During today's magnitude 5.8 earthquake in Virginia, perceptible shaking was reported as far away as South Carolina and Maine. So why doesn't the same thing happen during California's much more frequent earthquakes along the San Andreas Fault? It comes down to a difference in crust. Density and temperature are primary controls on how far seismic waves can propagate through rock before dissipating

On the East Coast, the continental crust is older, colder, and denser. The coastline hasn't been tectonically active since Pangaea split apart, back when dinosaurs still roamed the Earth.

Contrast that with the West Coast, which is still tectonically active today, from the San Andreas Fault in Southern California to the subduction zone in the Pacific Northwest. West of the Rockies, the continent has been undergoing extension for the past 17 million years or so. That has stretched the crust, making it thinner and bringing hot mantle rock closer to the surface, which has warmed things up.

Because of these differences, shaking can be transmitted much farther (about three times the distance) through the colder, denser eastern crust.