Why tsunamis are dangerous

Earthquakes are commonly associated with ground shaking that is a result of elastic waves traveling through the solid earth. Note: In this 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.

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.

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. Several things happen as the local tsunami travels over the continental slope. Most meteorites burn as they reach the earth's atmosphere. However, large meteorites have hit the earth's surface in the distant past. This is indicated by large craters, which have been found in different parts of the earth.

Also, it is possible that an asteroid may have fallen on the earth in prehistoric times - the last one some 65 million years ago during the Cretaceous period. Since evidence of the fall of meteorites and asteroids on earth exists, we must conclude that they have fallen also in the oceans and seas of the earth, particularly since four fifths of our planet is covered by water.

The fall of meteorites or asteroids in the earth's oceans has the potential of generating tsunamis of cataclysmic proportions. Scientists studying this possibility have concluded that the impact of moderately large asteroid, km in diameter, in the middle of the large ocean basin such as the Atlantic Ocean, would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. On both sides of the Atlantic, coastal cities would be washed out by such a tsunami.

An asteroid kilometers in diameter impacting between the Hawaiian Islands and the West Coast of North America, would produce a tsunami which would wash out the coastal cities on the West coasts of Canada, U.

Conceivably tsunami waves can also be generated from very large nuclear explosions. However, no tsunami of any significance has ever resulted from the testing of nuclear weapons in the past. Furthermore, such testing is presently prohibited by international treaty. Click the link below to view a movie that shows a physics-based computer simulation of the tsunami generated by the impact of the Chicxulub asteroid 65 million years ago.

This asteroid impact is thought to responsible for the extinction of the dinosaurs. Tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water. Each region of the world appears to have its own cycle of frequency and pattern in generating tsunamis that range in size from small to the large and highly destructive events. Most tsunamis occur in the Pacific Ocean and its marginal seas.

The reason is that the Pacific covers more than one-third of the earth's surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire" - where most earthquakes occur off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America. Many tsunamis have also been generated in the seas which border the Pacific Ocean. Tsunamis are generated, by shallow earthquakes all around the Pacific, but those from earthquakes in the tropical Pacific tend to be modest in size.

While such tsunamis in these areas may be devastating locally, their energy decays rapidly with distance. Usually, they are not destructive a few hundred kilometers away from their sources. That is not the case with tsunamis generated by great earthquakes in the North Pacific or along the Pacific coast of South America.

On the average of about half-a-dozen times per century, a tsunami from one of these regions sweeps across the entire Pacific, is reflected from distant shores, and sets the entire ocean in motion for days.

For example, the Chilean tsunami caused death and destruction throughout the Pacific. Hawaii, Samoa, and Easter Island all recorded runups exceeding 4 m; 61 people were killed in Hawaii. In Japan people died. Although not as frequent, destructive tsunamis have been also been generated in the Atlantic and the Indian Oceans, the Mediterranean Sea and even within smaller bodies of water, like the Sea of Marmara, in Turkey. In , a large earthquake along the North Anatolian Fault zone, generated a local tsunami, which was particularly damaging in the Bay of Izmit.

In the last decade alone, deadly tsunamis have occurred in Chile , , Haiti , Indonesia , , , , Japan , Peru , Samoa - American Samoa - Tonga , Solomons Of these, only Indonesia and Japan caused deaths at distant shores.

In the deep ocean, tsunami wave amplitude is usually less than 1 m 3. The crests of tsunami waves may be more than a hundred kilometers or more away from each other. Therefore, passengers on boats at sea, far away from shore where the water is deep, will not feel nor see the tsunami waves as they pass by underneath at high speeds. The tsunami may be perceived as nothing more than a gentle rise and fall of the sea surface.

There are three factors of destructions from tsunamis: inundation, wave impact on structures, and erosion.

Strong, tsunami-induced currents lead to the erosion of foundations and the collapse of bridges and seawalls. Flotation and drag forces move houses and overturn railroad cars. Considerable damage is caused by the resultant floating debris, including boats and cars that become dangerous projectiles that may crash into buildings, break power lines, and may start fires.

Fires from damaged ships in ports or from ruptured coastal oil storage tanks and refinery facilities, can cause damage greater than that inflicted directly by the tsunami. Of increasing concern is the potential effect of tsunami draw down, when receding waters uncover cooling water intakes of nuclear power plants. Destruction of Hilo, Hawaii harbor pier during Aleutians Islands tsunami.

Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the ocean's depth. A tsunami is made up of a series of very long waves. The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond. The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event.

Beaches, harbors, bays, and river mouths are at the greatest risk. If you are in the white area, stay where you are. Each horizontal dashed-line is 12 hours. Each vertical dashed-line is about 1. The tidal fluctuation blue line affects the total water height. For example, a tsunami arriving on a high tide will cause more innundation and damage than on a low tide. Interactive Tsunami Hazard Map. Sign up for Emergency Alerts.

Redwood Coast Tsunami Work Group rctwg humboldt. Skip to main content. Toggle navigation Menu. What is a tsunami? A series of waves or surges that is most often caused by earthquake fault movement beneath the sea floor.

A tsunami maylooksmall from a distance, but bythe time you realize how big it is,you might not be able to reach high ground. The water moves much faster than it appears. Tsunamis penetrate far onto the shore and can travel miles up coastal rivers. There is no curling wave or face to surf.

When this occurs buildings can settle, tilt, or shift. Landsliding can occur during an earthquake where shaking reduces the strength of the slope. These hazards can usually be reduced or eliminated through established engineering methods. The law requires that property being developed within these zones be evaluated to determine if a hazard exists at the site. If so, necessary design changes must be made before a permit is granted for residential construction.

Being in an SHZ does not mean that all structures in the zone are in danger. The hazard may not exist on each property or may have been mitigated. Mapping new SHZs in urban and urbanizing areas is ongoing statewide. Current zones, as established by the California Geological Survey, are indexed at www. Earthquake Fault Zones EFZs recognize the hazard of surface rupture that might occur during an earthquake where an active fault meets the earth's surface.

Few structures can withstand fault rupture directly under their foundations. The law requires that within an EFZ most structures must be set back a safe distance from identified active faults. The necessary setback is established through geologic studies of the building site.