Life of a Tsunami

Life of a Tsunami
How are tsunamis formed...

Panel 1—Initiation: Earthquakes are typically linked to ground shaking caused by elastic waves moving through the solid earth.

However, in the vicinity of submarine earthquakes, the seafloor experiences a "permanent" uplift and down-drop, which causes the entire water column to oscillate vertically. The potential energy generated from elevating water above the mean sea level is subsequently converted into the horizontal movement of the tsunami wave (kinetic energy). In the example illustrated above, the earthquake rupture took place at the base of the continental slope in relatively deep waters. There are also instances where the earthquake rupture occurs beneath the continental shelf in significantly shallower waters.

Note: In the accompanying figure, the waves are significantly exaggerated in comparison to the water depth. In the open ocean, the waves typically reach a height of only a few meters, extending over distances of many tens to hundreds of kilometers.

Panel 2—Split: Within a few minutes following the earthquake, the initial tsunami (Panel 1) divides into two distinct tsunamis: one that moves out into the deep ocean (distant tsunami) and another that heads towards the nearby coastline (local tsunami). The height above mean sea level of these two oppositely directed tsunamis is roughly half that of the original tsunami (Panel 1). (This is somewhat altered in three dimensions, but the fundamental concept remains.) The velocity at which both tsunamis propagate is proportional to the square root of the water depth. Consequently, the deep-ocean tsunami travels at a faster rate than the local tsunami near the shore.

Panel 3—Amplification: As the local tsunami progresses over the continental slope, several phenomena occur. The most apparent is the increase in amplitude. Additionally, the wavelength diminishes. This leads to the steepening of the leading wave—an essential factor influencing wave runup at the coast (as shown in the next panel). It is important to note that the initial part of the wave that reaches the local shore is a trough, which will manifest as the sea receding significantly from the shore. This serves as a common natural warning sign for tsunamis.

Note also that the deep ocean tsunami has traveled much farther than the local tsunami because of the higher propagation speed. As the deep ocean tsunami approaches a distant shore, amplification and shortening of the wave will occur, just as with the local tsunami shown above.

Panel 4—Runup: Tsunami runup takes place when a peak of the tsunami wave moves from the near-shore area onto the land. Runup serves as a measurement of the height of the water observed onshore above a reference sea level.

With the exception of the most significant tsunamis, such as the 2004 Indian Ocean event, the majority of tsunamis do not produce massive breaking waves (unlike typical surf waves at the beach that curl as they approach the shore). Instead, they arrive similarly to very strong and rapidly moving tides (i.e., powerful surges and swift fluctuations in sea level). A considerable portion of the destruction caused by tsunamis is attributed to strong currents and floating debris.

The limited number of tsunamis that do break often create vertical walls of turbulent water known as bores. Tsunamis frequently travel much further inland than standard waves.

Do tsunamis cease once they reach land? 

No! Following runup, a portion of the tsunami's energy is reflected back into the open ocean and dispersed by abrupt changes in the coastline. Furthermore, a tsunami can produce a specific type of coastal trapped wave referred to as edge waves, which move back and forth parallel to the shore. These phenomena lead to multiple arrivals of the tsunami at a specific location along the coast, rather than a single wave as indicated by Panel 3. Due to the complex behavior of tsunami waves near the coastline, the first runup of a tsunami is often not the most significant, highlighting the necessity of avoiding a return to the beach many hours after the initial impact of a tsunami. How are tsunamis generated..