The quake was moderate, notes Andre Filiatraut, who heads the Multidisciplinary Center for Earthquake engineering research at the State University of New York at Buffalo. But "it is significant because we haven't had very many earthquakes of this magnitude occurring in the eastern United States or eastern Canada."
The relatively low frequency of moderate-to-strong earthquakes in the eastern US compared with shaking and volcanism along the the West Coast is due to significant differences in the the geological activity taking place beneath their residents' feet.
In California's case, the western edge of the state straddles an active boundary between two enormous plates in Earth's crust – the North American plate and the Pacific Plate. The San Andreas fault and its tributaries mark the boundary with plenty of shaking and mountain-building as the Pacific Plate grinds against its way north against the continent.
The eastern seaboard, by contrast, sits in the middle of the North American Plate, but 200 million to 300 million years ago it was along an ancient plate boundary that ran through a connected Europe, Africa, and North America.