Looking for earthquakes: From the riverbed to the research lab| June 25, 2012
Last August, a fault slipped in central Virginia. The resulting 5.8 magnitude earthquake rumbled through the old, cold rocks of the Piedmont, broke buildings in the District of Columbia 80-plus miles away and contributed to more than a few existential-type pauses along the entire eastern seaboard. As one local writer commented, almost immediately “the perceived seismic hazard” for the major population centers of Philadelphia, New York City and Boston was raised.
“It kicked us in the pants,” said William & Mary geologist Chuck Bailey.
Within minutes, W&M news-marketing director Suzanne Seurratan had Bailey on the phone trying to direct media calls. The questions: “What happened?” “Will it happen again?” “When?” Between interviews, Bailey prepared his blog post “It’s not my fault.” It began, “This afternoon, our Geology faculty meeting was adjouned by a motion from the floor.” He went on to report that the quake was felt for 20 seconds in Williamsburg at 1:51 p.m. Its epicenter was about 60 kilometers from Richmond. It occurred in the central Virginia seismic zone—an area of modest but persistent seismic activity.
Bailey also predicted: “This could be quite a semester to study the Earth.”
Step in John Hollis ’12, a one-time psychology major who had turned to geology late in his undergraduate career. Under Bailey’s tutelage, Hollis began working the Hylas Fault Zone, one of many that riddle the region. The Hylas, unlike the unknown fault that slipped more than two miles underground in August, was exposed. It could be seen along the fall line of the North Anna River near Doswell, Va., as reported in Bailey’s blog post “Searching for Hylas.”
During a subsequent excursion, discovery of what appeared to be a 15-centimeter wide vein of pseudotachylyte in a rock located approximately 20 meters from the current riverbed generated the most excitement. Pseudotachylyte occurs when a fault slips and frictional heating melts the surrounding rocks, Hollis explained. The August 2011 earthquake probably was too small to generate melting; the fact that the discovered sample appeared so large indicated the quake that produced it was perhaps 10-to-50 times larger. Such an event today, he said, would devastate the entire region.
During the coming year, Hollis will continue his study as a research fellow at the university. Employing geochemical and geochronological analyses, he will attempt to determine when the Hylas last ruptured.
According to Bailey, that research extends the geology department’s interest in establishing criteria by which to recognize potentially active faults in Virginia, which is nearly 3,ooo kilometers from the Mid-Atlantic Ridge, the major tectonic plate boundary.
“Virginia is riddled with these faults, most of which were considered ancient faults,” Bailey said. “The trouble is we don’t know which ones are active and which ones are old, dead, extinct.”
Concerning the current Hylas investigation, Bailey said, “We can look at structures and textures there. They may be a good analog for the processes involved in the 2011 earthquake.
“The volume of pseudotachylyte indicates large earthquakes in the Hylas zone that were many times larger than the August 2011 quake,” Bailey continued. “The question is, When did that large quake occur? If it were 2 million years ago, that is one thing. If it occurred 2,000 years ago, well then … .”