“Highly unusual, once in a century” 5.8 Virginia earthquake leaves geologists puzzled

August 24, 2011 – VIRGINIA – The strong earthquake that rattled the eastern United States on Tuesday was highly unusual in its severity, though it was centered in a part of Virginia known for smaller quakes, seismologists said. The initial earthquake, which registered a magnitude of 5.9 just before 2 p.m. EDT, was felt from the Carolinas to New England. “One of this size is highly unusual,” said Karen Fisher, a professor of seismology at Brown University and president-elect of the seismology section at the American Geophysical Union. “This is the largest earthquake by far that I am aware of occurring there in recent history.” The quake’s epicenter in central Virginia is in an area the U.S. Geological Survey calls the Central Virginia Seismic Zone, Fisher said by phone from Providence, Rhode Island. While the strength of the quake was unusual, the wide spread of the shock waves was common for the East Coast, according to Peggy Hellweg, a research seismologist at the Berkeley Seismological Laboratory in California. This is largely due to the difference in the terrain, Hellweg said in a telephone interview. “Our ground is all of this chopped-up stuff … like a pile of marbles,” Hellweg said of California’s geology, which means that the waves from a quake don’t spread out all that far. In the Eastern United States, she said, “What you’ve got there is gorgeous bedrock and … the waves propagate beautifully.” Another difference between the East and West U.S. coasts is that the West Coast is over the boundary between two active tectonic plates, the North American and the Pacific, and the force of these plates sliding against each other generates quakes regularly, Fisher said. In the East, she said, “There’s no driving engine in terms of the two plates sliding past each other … so that’s why it’s much more unusual.” David Oppenheimer, a seismologist with the U.S. Geological Survey, said aftershocks should be expected for months. “You’ll feel magnitude 3 earthquakes over great distances … I would expect tens of magnitude 3s for earthquakes of this size,” Oppenheimer said from Menlo Park, California. –Reuters

Dormant faults have started to awaken: The outer rocky shell of the Earth, known as the lithosphere, is colder on the East Coast than in California, which is well known for experiencing frequent earthquakes. “So when something shakes, it is like hitting a bar of steel, it rings pretty well. Whereas on the West Coast, the rocks are higher temperature and it is more like hitting something quite a bit softer,” he said. Lucy Jones, a USGS spokeswoman, said the West Coast crust is broken up by active faults so it “doesn’t do as good of a job of transmitting the energy.” “On the East Coast, you have this old, hard, cold crust that does a lovely job of transmitting the waves like a solid bell,” she said on CNN, so that an earthquake “can definitely be felt hundreds of miles away.” The U.S. East Coast has plenty of fault lines, but they are ancient, and are inside a creaky plate that is under pressure from being jostled and pushed by other plates, experts said. Occasionally, pressure builds up and stresses will be released in earthquakes, like the one on Tuesday. “They are faults that used to be very active faults hundreds of millions of years ago, unlike the faults on the West Coast… (that) are active today,” Jordan said. Jack Boatwright, a seismologist with USGS, told AFP that one aftershock of 2.8 magnitude was recorded in the hour following the quake. Other parts of the world that are similar to the US East Coast in terms of earthquake dynamics would include India, as well as some parts of Russia and Australia, he added. “In India, that large triangle is relatively old, so we think that it conducts energy similarly,” he said. Other differences between East and West Coast quakes are the sounds they make — residents of California are less likely to hear banging associated with a big quake unless they are very near the epicenter, he said. –Physics.org