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All Islands Health Talk About Tsunamis in Hawaii

About Tsunamis in Hawaii

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About Tsunamis in Hawaii

"Tsunami" is the Japanese term meaning wave in the harbor, and is the common word for tidal wave in Hawaii.

About 50 tidal waves - also called tsunamis - have been documented in the Hawaiian Islands since the early 1800's. Two out of Seven of the major tsunmai were caused by local seismic activity.

In 1946 and 1960 the most destructive and deadly tsunami struck the northeast coast of the big island.

1946 Big Island Tidal Wave

Deadly April fools Day 1946. On April 1st 170 people died when a tidal wave originated in the Aleutian islands and struck Hawaii without warning. Near Hilo and Laupahoehoe it reached an average height of 30 feet. The maximum height of the 1946 tidal wave was 55 feet in the Pololu Valley.

There is a memorial monument in Laupahoehoe Point Park where wenty-three students and four teachers died after the school children went out on the beach collecting fish deposited by the first two waves when the third and deadly 50 foot wave struck, dragging them all out to sea.

1960 Big Island Tidal Wave

An earthquake near Chile created a tidal wave as high as a two-story building when it struck the Big Island of Hawaii on May 23, 1960.

Sixty-one people were killed and the damages were estimated at $50 million USD.

In South America, the term "maremoto" is frequently used. However the use of the word "tsunami" is most commonly accepted by scientists and by most of the lay public in Pacific basin countries.

Why prepare for tsunamis?

All tsunamis are potentially dangerous. Twenty-four tsunamis have caused damage in the United States and its territories in the past 200 years. Since 1946, six tsunamis have killed more than 350 people and caused significant property damage in Hawaii, Alaska, and along the West Coast. Tsunamis have also occurred in Puerto Rico and the Virgin Islands.

Hawaii was the first US State to be declared "Tsunami Ready" in 2005.

Unlike hurricane season, a tsunami can occur during any season of the year and at any time, day or night.

When a tsunami comes ashore, it can can travel upstream in coastal estuaries and rivers, and valleys, causing destruction by sending waves and debris inland beyond the immediate coastline

How can I protect myself from a tsunami?

Know the Tsunami history of your area. If you are in a coastal community and feel the shaking of a strong earthquake, you may have only minutes until a tsunami arrives. Do not wait for an official warning. Instead, use common sense and don't panic. Protect yourself from falling objects, quickly move away from the water and to higher ground. If the surrounding area is flat, move inland and up. Once away from the water, listen to a local radio or television station or NOAA Weather Radio for information from the Tsunami Warning Centers about further action you should take.

Even if you do not feel shaking, if you learn that an area has experienced a large earthquake that could send a tsunami in your direction, listen to a local radio or television station or NOAA Weather Radio for information from the Tsunami Warning Centers about action you should take. Depending on the location of the earthquake, you may have a number of hours in which to take appropriate action.

What is the best source of information in a tsunami situation? All Hawaii coastlines have Civil Defense Sirens which are tested each month. Turn on your radio or television to any station when the siren is sounded and listen for emergency information and instructions. Maps of tsunami-inundation areas and evacuation routes can be found in the front of local telephone books in the Disaster Preparedness Info section.

As part of an international cooperative effort to save lives and protect property, the National Oceanic and Atmospheric Administration’s National Weather Service operates two tsunami warning centers: the West Coast/Alaska Tsunami Warning Center (WC/ATWC) in Palmer, Alaska, and the Pacific Tsunami Warning Center (PTWC) in Ewa Beach, Hawaii. The WC/ATWC serves as the regional Tsunami Warning Center for Alaska, British Columbia, Washington, Oregon, and California. The PTWC serves as the regional Tsunami Warning Center for Hawaii and as a national/international warning center for tsunamis that pose a Pacific-wide threat.

Tsunami warnings are broadcast on local radio and television stations and on NOAA Weather Radio. NOAA Weather Radio is the prime alerting and critical information delivery system of the National Weather Service (NWS). NOAA Weather Radio broadcasts warnings, watches, forecasts, and other hazard information 24 hours a day on more than 650 stations in the 50 states, adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands, and the U.S. Pacific territories.

The NWS encourages people to buy a weather radio equipped with the Specific Area Message Encoder (SAME) feature. This feature automatically alerts you when important information is issued about tsunamis or weather-related hazards for your area. Information on NOAA Weather Radio is available from your local NWS office or online.

Carry the radio with you when you go to the beach and keep fresh batteries in it.

For the TWS, tsunamis can be categorized as local, regional, or Pacific-wide, with those terms being used to describe the extent of potential destruction relative to the tsunami source area. Local tsunamis will often be associated with tsunami generation by submarine or subaerial landslides or volcanic explosions.

An example would be the awesome local tsunami of July 9, 1958, at Lituya Bay, Alaska, where wave run-up exceeded 485 meters but the destruction was confined to a very limited area.

Regional tsunamis are by far the most common. Destruction may be limited in areal extent either because the energy released was not sufficient to generate a destructive Pacific-wide tsunami, or because the geomorphology of the source area limited the destructive potential of the tsunami.

Pacific-wide tsunamis are much less frequent, but of far greater destructive potential in that waves are not only larger initially, but in transit across the Pacific basin, many distant coastal areas are subject to destructive impact. For example, the tsunami of May 22, 1960, spread death and destruction across the Pacific from Chile to Hawai`i, Japan, and the Philippines.

A tsunami is a system of gravity waves formed in the sea as a result of a large-scale disturbance of sea level over a short duration of time. In the process of sea level returning to equilibrium through a series of oscillations, waves are generated which propagate outward from the source region.

A tsunami can be generated by submarine volcanic eruptions, by displacement of submarine sediments, by coastal landslides into a bay or harbor, by meteor impact, or by vertical displacement of the earth's crust along a zone of fracture which underlies or borders the ocean floor.

The latter is by far the most frequent cause of tsunamis and for all practical purposes the primary cause of tsunamis capable of propagation across an ocean basin.

The rupture of the earth's crust will also generate a major earthquake which can be detected and measured by seismic instrumentation throughout the world. However, not all major coastal or near-coastal earthquakes produce tsunamis. At present, there is no operational method to determine if a tsunami has been generated except to note the occurrence and epicenter of the earthquake and then detect the arrival of the characteristic waves at a network of tide stations.

When a major earthquake occurs, the resultant energy released into the earth will propagate over a wide range of frequencies and velocities. Even though the earth movements discernible to the viewer may be confined to the general region of the earthquake origin, the various seismic wave phases propagating throughout the earth result in small, but measurable, ground motion which can be detected by a seismometer. A seismograph then provides a visual record of the ground motion at that station.

For the purposes of the Tsunami Warning System, consideration is given to three significant seismic wave phases. The first, the P-wave, is a compressional wave traveling through the earth's interior at a velocity varying from approximately 8.0 km/second near the crust-mantle interface to about 13.5 km/second at the mantle-core interface. As such it is the first seismic phase to be recorded at any one seismic station and is the first indication that a distant earthquake has occurred. The location of the earthquake can be determined by assuming the "best fit" of the pattern of P-wave arrivals at several stations compared to a standard table of P-wave arrival times for various distances and depths of earthquake focus or, in the case of local earthquakes in or near the limits of a relatively small area seismic station network, compared to the calculated arrivals based on a local crustal seismic velocity model.

The second seismic phase of importance is the S-wave, or Secondary wave. This phase travels through the earth's interior as a shear wave, following approximately the same travel path as the P-wave but at a reduced velocity varying from approximately 6.7 km/second near the crust-mantle interface to about 8.0 km/second near the core. These seismic wave phases are classified as body waves due to their propagation through the earth's interior. In addition to providing a location, body waves are useful in determining the size of an earthquake, especially when the eathquake's focus is deep within the earth.

The third set of seismic phases to be considered are the surface waves resulting from ground displacements propagating outward along the surface of the earth. These are observed at a seismic station as local or regional surface waves and are the basis for measuring magnitude on the Richter scale. This is a logarithmic scale devised by Charles Richter to use the amplitude of the trace recorded on a seismograph and the distance from the epicenter to assign a somewhat consistent indication of size to a particular earthquake as measured at different stations. Beno Gutenberg extended the Richter scale to include distant Love and Raleigh surface waves. Though it is a logarithmic scale to the base 10, this is merely a reference to the Richter scale value being incremented as a logarithmic function of the trace deflection as recorded on the seismograph and the distance of the station from the epicenter. The actual energy released for each increment of the Richter scale is a factor of 32. Thus a magnitude 7.0 earthquake will release 32 times as much energy as a magnitude 6.0, and the energy release for a magnitude 8.0 is more than 1000 times greater than a 6.0.

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 kilometers per hour (300 to 600 miles per hour). The distance between successive crests can be as much as 500 to 650 kilometers (300 to 400 miles); however, in the open ocean, the height of the waves may be no more than 30 to 60 centimeters (1 or 2 feet), 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 topographic features modify both the wave form and rate of advance. The tsunamis are waveform 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 transmitted by a tsunami.

The successive waves of a tsunami in the deep sea have such great length and so little height they are not visually recognizable from a surface vessel or from an airplane. The passing waves produce only a gentle rise and fall of the sea surface. During the April 1946 tsunami at Hawai`i, ships standing off the coasts observed tremendous waves breaking on shore but did not detect any change in sea level at their offshore locations.

Upon reaching shallower water, the speed of the advancing wave diminishes, its wave length decreases, and its height may increase greatly, owing to the piling up of water. Configuration of the coastline, shape of the ocean floor, and character of the advancing waves play an important role in the destruction wrought by tsunamis along any coast, whether near the generating area or thousands of kilometers from it. Consequently, detection of relatively small tsunamis at any locality warrants immediate reporting -- through the TWS -- to spread the alarm to all coastal localities of approaching potentially dangerous waves.

At present, detection of tsunamis is possible only near shore where the shoaling effect can be observed. The first visible indication of an approaching tsunami is often a recession of water caused by the trough preceding an advancing wave. Any withdrawal of the sea, therefore, should be considered a warning of an approaching wave. A rise in water level also may be the first event. Tide-gauge records of the Chilean tsunami of May 22, 1960, generally showed a rise in water level as the first indication of this tsunami. This rise amounted to about one-half the amplitude of the following decrease in water level. Under certain conditions, the crest of an advancing wave can overtake the preceding trough while some distance offshore. This causes the wave to proceed shoreward as a bore -- a wave with a churning front.

The force and destructive effects of tsunamis should not be underestimated. At some places, the advancing turbulent front is the most destructive part of the wave. Where the rise is quiet, the outflow of water to the sea between crests may be rapid and destructive, sweeping all before it and undermining roads, buildings, and other works of man with its swift currents. Ships, unless moved away from shore, can be thrown against breakwaters, wharves, and other craft, or washed ashore and left grounded during withdrawals of the sea.

In the shallow waters of bays and harbors, a tsunami frequently will initiate seiching. If the tsunami period is related closely to that of the bay, the seiche is amplified by the succeeding waves. Under these circumstances, maximum wave activity often is observed much later than the arrival of the first wave.

A tsunami is not one wave, but a series of waves. The time that elapses between passage of successive wave crests at a given point usually is from 10 to 45 minutes. Oscillations of destructive proportions may continue for several hours, and several days may pass before the sea returns to its normal state.

During the 101-year period from 1900 to 2001, 796 tsunamis were observed or recorded in the Pacific Ocean according to the Tsunami Laboritory in Novosibirsk. 117 caused casualties and damage most near the source only; at least nine caused widespread destruction throughout the Pacific. The greatest number of tsunamis during any 1 year was 19 in 1938, but all were minor and caused no damage. There was no single year of the period that was free of tsunamis.

17 percent of the total tsunamis were generated in or near Japan. The distribution of tsunami generation in other areas is as follows: South America, 15 percent: New Guinea Solomon Islands, 13 percent; Indonesia, 11 percent: Kuril Islands and Kamchatka, 10 percent; Mexico and Central America, 10 percent; Philippines, 9 percent; New Zealand and Tonga, 7 percent; Alaska and West Coasts of Canada and the United States, 7 percent; and Hawai`i, 3 percent.

 

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