How likely was the DC earthquake?

Last night at about 5 am there was an earthquake in the DC area that registered an unremarkable 3.6 on the Richter scale. It woke me up and was really an eerie feeling- kind of like a very big truck rumbling by, but definitely from underneath. Apparently there are small fault lines all over, it’s just a matter of how active they are.

How likely is it for something like this to happen? Has a really unlikely natural phenomenon ever hit your area? First we had snowmageddon, now this. WTF, nature.