For most Washington residents, an earthquake in the nation’s capital seems about as likely as a foot of snow in Malibu. And yet, as Tuesday’s temblor shows, no region in the country is actually immune from the movement of the earth’s crust.
As the 5.8 quake also reveals, East and West may be one geographical nation, but geologically speaking, they’re as far apart as Hollywood and New Hampshire. The difference in geology is a main reason why the moderate earthquake centered near rural Mineral, Va., could cause a four-foot-long crack in the Washington Monument and be felt as far away as Toronto, while Western quakes, even if they are stronger, tend not to travel as far.
(The Washington Monument remains closed while the National Park Service awaits a damage assessment from structural engineers.)
The age of the rock is the first and biggest reason for the difference between East and West, says Mark Castner, director of the Braun-Ruddick Seismograph Station at Canisius College in Buffalo, N.Y.
The Sierra Nevada mountain range that dominates the West Coast pushed up some 40 to 60 million years ago, whereas the Appalachians date back 400 million years or more. The relatively recent and ongoing activity in the West means that there are 10 times more fractures in the surface rock – which tend to dampen earthquake transmission – than in the East.
By contrast, the East coast mountains are much lower, with much more of the original rock now compressed into the valleys, creating a long and uninterrupted rock structure to transmit the power of the quake, Professor Castner says.
The phenomenon has been known for a long time and has been consistently observed for all moderate-to-large earthquakes in the central and eastern part of the continent, says Timothy Larson, a geophysicist at the Illinois State Geological Survey, Prairie Research Institute, at the University of Illinois at Urbana.
“We have fewer earthquakes here, but when we do have them, they affect larger areas,” he says.
All these differences mean “lessons learned” from the seismically active West must be modified for implementation in the East, he adds.
The biggest challenge scientists face is the lack of data for quakes in the eastern US, says University of Dayton geologist Allen McGrew.
“It is very difficult to do any meaningful statistics on them,” because scientists just don’t have the leverage of large numbers, he says via e-mail. “Forget calculating recurrence intervals or anything like that except in the most active zones in the East, like the New Madrid seismic zone.”
This also means that processes like weathering, erosion, and soil formation have a longer period in between quakes to “erase the evidence.” Moreover, in the wetter, more humid East, these processes act more quickly.
Eastern quakes are like the old-school college professor who erases with his left hand as he writes with his right,” Professor McGrew adds.
Geologists cannot even locate the faults that are responsible for the seismicity; they remain hidden from view in the underlying “basement” rocks, he says. “Such is the case for the Virginia earthquake, where no surface fault has previously been mapped despite the fact that it is a known seismic zone. “
The need for preparedness based on local geology may be the most important lesson, says Gregory Waite, a geologist at Michigan Technological University in Houghton.
Relatively moderate earthquakes, such as Tuesday’s event, provide a reminder of the potential for truly damaging earthquakes, he says, which makes the much less frequent quakes in the East all the more important to study carefully and prepare for.
“Much of what is done to understand the hazards from earthquakes is based on the occurrence of past events. These recent earthquakes provide important data for those studies,” he says.
Large earthquakes have occurred in the Midwest and along the East Coast during the past 200 years, inclding the New Madrid earthquakes of 1811 and 1812 and the Charleston earthquake of 1886, which were magnitude 7 or larger, he says.
“Similar events today would certainly result in loss of life, extensive damage, and widespread disruption of commerce,” he says. “While these events are extremely rare, it is important to be aware that they can and probably will occur again some time in the future.”