TSUNAMI CLASSIFICATION
Characteristics of the Tsunami Phenomena
A tsunami travels outward from the source region as
a series of waves. Its speed depends upon the depth
of the water, and consequently the waves undergo
accelerations or decelerations in passing respectively
over an ocean bottom of increasing or decreasing
depth. By this process the direction of wave propagation
also changes, and the wave energy can become
focused or defocused. In the deep ocean, tsunami
waves can travel at speeds of 500 to 1,000 kilometers
per hour. Near shore, however, a tsunami slows down
to just a few tens of kilometers per hour. The height
of a tsunami also depends upon the water depth. A
tsunami that is just a meter in height in the deep
ocean can grow to tens of meters at the shoreline.
Unlike familiar wind-driven ocean waves that are
only a disturbance of the sea surface, the tsunami
wave energy extends to the ocean bottom. Near
shore, this energy is concentrated in the vertical direction
by the reduction in water depth, and in the
horizontal direction by a shortening of the wavelength
due to the wave slowing down.
Tsunamis have periods (the time for a single wave
cycle) that may range from just a few minutes to as
much as an hour or exceptionally more. At the shore,
a tsunami can have a wide variety of expressions depending
on the size and period of the waves, the
near-shore bathymetry and shape of the coastline, the
state of the tide, and other factors. In some cases a
tsunami may only induce a relatively benign flooding
of low-lying coastal areas, coming onshore similar to
a rapidly rising tide. In other cases it can come onshore
as a bore - a vertical wall of turbulent water that
can be very destructive. In most cases there is also a
drawdown of sea level preceding crests of the tsunami
waves that results in a receding of the shoreline, sometimes
by a kilometer or more. Strong and unusual ocean
currents may also accompany even small tsunamis.
Destruction from tsunamis is the direct result of three factors:
inundation, wave impact on structures, and erosion.
Strong tsunami-induced currents have led to the erosion
of foundations and the collapse of bridges and seawalls.
Flotation and drag forces have moved houses and overturned
railroad cars. Tsunami associated wave forces
have demolished frame buildings and other structures.
Considerable damage also is caused by the resultant
floating debris, including boats and cars that become dangerous
projectiles that may crash into buildings, piers,
and other vehicles. Ships and port facilities have been
damaged by surge action caused by even weak tsunamis.
Fires resulting from oil spills or combustion from damaged
ships in port, or from ruptured coastal oil storage
and refinery facilities, can cause damage greater than that
inflicted directly by the tsunami. Other secondary damage
can result from sewage and chemical pollution following
the destruction. Damage of intake, discharge, and
storage facilities also can present dangerous problems. Of
increasing concern is the potential effect of tsunami
drawdown, when receding waters uncover cooling water
intakes associated with nuclear plants.
air-coupled tsunami Synonym for atmospheric tsunami.
atmospheric tsunami Tsunami-like waves generated by
a rapidly moving atmospheric pressure front moving
over a shallow sea at about the same speed as the
waves, allowing them to couple.
internal tsunami Tsunami wave manifested as an internal
wave and traveling along a thermocline.
local tsunami A tsunami which its destructive effects
are confined to coasts within a hundred km, of the
source, usually an earthquake and sometimes a landslide.
microtsunami A tsunami of such small amplitude that it
must be observed instrumentally and is not easily detected
visually.
near-field or local tsunami A tsunami from a nearby
source, generally less than 200 km away. A local tsunami
is generated by a small earthquake, a landslide
or a pyroclastic flow.
Numerical Modeling: snapshots of the water surfaces 10 minutes after
the initiation of the submarine landslide of the pyroclastic flow
(on the South-East part of Monserrat Island)
Pacific-wide tsunami A tsunami capable of widespread
destruction, not only in the immediate region of its generation, but across the entire Pacific Ocean
paleotsunami Research on paleotsunamis, events occurring
prior to the historical record, has recently been
taking place in a few regions around the Pacific. This
work is based primarily on the collection and analysis
of tsunami deposits found in coastal areas, and other
evidence related to the uplift or subsidence associated
with nearby earthquakes. In one instance, the research
has led to a new concern for the possible future occurrence
of great earthquakes and tsunamis along the
northwest coast of North America. In another instance,
the record of tsunamis in the Kuril-Kamchatka region
is being extended much further back in time. As work
in this field continues it may provide a significant
amount of new information about past tsunamis to aid
in the assessment of the tsunami hazard.
regional tsunami A tsunami capable of destruction in a
particular geographic region, generally within about
1000 km of its source. Regional tsunamis also occasionally
have very limited and localized effects outside
the region.
Most destructive tsunami can be classified as local or
regional, meaning their destructive effects are confined
to coasts within a hundred km, or up to a thousand
km, respectively, of the source -- usually an
earthquake. It follows that the majority of tsunami related
casualties and property damage also come from
local tsunami. Between 1975 and 1998 there have
been at least eighteen in the Pacific and its adjacent
seas resulting in significant casualties and/or property
damage.
Posted by Aneka Tips
on 17.36. Filed under