The text of the document was based on :
http://www.tsunami.noaa.gov/tsunami_story.html
The word tsunami is a Japanese word,
represented by two characters: tsu,
meaning, "harbor", and nami meaning,
"wave".
What is a Tsunami? Tsunami is a set of ocean waves caused by an impulsive disturbance in a body of
water such as a lake or ocean. If the disturbance is close to the coastline, local tsunamis would reach
coastal communities within a few minutes. A very large disturbance can cause local devastation AND
export tsunami destruction thousands of miles away.
What creates or generates a Tsunami? Tsunamis are most commonly generated by under-water fault
displacements triggered by subduction or thrust fault earthquakes. Major tsunamis are produced by
large (greater than 7 on the Richer scale), shallow focus (< 30km depth in the earth crust) earthquakes
associated with the movement of oceanic and continental plates. They most often occur in the Pacific,
where dense oceanic plates slide under the lighter continental plates. When these plates fracture they
provide a vertical movement of the seafloor that allows a quick and efficient transfer of energy from the
solid earth to the ocean.
Is there a difference between an wind-generated wave and a Tsunami: YES. Tsunamis are very long
from crest to crest (hundreds of meters) and cause deep penetration of flooding, while the large ocean
waves can exceed 25 meters from crest to trough but cause only limited local flooding, unlike the
massive and wide spread flooding from a tsunami.
Effects of Tsunamis: Tsunamis rank high on the scale of natural disasters. According to the NOAA
website about Tsunamis, since 1850 alone, tsunamis have been responsible for the loss of over 420,000
lives and billions of dollars of damage to coastal structures and habitats. Most of these casualties were
caused by local tsunamis that occur about once per year somewhere in the world. For example, the
December 26, 2004, tsunami killed about 130,000 people close to the earthquake and about 58,000
people on distant shores. Predicting when and where the next tsunami will strike is currently impossible.
Once the tsunami is generated, forecasting tsunami arrival and impact is possible through modeling and
measurement technologies. When a powerful earthquake (magnitude 9.3) struck the coastal region of
Indonesia in 2004, the movement of the seafloor produced a tsunami in excess of 30 meters (100 feet)
along the adjacent coastline killing more than 240,000 people. From this source the tsunami radiated
outward and within 2 hours had claimed 58,000 lives in Thailand, Sri Lanka, and India.
Are there other ways to generate a tsunami? Underwater landslides associated with smaller
earthquakes are also capable of generating destructive tsunamis. The tsunami that devastated the
northwestern coast of Papua New Guinea on July 17, 1998, was generated by an earthquake that
registered 7.0 on the Richter scale that might have triggered a large underwater landslide. Three waves
measuring more than 7 meter high struck a 10-kilometer stretch of coastline within ten minutes of the
earthquake/slump. Three coastal villages were swept completely clean by the deadly attack leaving
nothing but sand and more than 2,000 people dead. Other large-scale disturbances of the sea -surface
that can generate tsunamis are explosive volcanoes and slumping glaciers, and very rarely a meteorite
impact.
Warning Systems: After an earthquake scientist from around the world review data from seismograph
and DART® (Deep-ocean Assessment and Reporting of Tsunamis) data. Using this data Tsunami warning
are issued for at risk coastlines. Due to the variability of the tectonic plates and earth quakes this is not
an exact science. Forecasts are often conservative, causing people to evacuate when the tsunami runup is very low.
What is Tsunami Engineering: Tsunami Engineering is a subset of Civil and Coastal Engineering focused
on helping coastal communities be resilient to Tsunamis. In the US Tsunamis are rare and destructive
events. The goal of creating tsunami resilient communities is to provide life safety by creating
evacuation routes and structures to help people evacuate.
Vocabulary:
Inundation, or inundation distance—The horizontal distance inland that a tsunami penetrates, generally
measured perpendicularly to the shoreline.*
Runup or Run-up—
1. Difference between the elevation of maximum tsunami penetration (inundation line)
and the sea level at the time of the tsunami. In practical terms, runup is only measured
where there is a clear evidence of the inundation limit on the shore.*
2. Elevation reached by seawater measured relative to some stated datum such as mean
sea level, mean low water, sea level at the time of the tsunami attack, etc., and
measured ideally at a point that is a local maximum of the horizontal inundation. Where
the elevation is not measured at the maximum of horizontal inundation this is often
referred to as the inundation-height.*
Run-up elevation—The elevation above sea level of a tsunami at the limit of penetration
Tsunami hazard, Tsunami hazard assessment—
Documentation of tsunami hazards for a coastal community is needed to identify populations and assets
at risk, and the level of that risk. This assessment requires knowledge of probable tsunami sources (such
as earthquakes, landslides, and volcanic eruptions), their likelihood of occurrence, and the
characteristics of tsunamis from those sources at different places along the coast. For those
communities, data of earlier (historical and paleotsunamis) tsunamis may help quantify these factors.
For most communities, however, only very limited or no past data exist. For these coasts, numerical
models of tsunami inundation can provide estimates of areas that will be flooded in the event of a local
or distant tsunamigenic earthquake or a local landslide.*
Tsunami propagation—Tsunamis travel outward in all directions from the generating area, with the
direction of the main energy propagation generally being orthogonal to the direction of the earthquake
fracture zone. Their speed depends on the depth of water, so that the waves undergo accelerations and
decelerations in passing over an ocean bottom of varying depth. In the deep and open ocean, they travel
at speeds of 500 to 1,000 km per hour (300 to 600 miles per hour). The distance between successive
crests can be as much as 500 to 650 km (300 to 400 miles). However, in the open ocean, the height of
the waves is generally less than a meter (three feet) even for the most destructive teletsunamis, and the
waves pass unnoticed. Variations in tsunami propagation result when the propagation impulse is
stronger in one direction than in others because of the orientation or dimensions of the generating area
and where regional bathymetric and topographic features modify both the waveform and rate of
advance. Specifically, tsunami waves undergo a process of wave refraction and diffraction throughout
their travel, and are much more reflective from the land than wind-generated waves. Tsunamis are
unique in that the energy extends through the entire water column from sea surface to the ocean
bottom. It is this characteristic that accounts for the great amount of energy propagated by a tsunami.*