Since we tend to associate earthquakes with active plate margins, as occur along the Pacific Rim, in southern Asia and in the Mediterranean, many Americans along the Eastern Seaboard and Midwest were surprised by today's quake, centered in Virginia. But the truth is, there are few places on Earth (if any) that are immune to earthquakes.
Indeed, both New England and the Southeastern Piedmont are, geologically speaking, a puzzle of exotic terrains, plastered to the "stable" craton during the Precambrian and early Paleozoic Eras, hundreds of millions of years ago; for that matter, the craton itself has old suture lines from the earliest days of continent formation. Though relatively quiescent when compared to the active faults of the Pacific Rim, these cemented pieces remain under pressure as the Atlantic continues to open and both subduction and lateral compression occur along the West Coast. Minor readjustments go unnoticed but more powerful slips, especially when relatively shallow, get our attention. In addition, these old continental fragments, welded together for eons, conduct pressure waves better than the relatively loose contact zones out West and, as a result, are felt across a broader area; indeed, today's quake was felt from South Carolina to Toronto.
While quakes from old suture lines are more likely to be isolated and, in general, less powerful, aftershocks may occur as pressure is transferred down the fault line. Planning for and living with earthquakes is not just a problem for our neighbors in California and Alaska as today's quake (and yesterday's in Colorado) remind us.
