Virginia Tech Seismological Observatory

Richmond Earthquakes: July–August, 2009

There have been five small earthquakes instrumentally located in the Richmond vicinity recently:

• The first shock was magnitude 2.3 and occurred around midnight local time July 6th/7th, with its epicenter near Short Pump, VA. It was sharply felt in the local area and many felt reports were received.
• A second, smaller shock occurred on July 15th, a few miles to the north, and was not reported felt.
• Another small shock (magnitude 2.1) occurred on July 31st, near Ashland, and was not reported felt.
• The fourth shock occurred on Sunday, August 2nd. It was magnitude 2.3 and was felt in the Beaverdam, VA area and by some residents in Caroline County.
• The most recent shock occurred on Monday, August 3rd, and was located in almost the same location as the fourth quake near Beaverdam. It was magnitude 1.6 and may have been felt.

Sequences of small shocks with magnitudes similar to those noted above have occurred in the central Virginia seismic zone on several previous occasions during the past 30 years. A distinguishing characteristic of all those previous sequences has been a very small source volume. Earthquakes defining those previous sequences have occurred at essentially the same location, with shallow focal depths (less than approximately 5 km). The current sequence seems to be quite different, however.

In this case, the shocks have progressed northward with time, over a distance of several kilometers. The overall trend of these epicenters parallels the strike of major geologic structures in the Richmond area, in particular, faults associated with the Mesozoic Richmond Triassic basin and the Paleozoic Hylas zone. Focal depths are in excess of 5 km. If this current sequence of events persists and/or intensifies, it will warrant close attention from the perspective of seismic hazard assessment.