Department of Earth and Environmental Sciences
Holiday parties are a time for me to connect with old friends, meet new friends, and get asked a lot of questions about why the Earth quakes along the East Coast.
This year, fellow party-goers here in New England have been telling me that they felt the earthquake that occurred in Virginia on August 23 – and they want to know why a magnitude 5.8 earthquake occurred there. My inevitably disappointing answer: “Nobody really knows.”
They finally have the attention of a seismologist and get to ask the big question, and all they get is “Nobody really knows”? You call that an answer? Have they been cheated out of a great scientific story? I don’t think so. In fact, I would argue that it’s the mysteries of science that are the great and engaging stories. The stories about what we know are much less exciting to me than the explorations of what we don’t know. This is the realm of wonder, awe, suspense, excitement, and fun that makes science a great endeavor.
Now, I should clarify: It’s not that we don’t know anything about the cause of Eastern US earthquakes.
California, and the West Coast in general, is the place in the US that many people think of as “earthquake country.” Most California earthquakes occur near a major boundary separating two of the Earth’s tectonic “plates”, the North American plate and the Pacific plate. Nonetheless, earthquakes are well known to occur in plate interiors as well, and those earthquakes are called intraplate earthquakes. Intraplate earthquakes are common occurrences, globally as well as specifically in the Eastern United States.
A commonly espoused explanation for the cause of intraplate earthquakes is that “ancient zones of weakness” are being reactivated by forces currently active in plate interiors. In this model, preexisting faults and/or other geological features formed during ancient geological episodes persist in the intraplate crust, and, by way of analogy with plate boundary seismicity, earthquakes occur when the present-day stress is released along these zones of weakness. While this concept of reactivation of old zones of weakness is commonly assumed to be valid, in reality the identification of individual active geologic features has proven to be quite difficult. Unlike the situation for many plate boundary earthquakes, it is not at all clear whether faults mapped at the Earth’s surface in the Eastern United States are the same faults along which the earthquakes are occurring.
The magnitude 5.8 Virginia earthquake occurred in the “Central Virginia Seismic Zone”, a zone of seismicity where earthquakes have occurred before. But the specific cause of earthquakes in this seismic zone, including that of the recent magnitude 5.8, remains an enigma.
In 1875, an earthquake in this zone shook bricks from chimneys, broke plaster and windows, and overturned furniture. Based on felt effects and damage from the 1875 earthquake, it has been estimated to have had a magnitude of about 4.8. On December 9, 2003, a magnitude 4.5 earthquake occurred in this region causing minor damage, and over the years other small earthquakes have also occurred in the Central Virginia Seismic Zone.
So it is not surprising to seismologists that an earthquake would occur in Virginia, but why it occurred at that particular location, at that particular time, and why it was a magnitude 5.8 remains a mystery. And this is the kind of mystery that drew me to a life obsessed with the enigma of why earthquakes occur in the Eastern United States, deep in the interior of the North American plate.
Click here for a paper summarizing the Virginia earthquake: Mineral, Virginia, Earthquake Illustrates Seismicity of a Passive-Aggressive Margin, by Emily Wolin, Seth Stein, Frank Pazzaglia, Anne Meltzer, and Alan Kafka, Geophysical Research Letters, 39(L02305), doi: 10.1029/2011GL050310.