Tsunamis ~ Tidal Waves ~ Flooding

Tsunamis are tidal waves formed by underwater earthquakes or, much less frequently, by volcanic eruptions - meteor impacts - or underwater landslides. They that can exceed 400 miles per hours in the deep ocean.

In deep water a tsunami may only be inches - or a few feet high. But when it reaches a shoreline that energies becomes a wall of water that can be a mile high.

Since 1990, there have been 82 tsunamis, out of which 10 have claimed more than 4,000 lives.

According to researchers, there is a significant rise both in numbers of waves and in death tolls over the century. Up until the now - the average per decade has been 57. The increase in tsunamis reported is due to improved global communications; the high death are partly due to increases in coastal populations.

The word Tsunami comes from the Japanese tsu (harbor) and nami (wave). Appropriate naming, as some 80 percent of all tsunamis occur in the Pacific Ocean and Japan has suffered many, some coming from as far away as South America. Tsunamis are often incorrectly called tidal waves, but tides have nothing to do with them (though the damage may be worse if a tsunami hits at high tide).

A tsunami (pronounced tsoo-nah-mee) is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life.

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.

As a result of their long wave lengths, tsunamis behave as shallow-water waves. A wave becomes a shallow-water wave when the ratio between the water depth and its wave length gets very small. Shallow-water waves move at a speed that is equal to the square root of the product of the acceleration of gravity (9.8 m/s/s) and the water depth - let's see what this implies: In the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s, or over 700 km/hr. Because the rate at which a wave loses its energy is inversely related to its wave length, tsunamis not only propagate at high speeds, they can also travel great, transoceanic distances with limited energy losses.

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.

Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.

A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.

One of the worst tsunami disasters engulfed whole villages along Sanriku, Japan, in 1896. A wave more than seven stories tall drowned some 26,000 people. More than 30,000 people died in Java from a 1883 tsunami cause by a volcanic eruption.

Recent tsunamis:

1946: An earthquake in the Aleutian islands sent a tsunami to Hawaii, killing 159 people (only five died in Alaska).

1964: An Alaskan earthquake triggered a tsunami up to 20 feet tall that killed 11 people as far away as Crescent City, California and caused more than 120 deaths in all.

1983: 104 people in western Japan were killed by a tsunami spawned from a nearby earthquake.

On July 17, 1998: A Papua, New Guinea tsunami killed roughly 3,000 people. A 7.1 magnitude earthquake 15 miles offshore was followed within 10 minutes by a wave some 40 feet tall. The villages of Arop and Warapu were destroyed.


Science investigates freak waves November 2002 - BBC

Freak waves up to 60 metres high (200 feet) that rise up from calm seas to destroy ships do exist researchers argue.

US tidal wave risk

Arrows show the cracks at the top of the continental shelf

May 3, 2000 - BBC

Newly discovered cracks in the ocean floor off the East Coast of the US could cause a dangerous tidal wave in the future, US geologists believe.

This would throw a wave up to six metres (20 feet) high back on to the land.

In 1998, two strong earthquakes caused a series of tsunamis to hit the north coast of Papua New Guinea, killing 2,000 people. And the US East Coast suffered in 1929, when 51 people died after a 12m wave hit southern Newfoundland.

If these cracks collapsed, then rock and sediment would rush down the slope.

The scientists predict that within about 30 minutes, the sea level at the coast would actually drop by several metres at first, as the water rushed out to fill the space left by the slumped rocks.

Then, anytime up to an hour later, the sea would crash back in, up to six metres higher than normal.

The extent of flooding resulting from any tsunami would depend on the tide level at the time it struck. The worst case scenario would be a tsunami coinciding with a high tide.

8 Killed in Pacific Island Quake

November 27, 1999 - Sydney, Australia

An earthquake and tsunami struck one of the islands of the Pacific nation of Vanuatu today, killing at least eight people and causing widespread damage, authorities said.

The undersea earthquake, with a magnitude of 7.1, struck at about 1:10 a.m. today off Pentecost Island, crumbling homes and setting off landslides that wiped out roads and some telephone lines, said Leiwa Pakoa, operations officer at Vanuatu's National Disaster Management Office.

Five people were killed, either by landslides or crushed in falling buildings, Pakoa said.

The earthquake set off a tsunami, or giant wave, which crashed over the southern part of the island about 30 minutes later, destroying homes in villages along the coast. Three bodies were recovered in the area and two people were still missing at nightfall, Pakoa said.

About 100 people were injured, most in the earthquake, he said. Dozens were being evacuated to neighboring islands and to the capital, Port Vila, for treatment while medical teams rushed to the island were treating others on Pentecost.

``It is quite a populated island and the damage is very extensive - landslides, buildings collapsed and roads damaged,'' Pakoa said.

``The ground is still very soft,'' and authorities feared further landslides, he said.

Prime Minister Barak Sope expressed sorrow at the tragedy, Radio Australia reported.

The quake was felt through much of the nation, a group of about 80 islands 1,550 miles northeast of Sydney, but the worst-hit island was Pentecost, which has a population of about 12,000.

Pentecost is famous for its residents' so-called ``land dives.'' Divers jump from wooden scaffolds with vines tied to their ankles.

Vanuatu, about 1,550 miles northeast of Sydney, has a population of 190,000.

Mexican scientists develop warning system for deadly tsunamis

October 14, 1998 - AP

A new system developed by Mexican scientists could give coastal residents a chance to seek safety with advanced warning of oncoming earthquake-generated waves. The new system "may provide a reliable tsunami warning very rapidly," said Nikolai Shapiro of the Universidad National Autonoma in Mexico City.

The great waves, called tsunamis, have claimed untold lives over the years, including more than 2,000 victims this summer in the Pacific nation of Papua New Guinea. Hawaii, Mexico, South America and Indonesia are among areas that have been hit hard in the past.

Indeed, warnings could occur as quickly as five minutes after a quake, compared with the 10 minutes to 20 minutes now necessary. The university's scientists devised a seismic ratio that can help determine whether a quake will produce devastating waves. The system uses data generated by relatively simple and affordable seismograph stations. Tsunami, once known as tidal waves, result from earthquakes, landslides or volcanoes that occur beneath the ocean. They generate deep waves spreading out in all directions.

The waves may be little noticed at sea, but when they near shore the rising seafloor causes them to lift up, sometimes to massive heights that can sweep away entire coastal villages. When the waves must travel long distances, the Pacific Tsunami Warning Center in Hawaii can provide alerts. But when they originate near shore there is often little chance to warn people.

In studying earthquakes that occur off Mexico's west coast, those happening near a subduction trench -- where the plates that make up the Earth's surface come together -- are more likely to cause tsunami than those that occur closer to the coast. They also found that the more hazardous events near trenches produced fewer high frequency vibrations than the near-shore quakes. The researchers worked out a ratio of seismic waves that allowed them to tell quickly which quakes were near subduction trenches and which were near the coast. The main requirement is a broadband seismograph located within a few hundred miles of the quake.