Most earthquakes originate along the edge of the tectonic plates where compression, friction or subduction occur. Other common sites are along rift valleys or mountain ranges where buried fault lines are especially numerous.
But, on occasion, earthquakes strike in areas where there is little surface evidence of past or recent tectonic activity. Tremors may strike vast plains, rolling farmlands or flat lake country. In such cases, the quake originates deep below the surface, in the basement rock of the tectonic plate, and usually reflects the presence of an old suture line or aborted rift zone. Since covered by thick layers of sedimentary rock and surface deposits, these deep faults, often quiescent for thousands or millions of years, suddenly shift due to pressure change within the plate.
Almost all of western North America was pieced together by small plates and exotic terrains that were welded to the primary North American Plate. These numerous suture lines remain a common source of earthquakes and, as the Great Basin continues to stretch, the rifting of these old margins will intensify the tectonic activity. Other, more stable regions of the Continents once formed and reformed in a similar fashion and their long-dormant sutures are prone to an occasional rupture.
While the risk for earthquakes varies widely across the globe, no region is immune. We all live in earthquake zones!
