the Ridgecrest earthquake sequence in California that occurred July 4-6, 2019. The earthquakes were large enough that the fault rupture reached the earth’s surface. Here, research geologist Belle Philibosian was part of a USGS field team working with Gordon Seitz (CGS) and Stephan Bock (NAWS) in the Ridgecrest area on the NAWS documenting fault offsets through direct measurements using tools ranging from tape measures to mobile laser scanning. Photo credit: Katherine Kendrick, USGS   

The recent hazard update includes: (1) an updated catalog of past and recent earthquakes, (2) new ground-shaking models in the central and eastern U.S. that incorporate shaking data from recent earthquakes and computer simulations, (3) improved ground-shaking estimates at sites overlying deep soils near Los Angeles, San Francisco, Seattle, and Salt Lake City, and (4) more detailed shaking estimates for a wider range of building and soil types. The new model considers a broad range of shaking wavelengths, because long-wavelength (slower) ground shaking influences tall buildings and bridges, and short-wavelength (faster) ground shaking influences shorter structures. It also considers various soil and rock types because ground shaking is often amplified at soil sites, making the underlying geology a critical factor in assessing hazards. This new hazard model can help communities prepare for strong earthquake ground shaking in ways that balance mitigation costs and life safety, resulting in community resilience following a damaging earthquake.

The USGS National Seismic Hazard Model is incorporated into U.S. building codes, as well as in risk assessments. The timing of USGS hazard model updates is coordinated with building-code updates developed by committees of civil engineers. Those committees also provide user feedback that can improve the USGS model. Through such user engagement, the USGS has provided science-based hazard information that is factored into seismic codes for not only buildings, but also bridges, railways, power and sewage grids, dams, nuclear facilities, schools, and hospitals. The National Seismic Hazard Model is also used by federal, state, and local government agencies, insurance companies and other private businesses, land use planners, emergency response officials, the financial industry, and the general public. The USGS updates the model regularly so that these users can incorporate the best available science into their plans.

Image: Damage from 2011 Virginia Earthquake
Photo of chimney damage at a house in Louisa County, Virginia. This was a result from a magnitude 5.8 earthquake in Virginia on August 23, 2011.

More people live or work in areas of high or moderate seismic hazard than ever before, leading to higher risk of undesirable consequences from future ground shaking. For example, about 1 in 10 people in the U.S. now live in high-hazard areas where strong shaking is likely during their lifetimes. About a third of the population live in places where very strong shaking from rare earthquakes is anticipated. High risk is recognized along the west coast of the U.S., in parts of Nevada and Utah, and in parts of the central and eastern U.S. near Memphis and Charleston.

The higher risk is due to increases in population and improvements in how we estimate earthquake hazard. Results of the updated model show an uptick in ground shaking hazard in many (but not all) locations across the central and eastern U.S. (for example, Memphis, TN, St. Louis, MO and Chicago, IL), as well as near four urban areas overlying deep soils in the western U.S. Figure 2 shows damage from an earthquake in Virginia in 2011.

The map below (Figure 3​) shows a 2018 chance-of-damage map for 100 years with population density superimposed. Many populated centers are coincident with areas of higher ground shaking hazard, not only across the western U.S., where most earthquakes occur, but also within the central and eastern U.S. where earthquakes are less common.

2018 NSHM - Chance of Shaking Image
Map showing the chance of damaging earthquake shaking in 100 years from the 2018 NSHM. The shaking is equivalent to Modified Mercalli Intensity VI and is based on the average 1-second horizontal spectral response acceleration. Ground motions are computed for topographic-based VS30 values (Wald and Allen, 2007). Population density (from LandScan with 1 km x 1 km resolution [Dobson et al., 2000]) is superimposed on the map.

Useful Resources:

Get the latest in earthquake early warning with the ShakeAlert system.

Sign up to get earthquake notices from around the world with the USGS Earthquake Notification Service.

Finally, check out a recent scenario created by the USGS and other emergency management partners on the potential impact of a magntitude 7 earthquake along the Hayward Fault near San Francisco.

For more information about USGS Earthquake Science, maps, hazard models, and frequently asked questions, check out the USGS Earthquake Hazard Program website.




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