April 23, 2008 -- The newest U.S. earthquake hazard maps show a laundry list of seismic possibilities from sea to shining sea -- as well as in Hawaii and Alaska.
The maps by the U.S. Geological Survey (USGS) reflect the discovery of more earthquake faults in California, a new 7.4-magnitude-quake risk in Utah, as well as more ways to rock the Midwest, South Carolina and the Pacific Northwest than you can shake a stick at.
Unlike last week's forecast of possible ruptures along faults in California, the National Seismic Hazard Maps calculate the likely intensity of shaking at any given location -- caused by any regional fault giving way -- over a period of 50 years.
As a result, the new maps are vital for updating local building codes and a direct response to the old geologist truism: Earthquakes don't kill people, buildings kill people.
The good news from the new maps is that the potential for ground shaking in the Central and Eastern United States is thought to be 10 to 25 percent lower, based on the latest science of quake frequencies and models of how the ground moves.
Meanwhile a better understanding of the colliding plates in the Pacific Northwest, the quake history there and its similarity to the mega-quake-producing Sumatra collision zone has upped the risks of shaking from coastal Northern California up to and past the Canadian border.
"We usually update the maps every six years," said the USGS's Mark Peterson, who crisscrossed the country gathering expert opinions and the best new science to develop the 2008 maps. The six-year interval meshes well with the three-year interval between building code changes. The hazard maps are also used by insurance companies, Peterson said.
What the maps actually show is the possible horizontal or sideways motions of the ground caused by an earthquake in terms of fractions of one "g." A g is the downward tug of gravity felt on the Earth's surface.
In other words, if you are in an earthquake that's causing 0.5 g horizontal accelerations, your house would be jerked back and forth sideways with half the force that gravity is exerting to hold the building on the ground. That's something building engineers need to plan for.
The simple idea behind the maps is to estimate the frequency of earthquakes on any given fault and equate it to the degree to which the ground could shake.
"We tie it all together," said Art Frankel, a USGS scientist who has worked on earlier versions of the national quake maps.
That said, there's a whole lot of sweat and science by government, private industry and academic Earth scientists that goes into determining quake frequencies and how they will jolt the ground.
In addition to including the latest information about historical earthquakes, the maps reflect work on prehistoric quakes, ancient and current slip rates of faults and the degree to which a regions ground is deformed by geological forces. There are also a variety of complex models used to estimate the ground shaking.
All the maps are available without charge online.
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