The earthquake which occurred in central Virginia on 9 December 2003 was a double event. This can be seen be comparing the two figures below: one from an earlier event (5 May 2003 with M=3.9) in the same area as recorded at station BLA in Blacksburg, VA (epicentral distance 215 kilometers) and one for this event recorded at station FWV at Forest Hills, WV (254 kilometers).
The above figure, from the 5 May event, shows normal arrivals from an earthquake at this distance: a P-wave arrival at about 75 seconds followed about 25 seconds later by the larger amplitude S-wave energy. This time separation increases with epicentral distance.
The above figure, from the 9 December event, is like the above except that there are two sets of P-wave and S-wave arrivals. The P-wave and S-wave arrivals for the first event are at a little before 59 seconds and 88 seconds respectively, and those for the second event are about 12 seconds later for each arrival. Based on the relative amplitudes, the two events have the same magnitude to within 0.1 magnitude units.
The ground shaking which people feel comes from the S-wave arrivals. Many of the felt reports reported a duration of up to 30 seconds. If this were from a normal event (one with only a single set of arrivals), one would infer from this that the magnitude was larger than 4.5. Based on these records, we think that what the observers actually experienced was shaking from the two separate events. A few observers at more distant locations (Waynesboro and Blacksburg) report feeling two successive "bumps" with about a ten-second time separation.