Klaus Jacob, a senior research scientist at the Lamont-Doherty Earth Observatory of Columbia University, explains.
The rapid displacement of a significant volume of ocean water by some external physical process acting either from below at the ocean floor or from above impacting the water surface generates a tsunami. Gravity then provides the restoring force to smooth out the vertical displacement of the ocean surface, causing a wave to propagate away from the source of disturbance.
A variety of events can cause the required vertical displacement of water, including some (but not all) submarine earthquakes; submarine landslides; large calving icebergs; explosive volcanic eruptions in the ocean (or near its coast); slides of land into the ocean; the impact of a meteorite or comet into the ocean (or on land near the coast); even large explosions of ships in harbors can cause local tsunamis.
So why do some submarine earthquakes cause tsunamis but others do not? First, the quakes have to be sufficiently large. Noticeable tsunamis require earthquakes of about magnitude seven or larger and widely-damaging tsunamis usually require earthquake magnitudes of at least eight or greater. But whereas large-magnitude events are necessary, they alone are not sufficient to cause a tsunami. Also essential is that the ocean floor be deformed vertically. This deformation can include faults that intersect the ocean floor and have a vertical component of fault offset. (The fault offset is the distance the rock on one side of the fault slips--or is offset--against the rock on the other side of the fault.) For this reason strike-slip faults (like the San Andreas Fault in California, even where some of its segments run offshore), in which the plates tend to slip horizontally against each other, do not normally generate significant tsunamis. Because water is virtually immune to the horizontal shearing motion of the ocean floor in these regions, little water is displaced. Exceptions occur when a strike-slip earthquake subsequently triggers a large submarine landslide. The latter event can displace water vertically and thus generate a tsunami.
The most notorious tsunamigenic earthquakes occur at subduction zones. During a single large subduction earthquake, one plate (typically an oceanic plate) can slip as much as 20 meters (60 feet) beneath the leading edge of the overriding plate (often a continent or chain of volcanic islands). Preliminary analysis of global seismograms indicate that during the December 26, 2004, great Sumatra-Andaman earthquake the India plate may have indeed slipped past the Burma plate as much as 20 meters at one patch of the fault, but probably somewhat less at other segments. The vertical component of this total inclined slip on the fault that dips to the northeast at an angle of about 10 or 15 degrees is probably on the order of two meters. These very preliminary estimates will surely be refined in future careful studies when all available seismological, tsunamic, geodetic, marine geophysical and geologic, and other data can be jointly analyzed.
A deep oceanic trench usually marks the boundary between the two plates where one plate subducts below the other. In the area near and seaward of the trench, the ocean floor tends to be downthrust during the earthquake, while landward of the trench the leading edge of the overriding plate is raised by several meters. Often the edge of the overriding plate is sliced subvertically into a tilted stack of "imbricated" fault blocks, not unlike playing cards moving against one another inside a tilted deck. All the vertical components of the ocean floor deformation contribute to the tsunami, whether they represent a rise or fall of ocean floor during the quake. Whether the first of a succession of tsunami waves empties out a nearby harbor or coast, or, conversely, floods the coast with a surging wall of water, depends largely on whether the coast faces most closely the down-dropped portion of the ocean floor or, alternatively, the portion that has been uplifted.