-------- Original Message --------
Subject: Fw: USGS News Release/Earthquake Hazard Maps Show How the
Nation Shakes with Quakes
Date: Mon, 21 Apr 2008 15:40:05 -0500
From: John C Fouke <[log in to unmask]>
To: Maps-L
News Release
Contacts: Mark D. Petersen 303-273-8546
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Clarice Nassif Ransom 703-648-4299
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Earthquake Hazard Maps Show How the Nation Shakes with Quakes
Friday?s magnitude-5.2 earthquake in southern Illinois is a reminder that
earthquakes are a national hazard.
Today, scientists at the U.S. Geological Survey (USGS) are revealing how
shaky the nation is by releasing an updated version of the USGS National
Seismic Hazard Maps.
Earthquakes remain a serious threat in 46 of the United States. For some
areas such as western Oregon and Washington, the new maps contain higher
estimates for how hard the ground will shake compared to earlier versions
of the maps released in 1996 and 2002.
But for most of the United States, the ground shaking estimates are lower.
This revision incorporates new seismic, geologic and geodetic information
on earthquake rates and the manner in which the energy released in
earthquakes dies off with distance from the rupture.
National-scale maps of earthquake shaking hazards provide information
essential to creating and updating the seismic design provisions of
building codes used in the United States. The timing of the National
Seismic Hazard Map release is tied to the schedule for revising model
building codes that are developed by international code committees and
then considered by state and local governments for adoption. Cities and
counties rely on seismic design provisions in building codes to ensure
that structures such as buildings, bridges, highways and utilities are
earthquake resistant.
"The hazard maps released today incorporate more than a century of seismic
monitoring and decades of research," said USGS Director Mark Myers. "These
maps help policymakers and engineers make all of our structures ? from our
homes to our hospitals to the utilities that run beneath our feet ? better
able to withstand the earthquakes of tomorrow."
The National Seismic Hazard Maps consist of a series of maps and databases
describing ground shaking at many points across the country and have many
applications. They are used by insurance companies to set insurance rates
for properties in various areas of the country, by civil engineers to
estimate the stability and landslide potential of hillsides, by the U.S.
Environmental Protection Agency to set construction standards that ensure
the safety of waste-disposal facilities, and by the Federal Emergency
Management Agency to plan the allocation of assistance funds for
earthquake education and preparedness.
The geologic and geophysical data-collection, research and modeling
results that underpin the maps have been generated by USGS scientists as
well as their colleagues in academia, state government, and the private
sector funded by external grants from the USGS Earthquake Hazards Program
and other sources. The new maps represent the best available science as
determined by the USGS based on an extensive information gathering and
review process involving state and university experts nationwide.
Last week the USGS and its partners released a new earthquake rupture
forecast for California, the first ever such forecast done statewide. That
forecast focused on the likelihood of earthquakes happening on specific
faults. The National Seismic Hazard Maps take the information about those
faults and calculate the intensity of shaking that a given location could
potentially experience over a 50-year period. These shaking estimates
combine the effects from all possible earthquakes, both nearby and
distant.
The changes in earthquake ground shaking estimates are due principally to
the incorporation of new models on the strength of earthquake shaking near
faults, and the manner in which shaking decreases with distance. The
increased hazard in western Washington and Oregon is due to new
ground-motion models for the offshore Cascadia subduction zone. Because of
new models, ground motion estimates in the Central and Eastern United
States are about 10 ? 25 percent lower. Ground motion estimates in most of
California, Utah, Nevada, Arizona, Idaho and western Montana are as much
as 30 percent lower for shaking that affects taller multi-story buildings.
For those same areas, ground motion estimates remain nearly the same for
shaking that affects structures of one or a few stories. Because these
maps are done at a national scale, they do not take into account local
soil conditions and the depth of sedimentary basins, which can
significantly amplify shaking relative to bedrock. As a result,
site-specific factors are applied in the building codes to determine the
seismic design needed for individual structures.
The map included the following regional changes:
· Several new faults were included or revised as a source of
earthquake ground shaking in California, the Pacific Northwest and the
Intermountain West.
· The Wasatch fault in Utah was modeled to include the possibility
of a magnitude-7.4 earthquake, in addition to smaller earthquakes along
the fault.
· The model for earthquakes along the New Madrid Seismic Zone in
the Central United States includes a wider range of possible magnitudes
and return periods between major earthquakes. The model was also adjusted
to allow for sequences of earthquakes to occur in groups of three within a
few years time, similar to what occurred in 1811 ? 1812.
· Offshore faults were added as possible sources of earthquakes
near Charleston, S.C.
· For the Cascadia Subduction Zone, more weight was given to a
magnitude-9 earthquake that ruptures the length of the subduction zone,
versus multiple smaller magnitude-8 earthquakes that fill the zone over
the same 500-year time period.
For individuals and communities, knowing where earthquakes occur and their
effects is just the start down the road to preparedness. The USGS
encourages all citizens in earthquake-prone areas to follow the Seven
Steps to Earthquake Safety, which can be accessed at
http://www.earthquakecountry.info/roots/seven_steps.html.
The USGS National Seismic Hazard Maps are available at
http://earthquake.usgs.gov/research/hazmaps/
USGS provides science for a changing world. For more information, visit
www.usgs.gov.
Subscribe to USGS News Releases via our electronic mailing list or RSS
feed.
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