Colorado, Virginia, California: three earthquakes across the United States in three days. Aside from the apocalyptic questions some are raising, was this more than an earth-trembling coincidence? Or was there some scientific connection between these three events? First there was the magnitude 5.3 earthquake near Trinidad, Colo., the largest earthquake in the state since 1973. Then a magnitude 5.8 quake centered in Mineral, Va., jarred a region from Charleston, S.C. to Boston. Finally, there was a relatively mild 3.6 rattler in the San Francisco area Tuesday night. Whats going on here?
Experts say that while the Colorado and East Coast earthquakes were unusual, the first one did not trigger the second. Nor did the San Francisco quake have anything to do with the two that preceded it. They really are unrelated, says Meredith Nettles, a seismologist at Columbia University. There really is no causal connection. This is pure coincidence, concurs San Diego State University seismologist Tom Rockwell. Thats because small earthquakes dont change the state of stress very much in the crust of the earth, so the effects will be only local, he says.
While this weeks earthquakes made news with earthquake tremors and hurricane Irene on the way, some people in North Carolina wondered if a plague of locusts was next the earths shake-rattle-and-roll is going on all around us almost constantly. In just the past week in the US alone, there were about 700 earthquakes perceptible to detection equipment. And as the US Geological Survey puts it, there's a 100 percent chance of an earthquake today somewhere in the world.
For most Washington residents, an earthquake in the nations capital seems about as likely as a foot of snow in Malibu. And yet, as Tuesdays temblor shows, no region in the country is actually immune from the movement of the earths crust. As the 5.8 quake also reveals, East and West may be one geographical nation, but geologically speaking, theyre 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.