The Western Coast of North America is, geologically speaking, an active margin, where the Pacific Plate is scraping past the North American Plate (Southern California) and where remnants of the Farallon Plate are subducting beneath the North American Plate (Mexico and the Pacific Northwest). Intermittent powerful earthquakes occur along or near the coast and volcanic activity and tsunamis threaten the subduction zones.
The East Coast, by contrast, lies on the passive margin of North America, moving westward as the Atlantic Ocean opens, a process that has been ongoing for 150 million years. Prior to the Atlantic's opening, Earth's land masses had merged into the mega-Continent of Pangea (some 225 MYA); when Pangea broke apart, the new fractures did not always match the old suture lines (e.g. some Continental crust, formerly part of the African Plate, now form sections of our East Coast, primarily in Southern New England and Florida). As the Atlantic continues to open and pressure builds within the North American Plate, minor earthquakes develop along those old suture lines and their associated faults.
Yesterday's magnitude 3.6 earthquake, centered over Buzzards Bay off New Bedford, was felt across coastal Massachusetts, Rhode Island and Connecticut. Reported to be the strongest earthquake in Southern New England in several decades, it reflects geologic events that were set in motion back in the Mesozoic Era, when dinosaurs ruled the planet.
