|
|
Panel
1—Initiation:
Earthquakes are commonly associated with ground shaking that is a result of
elastic waves traveling through the solid earth. However, near the source
of submarine earthquakes, the seafloor is "permanently" uplifted
and down-dropped, pushing the entire water column up and down.
|
|
The
potential energy that results from pushing water above mean sea level is
then transferred to horizontal propagation of the tsunami wave (kinetic
energy). For the case shown above, the earthquake rupture occurred at the base
of the continental slope in relatively deep water. Situations can also
arise where the earthquake rupture occurs beneath the continental shelf in
much shallower water.
Note:
In the figure, the waves are greatly exaggerated compared to water depth! In
the open ocean, the waves are at most several meters high spread over many
tens to hundreds of kilometers in length.
|
|
|
Panel
2—Split:
Within several minutes of the earthquake, the initial tsunami (Panel 1) is
split into a tsunami that travels out to the deep ocean (distant tsunami)
and another tsunami that travels towards the nearby coast (local tsunami).
|
|
The
height above mean sea level of the two oppositely traveling tsunamis is
approximately half that of the original tsunami (Panel 1). (This is
somewhat modified in three dimensions, but the same idea holds.) The speed
at which both tsunamis travel varies as the square root of the water depth.
Therefore the deep-ocean tsunami travels faster than the local tsunami near
shore.
|
|
|
Panel
3—Amplification:
Several things happen as the local tsunami travels over the continental
slope. Most obvious is that the amplitude increases. In addition,
the wavelength decreases. This results in steepening of the leading
wave--an important control of wave runup at the coast (next panel).
|
|
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: As
the tsunami wave travels from the deep-water, continental slope region to
the near-shore region, tsunami runup occurs. Runup is a measurement of the
height of the water onshore observed above a reference sea level.
|
|
Contrary
to many artistic images of tsunamis, most tsunamis do not result in giant
breaking waves (like normal surf waves at the beach that curl over as they
approach shore). Rather, they come in much like very strong and very fast
tides (i.e., a rapid, local rise in sea level). Much of the damage
inflicted by tsunamis is caused by strong currents and floating debris. The
small number of tsunamis that do break often form vertical walls of turbulent
water called bores. Tsunamis will often travel much farther inland than
normal waves.
|
Do tsunamis stop once
on land? After runup, part of the tsunami energy is reflected back to the
open ocean. In addition, a tsunami can generate a particular type of wave
called edge waves that travel back-and forth, parallel to shore. These
effects result in many arrivals of the tsunami at a particular point on the
coast rather than a single wave suggested by Panel 3. Because of the
complicated behavior of tsunami waves near the coast, the first runup of a
tsunami is often not the largest, emphasizing the importance of not returning
to a beach several hours after a tsunami hits. For more information on
tsunami preparedness, see tsunami links.
|
