Photos 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. Related Links: |
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