As discussed in a previous blog, the geographic Continents due not often correspond to the borders of their respective tectonic plates. Since earthquakes and volcanoes are concentrated along the plate margins (where compression, friction and/or subduction occur), a given Continent is susceptible to these geologic events if it lies along or near these "active" margins.
North America has an active western margin and a passive eastern margin. Its plate, forming at the mid Atlantic Ridge, is inching westward. In the Pacific Northwest, this motion is causing the Juan de Fuca and Pacific plates to subduct beneath the advancing edge of North America; earthquakes and volcanoes are thus common along that margin. In the Southwest, the Pacific plate is scraping northwestward along the North American plate (at the San Andreas Fault), producing frequent earthquakes in that region.
By contrast, the East Coast of North America, lying atop the central portion of the Continental Plate, is not prone to geologic upheaval (though minor quakes can occur along old rifts and faults). Rather, this passive margin, characterized by a broad continental shelf, has experienced geographic alterations related to fluctuations in sea level. During periods of glaciation, when sea level was much lower, the coast was much further east than it is today. As the climate warmed and the sea level rose, the coast shifted west, leaving offshore islands and peninsulas of high ground; Long Island, Martha's Vineyard, Nantucket, Cape Cod, Nova Scotia and Newfoundland are examples of these remnant landscapes.
