On Monday, my Physical Geology students and I experienced a magnitude-2.0 earthquake during our final exam. The hypocenter was 1 km south and 6km down.

I pulled out the geologic map of the Annandale quadrangle (1986) by Avery Drake and A. J. Froelich to check for faults in that area. Here's a scan of the map:

You can see our campus in the northwest, and I've highlighted the epicenter of the quake with the red and green concentric circle. Interstate 495, the Capitol Beltway runs north-south through the center of this area. To the south and east of NOVA's campus, you can see that there's a mappable thrust fault (the Red Fox Thrust) which dips to the northwest, presumably under the epicenter and under the campus. However, the map provides no information on the angle of dip of this thrust. Is it steep enough to get to 6km depth a mere ~3km north of its surface trace? (The map's cross section shows it dipping at about 52 degrees, but that's pretty speculative.) Or if the dip is shallow, is there a deeper (perhaps parallel) thrust underneath it? (There is none shown within the map area, though there is one to the north of campus that dips to the north -- making it unlikely to be the culprit.) Alternatively, was Monday's quake caused by a new fault? Perhaps a normal fault which cross-cuts these Paleozoic thrusts?

No new answers -- only more questions...

Labels: earthquakes, news, nova