Since the massive earthquake off northwestern Sumatra triggered a deadly tsunami in December, 2004, a series of 20 or more major quakes have occured in the region, accompanied by hundreds of aftershocks. As one might expect, these seismic events are interrelated.
The islands of Sumatra and Java form the western side of the Indonesian Archipelago and sit atop the western edge of a southern extension of the Eurasian Plate. Just off their western coasts, the Australian Plate is sliding beneath the Eurasian Plate; in such subduction zones, tremendous friction develops between the Plates. As the pressure builds, the edge of the upper plate (in this case the Eurasian) is warped downward; eventually, the warped edge will rebound upward, break off or induce a fracture in the lower plate. Any of these scenarios will produce an earthquake and, depending upon the size and depth of the rupture, a tsunami.
While an earthquake follows a release of pressure at one point along the subduction joint, it also portends a buildup of pressure further down the line. This series of transmitted pressure, release events and associated earthquakes is what we have seen along Indonesia over the past three years. Though the timing of the next major quake remains uncertain, it will occur and another tragic tsunami could result.
