Earthquakes greater than a magnitude 3.0 have occurred in many locations in the central and eastern United States since 1974. USGS
New research reveals that earthquake-induced ground shaking travels much farther on the East Coast of the United States than previously realized.
December 4, 2012—New research released by the United States Geological Survey (USGS) earlier this month reveals that the August 23, 2011, magnitude 5.8 earthquake centered in Mineral, Virginia, produced ground shaking that traveled much farther than anything that had been previously recorded. Some on the West Coast—home to such major events as the 1906 San Francisco earthquake (magnitude roughly 7.7-8.3), the 1989 Loma Prieta earthquake (magnitude 6.9), and the 1994 Northridge earthquake (magnitude 6.7)—derided the East Coast for its reaction to the relatively small earthquake. But ground motion from the quake was felt from Canada to Florida, and from Virginia to Texas and Wisconsin.
The current study, which examined landslides triggered by the earthquake, was completed by USGS research geologists Randall W. Jibson, Ph.D., and Edwin L. Harp, Ph.D.
The Virginia earthquake was shallow and had an epicenter about 80 mi southwest of Washington, D.C. Using existing models, the researchers anticipated that the ground shaking would cause small landslides no farther than 37 mi from the epicenter and within an area no larger than 579 sq mi. What they found was that the maximum epicentral distance for small landslides triggered by the quake was 152 mi, more than four times what has been observed in previous earthquakes of this magnitude. Even more impressive, the area affected by landslides extended for 12,896 sq mi, according to Jibson, who wrote in response to questions posed by Civil Engineering online.
“Finding an area affected by landslides that was 20 times larger than previously seen in the worldwide data was a big surprise,” Jibson said. “Although we expected the area to be larger because it was in the eastern United States, where we know seismic waves travel farther, we certainly did not anticipate a factor of 20.”
The crust of the earth in the central and eastern United States is
both older and colder than it is in the western Unites States,
meaning that earthquakes centered in the east propagate seismic
waves more efficiently—and over much greater distances—than
those centered in the west. USGS
Jibson explained, “In the central and eastern U.S., the crust is what we call ‘old and cold’—it is older, denser, colder crust that has fewer faults and fractures, and therefore it propagates seismic waves more efficiently. Crust in the western U.S. is less dense, warmer, and highly disturbed by faults and other deformation, and this makes it less efficient in propagating seismic waves.”
The implications of the pair’s findings for the future are fundamental, according to Jibson. The equations used by scientists and engineers to predict ground motion from earthquakes along the East Coast need to be updated in terms of distance, strength, and how mountainous regions—such as the Appalachian Mountains in the 2011 event—can affect seismic waves. Such azimuthally dependent ground shaking will travel farther parallel to mountain ranges than perpendicular to mountain ranges, Jibson said.
Concrete knowledge of whether the impact of the 2011 Virginia Earthquake was an anomaly or an indicator of how much stronger and farther ground motion travels along the East Coast will come with additional research. “We need to carefully document other intraplate earthquakes,” said Jibson. “Of course, that means waiting for future central and eastern U.S. earthquakes, which occur much less frequently than West Coast earthquakes.” While the option of studying historical earthquakes also exists, it is not an easy task considering that the last quake of this size occurred more than 100 years ago—the approximately 5.9 magnitude Giles County, Virginia, earthquake in 1897—and researchers then were not able to study the quake in as much detail as is currently possible.
The new study will be published as “Extraordinary Distance Limits of Landslides Triggered by the 2011 Mineral, Virginia, Earthquake,” in the December 2012 issue of the Bulletin of the Seismological Society of America.
Previous coverage in Civil Engineering online has focused on the damage that the 2011 earthquake caused to the Washington Monument and National Cathedral in Washington, D.C. The official seismic assessment report for the Washington Monument was prepared by Wiss, Janney, Elstner Associates, Inc., (WJE) for the National Park Service and released this summer. A feature article detailing WJE’s postearthquake assessment will be published in the December issue of the magazine.