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.
Moderate earthquakes can sometimes launch huge tsunamis. An example is the 1946 earthquake that occurred off Unimak Island in the eastern Aleutian Island arc, just west of the tip of the Alaskan Peninsula. The earthquake's magnitude was "only" somewhere between 7.3 and 7.8, but a large submarine block slide triggered by the earthquake set off a Pacific-wide tsunami. A subduction quake acting alone to generate waves of comparable size would have needed to have a magnitude of nearly nine.
When in 1883 the island volcano Krakatau was blasted into pieces in the Straits of Sunda, Indonesia, a gigantic volcanic caldera formed on the seafloor, while the volcanic debris thrown into the air came down onto the surface of the sea surrounding the former island. This double motion from below and above set off a tsunami crashing onto the shores across the entire Indian Ocean, in many places substantially exceeding the tsunami heights from the December 26, 2004, earthquake off northern Sumatra. Volcano- or earthquake-induced slides of portions of the Hawaiian Islands into the Pacific, or from the Canary Islands into the Atlantic are feared to put at risk most of the coasts in and across each of these two ocean basins. While such events have low probability, they could have catastrophic consequences.
Humanity has not experienced a major global tsunami from a meteorite impact into an ocean. But probably one of the most extreme tsunamis in the geological history of the earth was caused by the impact of a meteorite near what is now known as the Chicxulub structure, on the Yucatan Peninsula in Mexico. This event occurred some 65 million years ago and caused the extinction of many species, including the dinosaurs, most likely from a global drop of temperatures due to volcanic debris and dust in the atmosphere shielding the earth's surface from the sun's energy. But the meteor impact and associated ballistic debris fallout also caused a global tsunami estimated to be on the order of 1,000 feet high. Evidence of this great wave continues to be found at the coasts of many continents, including the U.S. states facing the Gulf of Mexico.
A very local, but still deadly tsunami occurred on Dec 12, 1917, in the harbor of Halifax, Nova Scotia. The explosion of a World War I-ammunition-laden freighter in the harbor set off a local tsunami, causing a 60-foot high wall of water to rush into town.
When enclosed water bodies such as lakes, reservoirs or fjords are subjected to sudden displacements of their waters by rock or landslides, they can also cause deadly and often spectacular upward surges of water on the opposing shorelines. The declared record in historic time is held by a rockslide in Lituya Bay in southeast Alaska, triggered by the 1958 Fairweather Fault strike-slip earthquake with an approximate magnitude of eight. A huge rockslide had gained high speed as it descended into the narrow fjordlike Lituya Bay, which made the displaced water run up on the opposing rock wall of the fjord to heights of 1,700 feet (520 m). The surging water stripped millions of forest trees and all soil from the entire rock wall up to this height.