Tsunami Destructive in Nature

TSUNAMI

What generates tsunami?

Tsunami(s), also called seismic sea waves, are a series of waves generated by large, violent earthquakes occuring near the ocean. Not all earthquakes generate tsunamis.By far, the most destructive tsunamis are generated from large shallow earthquakes with an epicenter or fault line near or on the ocean floor. Vertical displacements of the earth's crust along the rupture, resulting from such earthquakes, can generate destructive tsunami waves which can travel across an ocean spreading destruction across their path. Similar displacements of the ocean floor can also be produced by volcanic eruptions and submarine avalanches, or submarine landslides. However, these sources are considered as point sources and , although destructive locally, the energy of the waves is rapidly dissipated as they travel across the ocean. The wave crests of a tsunami may be a hundred kilometers or more apart as they travel across the ocean. The height from trough to crest may be only a few centimeters or meters in the open ocean. A tsunami cannot be felt aboard ships in deep water.

Why are tsunami so destructive?

As the tsunami enters the shoaling water near the coast, its velocity decreases and its height increases. It is in these shallow waters that tsunamis become a threat to life and property, for they can crest to heights of of more than 30-50 meters and strike with devastating force.Finally, terminal height or run-up of the tsunami at the point of impact will depend on how the energy is focused, the travel path of the waves, the coastal configuration, and the offshore topography. Tsunami run-up is the vertical distance between the maximum height reached by the water on shore and the mean sea level surface. Tsunamis are among the most terrifying natural hazards known to man. They have been responsible for tremendous loss of life and property throughout history.

The May 1960 Chilean Tsunami?

The largest earthquake in recorded times in the southern hemisphere occurred on May 22, 1960 along the Chile Trench, an area of crustal subduction known for large historic earthquakes and tsunamis. The greatest damage was caused by the tsunami waves which were destructive, not only locally in Chile, but throughout the Pacific. There was tremendous loss of life and property in Hawaii, Japan and elsewhere in the Pacific. (Aerial Photo of the May 1960 tsunami damage at Isla Chiloe, Chile)

What was so devastating about the Great Alaskan Earthquake of 1964?

The Great Alaskan Earthquake of March 28, 1964 was the largest earthquake to hit the northern hemisphere in recorded history. Its magnitude was 8.3. It affected an area that was almost one thousand miles long and more than two hundred miles wide. It affected a great part of Alaska from Valdez,the entire Prince William Sound, Kenai peninsula, Kodiak Island all the way to the Trinity Islands. The earthquake itself caused areas to be lifted by as much as 30 feet while many other areas subsided greatly. There was tremendous destruction from the earthquake throughout the populated regions of Alaska. The eartquake generated a great tsunami which was extremely damaging, not only in Alaska, but along Vancouver Island, and in Northern California and Hawaii.. Hardest hit was Crescent City, California, where eleven persons lost their lives.

The tsunami waves affected the entire California coastline, but were particularly high from Crescent City to Monterey with heights on the open coast ranging from 7 - 21 feet. At Santa Cruz Harbor, the tsunami wave reached as high as 11 feet, sinking a hydraulic dredge and a 38 foot cabin cruiser and causing minor damage to the floating docks.Damage in San Francisco Bay was largely to pleasure boats. The highest damage was reported from marinas in Marin County where strong currents induced by the tsunami caused boats and floating piers to break loose and strike other craft.Damage at Noyo Harbor was primarily to floating piers and to commercial fishing vessels. Also, damage occurred in Los Angeles and Long Beach Harbors.

The estimated losses elsewhere along California were between $1,500,000 and $2,375,000 (1964 dollars), while at Crescent City tsunami damage was estimated at $7,414,000.The maximum wave at Crescent City was approximately 20 to 21 feet. Crescent City Harbor is one of the oldest. Lumbering and timber products are the major industries. The first of the four tsunami waves that struck Crescent City caused no significant damage other than flooding. The second and third waves were smaller than the first. The fourth was the largest of the waves and was preceded by a withdrawal of the water which left the inner harbor almost dry. The fast moving fourth wave capsized 15 fishing boats in the harbor. Three other boats disappeared, and eight more sunk in the mooring area. Several other boats were washed onto the beach. Extensive damage was inflicted to the piers. The tsunami waves covered the entire length of Front Street, and about thirty blocks of Crescent City were devastated. Lumber, automobiles, and other objects carried by the waves were responsible for a good portion of the damage to the buildings in the area. Fires started when the largest tsunami wave picked up a gasoline tank truck and slammed it against electrical wires. The fire spread quickly to the Texaco tank farm, which burned for three days.

What did people do before the International Tsunami Warning System was established?

Before the International Tsunami Warning System (ITWS) went into effect there were not effective warning systems to alert the countries of the Pacific of an impending tsunami threat.The tsunamis often struck populated areas with no warning or evacuation. The data was not always shared among nations of the Pacific. Communication channels had not been established for the sharing of such data. The lack of a warning system was responsible for extensive loss of life and property. It was the great destruction caused by the May 1960 Chilean tsunami which prompted a large number of countries and territories to join the Pacific Tsunami Warning System (TWS), at least by contributing data and information. It was the great Alaskan earthquake of 1964 which generated a devastating tsunami that precipitated the need for an International Tsunami Warning System (ITWS).