An earthquake occurs when pressure is released along a fault; this pressure builds up due to friction between two plates, whether related to direct compression, sideways scraping (e.g. the San Andreas Fault) or one plate dipping beneath the other (subduction). The sudden release of pressure and the resulting plate slippage transfers pressure to other points along that fault and to vulnerable sites on other intersecting faults.
In some locations this transferred pressure is of no immediate consequence while, in others, the additional pressure exceeds the tolerance of the plate friction, producing other earthquakes; these secondary quakes are commonly referred to as aftershocks. Following a major earthquake, there are always hundreds if not thousands of such aftershocks, many of which are too faint to be noticed; furthermore, each secondary quake produces its own aftershocks. While these subsequent events are less powerful than the inciting quake, they can be strong enough to produce major damage (especially in light of already weakened structures) and, if they occur along subduction faults, may trigger secondary tsunamis.
A significant concern in Japan is that the recent massive earthquake and its numerous aftershocks may have transferred a critical pressure load to vulnerable sites along the subduction zone that parallels that island nation; this could hasten the next rupture, sending a tsunami across major urban centers. Such an event could occur in another century or another week.
