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A t s u n a m i i s a s e r i e s o f w a v e s g e n e r a t e d w h e n a b o d y o f w a t e r , s u c h a s a l a k e o r o c e a n i s r a p i d l y d i s p l a c e d o n a m a s s i v e s c a l e . E a r t h q u a k e s , l a n d s l i d e s , v o l c a n i c e r u p t i o n s a n d l a r g e m e t e o r i t e i m p a c t s a l l h a v e t h e p o t e n t i a l t o g e n e r a t e a t s u n a m i . T h e e f f e c t s o f a t s u n a m i c a n r a n g e f r o m u n n o t i c e a b l e t o d e v a s t a t i n g .
The word "tsunami" is Japanese for "harbor wave", because tsunamis cause little or no visible effect in deep sea, and often Japanese fishermen would be out at sea fishing in deep sea when a tsunami came, and in the evening they came home and found their home village devastated by the tsunami, and thus they theorized that tsunamis only happen in harbors and elsewhere close inshore.
A l t h o u g h i n J a p a n e s e t s u n a m i i s u s e d f o r b o t h t h e s i n g u l a r a n d p l u r a l , i n E n g l i s h t s u n a m i s i s w e l l - e s t a b l i s h e d a s t h e p l u r a l . T h e t e r m w a s c r e a t e d b y f i s h e r m e n a l t h o u g h t h e y h a d n o t b e e n a w a r e o f a n y w a v e i n t h e o p e n w a t e r . A t s u n a m i i s n o t a s u b - s u r f a c e e v e n t i n t h e d e e p o c e a n ; i t s i m p l y h a s a m u c h s m a l l e r a m p l i t u d e ( w a v e h e i g h t s ) o f f s h o r e , a n d a v e r y l o n g w a v e l e n g t h ( o f t e n h u n d r e d s o f k i l o m e t r e s l o n g ) , w h i c h i s w h y t h e y g e n e r a l l y p a s s u n n o t i c e d a t s e a , f o r m i n g o n l y a p a s s i n g " h u m p " i n t h e o c e a n . T s u n a m i s h a v e b e e n h i s t o r i c a l l y r e f e r r e d t o a s t i d a l w a v e s b e c a u s e a s t h e y a p p r o a c h l a n d t h e y t a k e o n t h e c h a r a c t e r i s t i c s o f a v i o l e n t o n r u s h i n g t i d e r a t h e r t h a n t h e s o r t o f c r e s t i n g w a v e s t h a t a r e f o r m e d b y w i n d a c t i o n u p o n t h e o c e a n ( w i t h w h i c h p e o p l e a r e m o r e f a m i l i a r ) . H o w e v e r , s i n c e t h e y a r e n o t a c t u a l l y r e l a t e d t o t i d e s t h e t e r m i s c o n s i d e r e d m i s l e a d i n g a n d i t s u s a g e d i s c o u r a g e d b y o c e a n o g r a p h e r s .
Causes
Schematic of a Tsunami
A tsunami can be generated by any disturbance that rapidly moves a large mass of water, such as an earthquake, volcanic eruption, landslide or meteorite impact. However, the most common cause is an undersea earthquake. An earthquake which is too small to create a tsunami by itself may trigger an undersea landslide quite capable of generating a tsunami.
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Such large vertical movements of the earth's crust can occur at plate boundaries. Subduction earthquakes are particularly effective in generating tsunamis, and occur where denser oceanic plates slip under continental plates in a process known as subduction.
Sub-marine landslides (which are sometimes triggered by large earthquakes) as well as collapses of volcanic edifices may also disturb the overlying water column as sediment and rocks slide downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can uplift the water column and form a tsunami.
Waves are formed as the displaced water mass moves under the influence of gravity to regain its equilibrium and radiates across the ocean like ripples on a pond.
In the 1950s it was discovered that larger tsunamis than previously believed possible could be caused by landslides, explosive volcanic action and impact events. These phenomena rapidly displace large volumes of water, as energy from falling debris or expansion is transferred to the water into which the debris falls. Tsunamis caused by these mechanisms, unlike the ocean-wide tsunamis caused by some earthquakes, generally dissipate quickly and rarely affect coastlines distant from the source due to the small area of sea affected. These events can give rise to much larger local shock waves (solitons), such as the landslide at the head of Lituya Bay which produced a water wave estimated at 50 150 m and reached 524 m up local mountains. However, an extremely large landslide could generate a megatsunami that might have ocean-wide impacts.
Characteristics
Often referred to as "tidal waves", a tsunami does not look like the popular impression of "a normal wave only much bigger". Instead it looks rather like an endlessly onrushing tide which forces its way around and through any obstacle. Most of the damage is caused by the huge mass of water behind the initial wave front, as the height of the sea keeps rising fast and floods powerfully into the coastal area. The sheer weight of water is enough to pulverise objects in its path, often reducing buildings to their foundations and scouring exposed ground to the bedrock. Large objects such as ships and boulders can be carried several miles inland before the tsunami subsides.
Tsunamis act very differently from typical surf swells; they are phenomena which move the entire depth of the ocean (often several kilometres deep) rather than just the surface, so they contain immense energy, propagate at high speeds and can travel great trans-oceanic distances with little overall energy loss. A tsunami can cause damage thousands of kilometres from its origin, so there may be several hours between its creation and its impact on a coast, arriving long after the seismic wave generated by the originating event arrives. Although the total or overall loss of energy is small, the total energy is spread over a larger and larger circumference as the wave travels. The energy per linear meter in the wave is proportional to the inverse of the distance from the source.[citation needed] (In other words, it decreases linearly with distance.) This is the two-dimensional equivalent of the inverse square law, which is followed by waves which propagate in three dimensions (in a sphere instead of a circle).
A single tsunami event may involve a series of waves of varying heights; the set of waves is called a train. In open water, tsunamis have extremely long periods (the time for the next wave top to pass a point after the previous one), from minutes to hours, and long wavelengths of up to several hundred kilometres. This is very different from typical wind-generated swells on the ocean, which might have a period of about 10 seconds and a wavelength of 150 metres.
The actual height of a tsunami wave in open water is often less than one metre. This is often practically unnoticeable to people on ships. The energy of a tsunami passes through the entire water column to the sea bed, unlike surface waves, which typically reach only down to a depth of 10 m or so.
The wave travels across open ocean at an average speed of 500 mph. As the wave approaches land, the sea shallows and the wave no longer travels as quickly, so it begins to 'pile-up'; the wave-front becomes steeper and taller, and there is less distance between crests. While a person at the surface of deep water would probably not even notice the tsunami, the wave can increase to a height of sixstories or more as it approaches the coastline and compresses. The steepening process is analogous to the cracking of a tapered whip. As a wave goes down the whip from handle to tip, the same energy is deposited in less and less material, which then moves more violently as it receives this energy.
A wave becomes a 'shallow-water wave' when the ratio between the water depth and its wavelength gets very small, and since a tsunami has an extremely large wavelength (hundreds of kilometres), tsunamis act as a shallow-water wave even in deep oceanic water. 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/s2) and the water depth.
For example, in the Pacific Ocean, where the typical water depth is about 4000 m, a tsunami travels at about 200 m/s (720 km/h or 450 mi/h) with little energy loss, even over long distances. At a water depth of 40 m, the speed would be 20 m/s (about 72 km/h or 45 mi/h), which is much slower than the speed in the open ocean but the wave would still be difficult to outrun. However, a conjecture exists for velocities. The passing "hump" mentioned earlier is a "momentum flux" equal to density multiplied by the square of the velocity. This gives the transient pressure built up during the quake as equal to twice and in addition to the hydrostatic pressure. There is no proof for this.
Tsunamis propagate outward from their source, so coasts in the "shadow" of affected land masses are usually fairly safe. However, tsunami waves can diffract around land masses (as shown in this Indian Ocean tsunami animation as the waves reach southern Sri Lanka and India). It's also not necessary that they are symmetrical; tsunami waves may be much stronger in one direction than another, depending on the nature of the source and the surrounding geography.
Local geographic peculiarities can lead to seiche or standing waves forming, which can amplify the onshore damage. For instance, the tsunami that hit Hawaii on April 1, 1946 had a fifteen-minute interval between wave fronts. The natural resonant period of Hilo Bay is about thirty minutes. That meant that every second wave was in phase with the motion of Hilo Bay, creating a seiche in the bay. As a result, Hilo suffered worse damage than any other place in Hawaii, with the tsunami/seiche reaching a height of 14 m and killing 159 inhabitants.
December 26, 2004 Indian Ocean Earthquake
It had a magnitude of 9.15, triggered a series of lethal tsunamis on December 26, 2004 that killed approximately 230,000 people (including 168,000 in Indonesia alone), making it the deadliest tsunami in recorded history. The tsunami killed people over an area ranging from the immediate vicinity of the quake in Indonesia, Thailand and the north-western coast of Malaysia to thousands of kilometres away in Bangladesh, India, Sri Lanka, the Maldives, and even as far as Somalia, Kenya and Tanzania in eastern Africa. The disaster prompted a huge worldwide effort to help victims of the tragedy, with billions of dollars being raised for disaster relief.
There was no organized alert service covering the Indian Ocean. This was in part due to the absence of major tsunami events between 1883 (the Krakatoa eruption, which killed 36,000 people) and 2004. In light of the 2004 Indian Ocean tsunami, UNESCO and other world bodies have called for a global tsunami monitoring system.
Tsunami Wikipedia
Recorded Tsunamis in Past History - BC
List of historic tsunamis by death toll
Ancient Mediterranean Tsunami May Strike Again National Geographic - March 10, 2008
Until now, the main thinking has been that this quake -- as in the Indian Ocean
tsunami of December 26, 2004 -- occurred in a so-called subduction zone.
Sumatra Earthquakes and Tsunami Wikipedia - September 12, 2007
The wave that destroyed Atlantis- Was it a tsunami? BBC - April 21, 2007
Photo Gallery: Quake Lifts Island Ten Feet Out of Ocean National Geographic - April 11, 2007
Solomon Islands earthquake and tsunami on Monday 04/02/07 Wikipedia
Photo Gallery: Tsunami in the Solomons -- Before and After National Geographic - April 4, 2007
Australia's coast battered by tsunamis News in Science - February 21, 2007
Australia's coastline has felt the impact of almost 50 tsunamis in the past 150 years ...
Deadly Java Tsunami Caused by Slow-Moving Quake - 7.7 National Geographic - July 19, 2006
12/26/04 - Underwater 7.7 earthquake and tsunami Wikipedia
National Geographic Coverage
Tsunamis More Likely to Hit U.S. Than Asia
Did Island Tribes Use Ancient Lore to Evade Tsunami?
Tsunami Family Saved by Schoolgirl's Geography Lesson
Tsunami in Southeast Asia
BBC Coverage
Tsunami among world's worst disasters - December 26, 2004 - BBC
Tsunami throws up India relics BBC - February 2005
Ghosts stalk Thai tsunami survivors BBC
AP Coverage
Tsunami Reveals Ancient Ruins in India AP
Whatever they saw is back under water and out of sight. But a few hundred yards away, something else came to the surface. The tsunami scrubbed away six feet of sand from a section of beach, uncovering a small cluster of long-buried boulders carved with animals, gods and servant girls.
The Dec. 26 tsunami savaged hundreds of miles of shoreline across Asia. It killed at least 126,000 people in Indonesia and at least 31,000 in Sri Lanka. In India, 10,700 people are confirmed dead, with more than 5,600 missing.
Mahabalipuram, India is the capital of an ancient kingdom and famous for its elaborate Hindu temples, escaped mostly unscathed, with only three dead and limited damage.
And there's something else the tsunami gave back - tourists, drawn by heated headlines in the Indian media about a rediscovered Atlantis.
But what did those fisherman see? Archaeologists laugh at the tales of Atlantis and say it may take years of undersea exploration to uncover the truth. But nearly everyone around here knows the stories - cocktails of history and mythology that tell of the great port city that traded with China and Southeast Asia some 1,300 years ago.
This is a town made for legend. It is home to dozens of Hindu temples, baroque stone structures often covered with carvings. But legend speaks of its most famous temples: the Seven Pagodas, named for the vaguely pagoda-like style of Hindu temples in this part of India. Those temples, which according to myth are said to have once lined the shore, were so beautiful that the gods destroyed all but one - the so-called Shore Temple, a magnificently carved complex that is now considered a national treasure.
Some fishermen insist they saw more than the six vanished temples when the waters fell back. "There must have been at least 20," said Sunderasan, a young man, gesturing toward the sea. "We had no idea there were so many out there."
Archaeologists say excavations on shore and at sea were already under way before the tsunami struck, and that divers made promising finds of barnacle-encrusted blocks that appear man-made. "From an archaeological perspective, maybe the tsunami was good. We found some new things," said one man, pointing to the exposed boulders.
July 23, 1998 - AP - Papua New Guinea
A village of low huts made of woven coconut palm leaves and sticks rises from a hill well behind the ruins of beachfront villages smashed by a huge and deadly wave. Survivors, battered, without possessions, many badly injured or dying, scrambled into the hills after the tsunami Friday that killed perhaps as many as 6,000 people in several coastal villages. The new, nameless village depends on emergency supplies of rice, flour and water delivered by air and on the meager pickings of a few vegetable gardens and whatever can be foraged from the forest. Unused fishing traps lie around the huts.
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