According to the data I've seen, it has been recorded as a 5 point 9. The epi centre being underground at Mineral in VA. Why it was felt in such a big area (including parts of southern Canada) is because of it being at a high level (a few miles under ground-level).
Between the epi centre and NY there is a fault line, in global terms it is a fairly minor one. The last noted earthquake being 114 years ago. The number of after shocks are only half a dozen or so. I say only half a dozen or so, as compared with Christchurch in New Zealand's South island. That has had about 8,000 aftershocks in the last 11 months.
On the down side, I hope all the residents of east coast America are getting ready, in case Irene heads your way. The last big one was Andrew in 1992, that cyclone decimated many communities on the coast.