The Magnitude 4.8 earthquake near Whitehouse Station, New Jersey, on April 5, was caused by a rather shallow, intra-plate slip along an old fault. Though not uncommon in the Eastern and Central U.S., any quake not along the Pacific Coast garners significant attention.
Indeed, major earthquakes in North America generally occur along the Continent's Pacific edge where remnants of the Farallon Plate are subducting beneath the North American Plate or (in Southern California) where the Pacific Plate is sliding past the North American Plate. The North American Plate, itself, is forming along the mid-Atlantic Ridge and is thus creeping westward; the Eastern Coast of the U.S. is a Passive Margin, defined primarily by the level of the sea.
Nevertheless, old fractures, faults and sutures remain beneath that region, having formed when the Appalachians rose or when our Continent formed from smaller fragments of crust. When pressure builds, likely from geologic events along the Pacific Coast, these old joints may slip or rupture, producing an earthquake and its subsequent aftershocks.
