If you are referring to the earthquake that occured on 08/24/2011 then the answer is actually very simple.
Due to the fact that the continental plate in which the eastern coast of the united states is made of is older it therefore means it is also more dense (solid). Seismic waves as we have learnerd through scientific study that the plates that are denser allow waves to move faster and those that are less dense such as the asthenosphere causes waves to slow down due to decrease in density. In other words the energy cannot move from one particle to the next because they are not touching as closely. Because of all of the above the east coast allows seismic waves to travel much farther and faster and lose very little energy in the process. Where in the west coast lets say anything west of the rockies the plate is a bit newer and less dense.