By Dr. Robert Thorson
How long is one month?
Long enough for mass culture to completely forget about the strongest earthquake to rattle the Eastern U.S. in decades, one felt by more people than any other in U.S. history.
It was certainly exciting while it lasted. Millions of Americans heard thunks, rattles, buzzes and clatters. Physically they felt slams, vibrations and some swaying. Countless buildings were evacuated, among them the White House, the U.S. Capitol, New York City Hall and untold urban office buildings, their occupants clogging the streets, talking and texting. My phone began ringing off the hook with requests for comment on a truly newsworthy event.
Yet within minutes, the forgetting began. The Dow Jones Industrial Average, despite plunging 60 points immediately following the quake, rebounded with a gain for the day. With no terrorist connection, little damage and little injury, there was nothing to give the story legs. And now, exactly one month later, it’s as if nothing had happened.
Unless, of course, you work with natural hazards like I do. If so, you know that the main issue with this class of hazard is not the physical science and engineering involved, but the fading of collective memory within a geological nanosecond.
The Federal Emergency Management Agency has not forgotten. Its report, “Estimates of Annualized Earthquake Losses in the United States” (2008), outlines the probability and possible severity of earthquakes occurring at intervals up to 1,000 years. In the agency’s analysis, New York, Boston and the cities in between remain hot spots of seismic risk. Though the physical geological hazard is fairly low, the concentration of population and investment in infrastructure is exceptionally high. So, although most people are busy forgetting, policy-makers and insurance corporations are busy remembering that we do have a problem.
For example, greater New York ranks fourth in the nation for the number of households likely to be displaced by earthquakes per year (204); eighth for the mass of demolition debris needing removal (41,000 tons); 12th for direct casualties (two deaths) and, most important in today’s economy, 21st for the cost of repairing the damage, about $30 million per year.
Damaging earthquakes do take place in the East. This summer’s Virginia quake was preceded by a stronger one there in 1897. Charlestown, S.C., was heavily damaged in 1886, its railroad tracks twisted like ribbons. Connecticut’s largest tremor shook the Moodus section of East Haddam in 1791, and would have caused considerable damage if it occurred that year in Hartford. The 1755 Cape Anne earthquake north of Boston toppled chimneys in western Connecticut.
According to the U.S. Geological Survey, the magnitude 5.8 earthquake that struck Mineral, Va., on Aug. 23 was caused by a rupture within the shallow crust. Long ago, this Central Virginia Seismic Zone was an active fault being thrust, stretched and sheared along a tectonically active plate margin. In the modern geological era, however, this zone is merely localizing the effects of strong compression affecting much of the crust beneath eastern North America.
Because this crust is old, thick and cold, strong shaking usually takes place only near the epicenter. Beyond that, seismic energy travels great distances with little effect. , other than generating short-term excitement.
The Virginia earthquake fits this pattern. Based on 26,392 responses and, though felt from Alabama to Ontario, it caused serious shaking and moderate to heavy damage only in the immediate vicinity of the epicenter (Intensity of VIII on the Modified Mercalli scale). Intensities fell to IV to V (moderate to light shaking and very light to no damage) within about 30 miles of the epicenter.
Because of their geological setting, eastern earthquakes pose a risk similar to eastern tornadoes. Though the probability of experiencing damage is low at any given time and place, the potential for serious localized damage is very real and quite high. To be prudent, our building codes should plan for the worst and expect the best. This, of course, takes money we don’t seem to have.