Generation of a Tsunami

Unlike normal ocean waves where energy dissipates with increasing depths, a tsunami consists of a series of waves that move through the entire water column. Reverse faults are the primary cause of the largest tsunamis.

Reverse faults are where two plates collide and one plate is lifted over the other plate. While these plates are generally in constant motion it is at the junction of their boundaries where the plates are locked together and the stress between them begins to build (#1 on image below).

As the two plates remain locked together stress continues to build at their intersection. As a result, the sea floor is pulled down (#2) with a corresponding rise on the continental side of the fault (#3).

Increasing stress between the two locked plates further deepens the sea floor (#4) and enhances the bulge on the continental side of the fault (#5).

At some point, the stress between the two plates becomes too much and the earth between the two plates move releasing the pressure (#6). This is felt as an earthquake.

As the earth moves it lifts the entire column of water above the fault (#7). Conversely, the bulge on the continental side of the fault subsides (#8) causing that column of water to sink as well (#9).

It is this sinking water, moving ahead of the bulge of lifted water, that is responsible for the often observed 'pulling away' of water from the normal shoreline as the tsunami approaches.

This occurrence often lulls the unsuspecting person to venture far seaward (to observe coral formations or hunt for souvenirs) and into grave danger as they are not able to escape when the crest of the tsunami approaches.

As the water seeks to return to a common level waves of rising and lowering water heights through the entire water column (the tsunami) radiate out from source of the earthquake. The depth of water directly effects tsunami waves as they move faster and have much longer wave lengths and periods in deep water (#10).

As the tsunami moves into more shallow water the speed of the tsunami diminishes and height of the waves grow (#11). When a tsunami finally reaches shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a 'bore' which a step-like wave with a steep breaking front. Also, the first wave may not be the largest in the series of waves.

Reefs, bays and the direction they face, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. One coastal area may see little or no damaging wave activity while in other areas destructive waves can be large and violent.

As such, travel/arrival times can be computed with water depth as the only limiting factor without any knowledge of the tsunami's height or wavelength.

Since the tsunami's velocity is directly related to the water's depth, waves move the fastest in deep water and decrease in velocity when the wave moves into shallow water.

As the waves moving into shallow water, the time between the crests (called 'period' of the wave) decreases yet the energy does not decrease. With no change in energy the height of the wave therefore increases.

The end result is a tsunami wave, unnoticed in the deep ocean, increasing in height up to 100 feet (30 meters) as it arrive onshore.

Because there is no change in the tsunami wave's period there is an additional threat with these events. One can be lulled into thinking a tsunami event is over after the first wave passes.

Yet there may be several more waves offshore and can be up to two hours between the next tsunami wave arrives. Therefore, if you are caught in a tsunami you need to wait several hours for ALL waves to pass.