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Prior to the tsunami of 26th December 2004, triggered by the most powerful earthquake in the world for the last forty years, there have been no major natural disasters in Thailand for generations.
Unconfirmed scientific reports indicate a comparable tsunami hit the Andaman coastline of Thailand some 600 years ago. Details of a smaller 1907 tsunami in the region are sketchy.
Historical earthquakes close to the Andaman islands, in the Indian ocean, due west of the Andaman coastline include:-
4th Jan 1907 (strength 7.8), 11th Oct 1914 (strength 7.2), 26th June 1941 (strength 8.7), 17th May 1955 (strength 7.0), and 2nd Nov 2002 (strength 7.6).
In particular the earthquakes of 1941 (S. Andaman island), and 1881 (Nicobar Islands) generated tsunamis.
The tsunami was such a tragic event, it almost defies comprehension.
In actual fact, on the basis of subsequent analysis of additional seismograms from around the world, scientists have determined the earthquake's magnitude to be 9.3 and not 9.0, as originally estimated.
Therefore, the calculated energy release was three times larger than originally thought. The revised estimate makes this quake to be the second largest ever instrumentally recorded by modern seismic instruments.
(The largest earthquake ever recorded, which measured 9.5, was in Chile on May 22, 1960).
This great tsunamigenic earthquake had a focal depth of approximately 30 km (originally reported as being only 10 km).
In addition, the earthquake had the largest ruptured fault (1200 km - essentially from Sumatra to the Andaman islands) ever observed, and covered a time span along the fault of 500 seconds (just under 9 minutes).
Initial estimates of the magnitude of the earthquake were grossly underestimated which unfortunately shows the limitations of conventional magnitude estimators for really great earthquakes.
A Harvard magnitude reading of 9.0 was only available after a few hours. And a final magnitude calculation of 9.3 was determined weeks later (by free oscillation calculations).
And so this prevented (in part) a timely alert of the coasts at risk, and highlights the need for a fast non-saturating magnitude recorder for large earthquakes. This is being worked on.
But at the time, estimators for earthquakes greater than 8.0 (and also slow-rupture earthquakes) came up with a somewhat inaccurate figure.
Accordingly the probability of a tsunami (and its strength) were also underestimated.
The critical magnitude to generate tsunamis is often regarded as about 7.5.
The maximum tsunami wave height at Khaolak has been reported to be 11.6 meters.
Average wave heights were as follows:
Khao Lak: 6 - 10 meters
Phuket (west coast): 3 - 6 meters Phuket (east coast): 3 meters
Phi Phi islands: 4.5 - 6.0 meters
The tsunami at KhaoLak was exceptionally strong. It has been suggested from field survey reports that a lot of tsunami energy was trapped at the nearby Similan islands, and this energy
headed east to Khaolak.
In contrast to Phuket, which was relatively unscathed due (maybe) to the topology of the island and its bays, the Andaman coastline from Poseidon beach, through KhaoLak proper, Khuk Khak beach, the Pakarang peninsular, Bangsak, Baan Namken, and upto within ten kilometers of Takuapa,
suffered quite extensive damage. The waters breached highway 4 in a few places. In particular, Bang Niang beach, Khuk Khak beach. the Pakarang peninsular and Bangsak beach were quite badly hit.
The destruction to buildings including internationally acclaimed hotels was very severe. (And in terms of the general landscape, in the very short term at least, only the most robust of vegetation and trees were able to withstand the force of the waves
and the after-effects of the salt water.)
So very very sad and painful for so many people ...
At 00.59 hrs GMT on 26th December 2004, a 9.3 magnitude earthquake tore apart the seafloor 160 km off the northwest coast of Sumatra, at part of the boundary (the 1,200 km Andaman-Sumatran trench) of two of the world's mighty tectonic plates. The lower plate, which carries India, (and which is part of the bigger Indo-Australian plate) has been moving slowly north/northeast, at a speed
which has been compared to the growth of our fingernails (2 inches per year). It is continually being forced (subducted) beneath the upper plate (known as the Burma plate which is a tongue shaped part of the bigger Eurasian plate carrying most of South East Asia). The plates meet at oblique angles, which means the movement is not a smooth, frictionless glide but a torturous buildup of stresses that can result in dramatic perpendicular slips.
Before the fault ruptured, the edge of the upper Eurasian plate was actually being dragged downward by the descending Indo-Australian plate.
Released by the rupture of the fault, the edge of the upper plate sprang back up, lifting the ocean floor and setting off the tsunami. The fault slipped by as much as 20 metres in a few places.
This type of earthquake is very powerful and is referred to as a megathrust. Over 100 years of accumulated stress was released by the earthquake and billions of tonnes of seawater was subsequently displaced to form the tsunami.
An exceptionally long (estimated at 1,200 km) faultline slipped typically about 10 - 15 m along the subduction zone where the India Plate dives under the Burma Plate. The slip took place in two phases over a period of several minutes.
It is further estimated that 30 cubic km (7 cubic miles) of water was displaced. The tsunami waves radiated outwards along the entire 1,200 km length of the rupture, as opposed to originating from a point source.
Due to the complex rupturing of the sea floor, some tsunami waves effectively travelled with the crest first whilst others travelled with the trough first. When a trough reaches the coastline, it causes the sea to withdraw and disappear from the beaches. This is one of the classic warning signs of an approaching tsunami.
Tragically many people were unaware of this sign, and were attracted by the fish left flapping on the sand. Minutes later the crest waves would arrive and the full force of the tsunami caused a thousand tonnes of water to crash down on each metre of beach.
Although not helpful to people on the beach, it has been reported that elephants seemed to sense that something was coming. They began acting strangely, stamped the ground and tugged at their chains until they broke away and headed to the hills. Elephants have special bones in their feet that enable them to sense seismic vibrations long before humans can.
By all accounts, the regional disaster centres of Takuapa and Phuket performed wonders under the circumstances, and the caring and helpful nature of the fantastic Thai people shone through quite strikingly.
The Thai army and medics did a truly excellent job in setting up and administering relief centres, donation centres, and temporary accomodation.
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Photo above:
Official sign indicating the tsunami height at Bang Niang (Chong Fah) route 4 (Petchkasem Road)
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Photo above left:
Fishing boat washed up in the middle of the destroyed village of Baan Namken
Photo above right:
Police patrol boat in the jungle, east of route 4, 1 km inland at Baan Niang Khaolak
Photo below:
Royal Thai Navy frigate (KraBuri) beached at Thap Lamu pier, south of Khaolak
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According to some unofficial estimates, it could take upto December 2005 before Khaolak is again completely restored to its original charm and potential. The local Thai people who contribute so much to making
your vacation such a joy, and who have lost family members, friends, and their livelyhood, naturally hope this is a conservative estimate . Your visit would be so welcome.
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History of earthquakes in the region
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The following diagram illustrates the history of earthquakes in S.E. Asia, courtesy of Dr. Smith Dharmasaroja, a meteorologist and seismologist, and initially the Head of Thailand's National Disaster warning Centre.
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Indian Ocean Tsunami Early Warning System program
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As a result of the 2004 tsunami, a real-time tsunami detection system is gradually being put in place across the Indian Ocean.
A total of 22 tsunami detection stations are envisaged. This is part of an end-to-end warning system, for the whole region, and which includes tide gauges, communications upgrades, modeling, and dissemination systems for five countries - Thailand, Indonesia, India, Sri Lanka, and the Maldives.
A typical detection station consists of a bottom pressure sensor unit (a tsunameter) that is anchored to the sea floor, and a companion moored surface buoy.
An accoustic link transmits data from the pressure unit to the surface buoy, and then satellite links relay data to ground stations.
The initial ground stations in the chain (until an alternative regional ground station capability is established) are the NOAA Pacific Tsunami Warning Centre in Hawaii and the Japan Meteorological Agency.
These two ground stations provide tsunami advisory and watches alerts to 27 Indian Ocean countries on an interim basis.
The individual countries then determine if and how they issue a warning to their publics.
The tsunami detection stations in the ocean can measure natural tidal cycles, but these effects are predictable and easily subtracted out.
Tsunami buoys do not register minor surface-wind waves, however. This is because of their wavelength.
Wind waves only affect the water column as deep as half their wavelength, which can be up to a
few hundred meters. Tsunamis, however, have wavelengths approaching 120 nautical miles.
These tsunami waves affect the water column all the way down to the seafloor and the awaiting
tsunami detection system.
Bear in mind that a tsunami travels at a speed of approx 700 km/hour (similar to that of a jet airliner).
One of the main earthquake fault lines runs north from Sumatra, Indonesia to the Andaman islands/Nicobar islands (west of Phuket) and further.
The distance from the tip of Sumatra to the Andaman Islands is roughly 1,200 kms.
A tsunami wave in the middle of the Ocean is hardly noticeable to the naked eye due to the vast depth of the water, and there is no danger to shipping.
The problem of course is when the tsunami runs out of water depth and hits the coast.
There are typically six small but measurable tsunamis every year worldwide. But a significant tsunami with waves over 4 meters typically only occurs on decadal scales.
The tsunami data, from the ocean detection stations, can also be combined with seismic data ingested into a scientific forecast model to generate accurate tsunami forecasts for all of the coastal areas in the region.
The forecast modeling technology includes data from the tsunamis of 1881, 1941 and 2004.
In effect, this overall combined approach leads to a computer and web based detailed picture, analysis and scenario all along the coast. The system is designed to give at least a one hour warning prior to the tsunami reaching land.
In a landmark first step towards the Indian Ocean project, the USA and NOAA donated under a memorandum of understanding the first Dart II buoy system to Thailand.
It was deployed in December 2006 about 1,000 km north west of Phuket, roughly midway between Thailand and Sri Lanka, and close to the Nicobar islands. Thailand is responsible for its upkeep.
This should give the capability to provide a one hour warning to most of the countries in the upper basin (of the Bay of Bengal/Indian Ocean).
Under a similar arrangement between Indonesia and USA, the USA donated a second Dart II buoy system off the coast of Sumatra - this was due to be installed in April 2007. (This buoy was reportedly later damaged by fishermen).
The status of the remaining 20 buoy systems is still ongoing.
DART II performance characteristics:
Reliability and data return ratio: Greater than 80%
Maximum deployment depth: 6000 meters
Minimum deployment duration: Greater than 1 year
Operating Conditions Beaufort 9 (survive Beaufort 11)
Maintenance interval, buoy Greater than 2 years
Maintenance interval, tsunameter Greater than 4 years
Sampling interval, internal record: 15 seconds
Sampling interval, event reports: 15 and 60 seconds
Sampling interval, tidal reports: 15 minutes
Measurement sensitivity: Less than 1 millimeter in 6000 meters; 2 x 10-7
Tsunami data report trigger Automatically by tsunami detection algorithm
On-demand, by warning center request
Reporting delay: Less than 3 minutes
Maximum status report interval: Less than 6 hours
Batteries in tsunameter:
The tsunameter computer and pressure measurement system uses an Alkaline D-Cell battery
pack with a capacity of 1560 watt-hours. The acoustic modem in the tsunameter is powered by
similar battery packs that can deliver over 2,000 watt-hours of energy. These batteries are
designed to last for four years on the seafloor; however, this is based on assumptions about the
number of events that may occur and the volume of data request from the shore. Battery
monitoring is required to maximize the life of the system.
Batteries in surface buoy:
The buoy's fiberglass well houses the system electronics and power supply, which is made up
of packs of D-cell alkaline batteries. The computer and Iridium transceiver are powered by 2,560
watt-hour batteries; the acoustic modem is powered by 1,800 watt-hour batteries. These
batteries will power the buoy for at least two years.
The buoy is designed to mitigate the potentially dangerous build up of hydrogen gas that is
naturally vented from alkaline cells. Design features include: 1) hydrogen getters (such as those
from HydroCap Corp); 2) pressure relief valves; and 3) spark-free components such as
fiberglass or plastic.
Tsunami Detection Algorithm:
Each DART II tsunameter is designed to detect and report tsunamis autonomously. The
Tsunami Detection Algorithm works by first estimating the amplitudes of the pressure
fluctuations within the tsunami frequency band, and then testing these amplitudes against a
threshold value. The amplitudes are computed by subtracting predicted pressures from the
observations, in which the predictions closely match the tides and lower frequency fluctuations.
If the amplitudes exceed the threshold, the tsunameter goes into Event Mode to provide detailed
information about the tsunami.
Surface Buoy details:
The DART II surface buoy relays information and commands from the
tsunameter and the satellite network. The buoy contains two identical electronic systems to
provide redundancy in case one of the units fails. The Standard Mode transmissions are
handled by both electronic systems on a preset schedule. The Event Mode transmissions, due
to their importance and urgency, are immediately transmitted by both systems simultaneously.
The surface mooring uses a 2.5 m diameter fiberglass over foam disk buoy with a displacement
of 4000 kg. The mooring line is 19 millimeter eight-strand plaited nylon line with a rated breaking
strength of 7100 kg, and is deployed to maintain a tight watch circle, keeping the buoy
positioned within the cone of the acoustic transmission. In temperate areas where fish tend to
aggregate and bite lines, wire rope is use on the upper few hundred meters of the mooring.
Two downward-looking transducers are mounted on the buoy bridle at a depth of 1.5 meters
below the sea surface. A multi layered baffle system of steel, lead, and syntactic foam shields
the transducers from noise, and cushions them with rubber pads for a soft mount.
NOAA provides technical support to the US Indian Ocean Tsunami Warning System (IOTWS). Specifically NOAA provides support to the UN Intergovernment Oceanographic Commission (IOC).
USAID is the US Agency for International Development.
In May 2009, it was announced that Thailand would purchase a SAIC (Science Application International Corporation) Tsunami Buoy (STB) System.
This is an enhanced and commercial version of the buoy system currently in use by the National Oceanic and Atmospheric Administration's (NOAA) DART II detection system.
It follows DART test and evaluation procedures and meets DART operational and performance standards.
It is also compatible with established DART design and integrates into global tsunami warning networks.
Subsequently, Thailand announced that it would in fact be buying three new buoys at a cost of about 200 million baht.
One will replace the original now defunct USAID-donated one near the Nicobar islands, and by the end of 2010 two more buoys will be placed in the Andaman Sea closer to the Thai mainland, such that they will better cover the
country's southern coastline.
In late 2009 after the monsoon season, the original Nicobar buoy, which suffered battery problems (and stopped transmitting on 11th September 2009) was replaced on 17th December 2009. This original buoy will be analysed and repaired in the USA and then returned to Thailand, at which point it will be used as a spare and rotated annually.
The vessel MV Seafdec (under the supervision of the Royal Thai Navy, US buoy specialists and NDWC staff) was used for the mission. The same vessel was used for the original installation in December 2006.
The SEAFDEC organisation is the South East Asia Fisheries Development Centre and often conducts several research surveys in the national waters of member countries in the Indian Ocean.
The MV Seafdec was donated by Japan in 1992 to SEAFDEC.
Experts say Thailand has been at the forefront of efforts to coordinate regional information-sharing on disaster warnings. Thailand has been one of the stalwarts of the regional tsunami warning system and pushed it forward.
They led the commitments in 2005 after the tsunami and have been a key supporter since then.
Thailand's fledgling National Disaster Warning Centre (NDWC) is growing but not without hiccups. Press reports say that there seems to have been some confusion regarding oversight of the NDWC and its legal status and funding.
It was established in 2005 and the centre is designed to process information on seismic activity in the region and issue warnings, particularly for tsunamis that may hit
the country's Andaman Sea coastline.
In the region, other organisations involved include the Asian Disaster Preparedness Centre (ADPC) (a nonprofit organisation), and the UN Educational, Scientific and Cultural Organisation (UNESCO) and the Intergovernmental
Oceanographic Commission (IOC). The aim is to set up a system that connects all of Asia's national disaster warning centres.
For comparison purposes, observers have pointed out that the Pacific tsunami warning centre in Hawaii took approximately 10 years to set up with far fewer countries.
In the Indian Ocean region, a lot more people are involved.
Tsunamis can be caused by submarine landslides and volcanic eruptions as well as earthquakes. The Indian Ocean Early Warning system issues a warning only after the tsunami has been initiated.
Another promising research area for enhancing our predictive capability is holistic geophysical forecasting. This makes use of the fact that the sizes of tsunami-related disturbances are so large and so powerful that they disturb the solid earth, the oceans and the atmosphere.
These disturbances do not just lead to mechanical forces and release of heat, as in storms, but they also affect electrical, magnetic and molecular processes, especially higher up in the atmosphere.
Modern instruments have become so sensitive that they can measure magnetic fields one millionth of the strength of the earth's magnetic field, so that tremors in the lithosphere can be detected long before large earthquakes and tsunamis actually occur.
Research at the Geoelectromagentic Research Center in Moscow also confirms that the motions in tsunami waves, once initiated, can be detected over many hundreds of kilometers from distant measurements of weak, slowly changing magnetic fields.
It is well known that scientists feel in rather different ways about the problem of earthquake prediction - some are skeptics, others are optimists.
In 2003 came the discovery of a principally new physical effect of pre-strong earthquake three-dimensional gravitational field variation sensed at huge distances (1000 km to 10,000 km away from the earthquakes). This effect was employed in creating a principally new physical instrument - a patented torsion detector for three-dimensional gravitational variations.
The Global Network for the Forecasting of Earthquakes (GNFE) was established in 2008 and uses this principally new technology for short term pre-earthquake forcasting.
Three automatic digital earthquake prediction stations, "atropatena", were established in Baku (Azerbaijan), Islamabad (Pakistan) and Yogyakarta (Indonesia) and combined into a global network. They continuously transmit information regarding unusual gravitational precursors of earthquakes to the central database in the USA.
Based on all this data, the Global Network allows short term prediction of earthquakes all over the Eastern Hemisphere with a 90% probability, ( according to its website at http://seismonet.org).
GNFE participating countries (Atropatena station owners) and partners (parties to cooperation agreements) include England, Austria, US, Indonesia, Azerbaijan, Pakistan, Turkey, Kazakhstan, Uzbekistan.
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Tsunami Early Warning system - December 2010
In June 2010, the aforementioned Dart II bouy went adrift due to a break in its anchoring cable.
It has been suggested that a large vessel tied up to it, causing the cable to snap under the increased force. The buoy itself was not badly damaged and was recovered in July 2010.
This bouy is officially called Station 23401 under the US Government's National Data Buoy Center scheme, which allows anyone to monitor water column and
tsunami event data in real time over the Internet at the following website:
http://www.ndbc.noaa.gov/station_page.php?station=23401
The mishap reduced to zero the number of functioning tsunami direct detection units off the Thai coast.
However the NWDC still has access to the water level data from the Royal Thai Navy's water level monitor on Ko Miang (Similan Islands).
Thailand's tsunami early warning system is maintained by Bangkok-based firm Raydant International, under a contract with the National Disaster Warning Center.
Raydant also operates the network of (over 100) tsunami warning towers in the Andaman coast region.
In November 2010, Raydant announced that the company had already placed a request with the Ministry of Information and Communication Technology (MICT)
for funds to purchase 4,500 meters of replacement cable to moor the buoy to the seabed some 3,000 meters
beneath the surface.
Raydant said it will take several months before the tsunami buoy that went adrift in early June can be
redeployed in its old position, 600 nautical miles west-northwest of Phuket.
Once funding is approved, the special cable must be manufactured abroad and shipped to Thailand.
It will take the company three months to produce the cable and another month for shipping.
This Dart II bouy is often referred to as the Raydant unit, as opposed to the first original NOAA Dart-II bouy,
which as of December 2010 is still in the USA undergoing maintenance.
It is expected that the Raydant unit will be operational again in early 2011, whilst the NOAA unit will be used as a rotating spare for annual on-shore maintenance.
Meanwhile, in March 2010, Platt Nera Co. Ltd ordered two Poseidon Class tsunameters from Italian Company Envirtech, related buoys, software and services to establish the new Thai Tsunami Warning System, marine segment.
Collected data will be managed by NDWC (National Disaster Warning Center) in Bangkok.
The Thai government decided to acquire Envirtech instrumentation after a long time evaluation of the available state
of art technology.
The marine segment will be installed in the Andaman Sea off shore Phuket and Surin Islands.
In August 2010, the Factory Acceptance Test was successfully held in Venice in the presence of a NDWC (National Disaster Warning Center) delegation.
The tests involved the Envirtech Poseidon Class tsunameters, the Control Center software and Data buoys payloads.
The Thai delegation also visited the Envirtech microwaves tide station, some ISPRA (Italian Environment Agency) settlements in Venice Lagoon and the INGV earthquakes control room in Rome.
The two new Envirtech systems are not part of the US Government's National Data Buoy Center, which allows anyone to monitor water column and
tsunami event data in real time over the Internet.
Accordingly the NWDC will need to set up a new website where real time data (for the Envirtech systems) will be posted for the public to see.
The Envirtech systems are of modern technology in accordance with international standards.
And so, as planned, two Envirtech Tsunami-Buoys and related Poseidon Class Tsunameters have been deployed in the Andaman Sea region to improve the Thai Tsunami Warning System capabilities.
Both systems are working properly transmitting real time data to the NDWC headquarter located in Bangkok. Bottom units have been deployed on seabed between 600 and 2600 meters depth, respectively,
off-shore Phuket and Surin Islands, far enough to give time to raise an alarm in case of an assessed tsunami approach.
Collected data, actually under evaluation by the National Disaster Warning Center personnel, will be availble for free website access to the international community via the WMO-GTS
(World Meteorological Organisation's Global Telecommunicatons System).
Each sea bottom unit collects sea level data with an accuracy of 1 mm per million and transmits it to the surface buoy via acoustic telemetry.
(The first is a shallow water float which was installed on 12th December 2010, about 124 nautical miles south/south-west of Phuket in the direction of Malaysia.
It carries identification advice to passing shipping in Malay and English).
(The second is a deep-water bouy which was installed on 19th December 2010 about 130 nautical miles north of the Surin Islands in the direction of Burma.
It carries identification advice to passing shipping in Burmese and English).
In summary then, by early 2011, Thailand will have three tsunameters operating under two different monitoring systems.
The use of two different monitoring systems will provide a kind of a failsafe if one system goes down.
There will be the Dart II tsunameter / NOAA system using one bouy and one rotational spare.
And there will be the two Envirtech tsunameter system.
Please visit the following website for information on tsunami detection and tsunami warning systems :
http://www.meted.ucar.edu/hazwarnsys/twcrg/Chapter5.pdf
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Tsunami Early Warning System in 2005
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Patong beach Phuket is the first of all Andaman resorts to have a newly-installed world-class emergency warning system. It emits a 121-decibel alert that can be heard 1,500 meters away.
Warnings will be issued in several languages.
The system, is linked to the Nonthaburi-based National Disaster Warning Center (NDWC).
The Patong system has been installed by Bangkok-based Ele Sat Engineering, using communications equipment from Swedish company Kockum Sonics and an Inmarsat satellite system.
In addition, the Similan islands tsunami detection station on Koh Mieng is now operational. Eight other such island-specific detection centres will be in operation by the end of 2005 and will include:
Tapao Noi and Racha Noi islands (Phuket province), Sikow island (Trang), Tarutao island (Satun), and the Surin Islands (Phang Nga).
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Basically about 80% of the tsunami early warning system, including technology to send alerts to mobile phones and television and radio stations, as well as watchtowers equiped with sirens, will be in place by May 2005
for all six coastal provinces affected by the Decemeber 26th 2004 tsunami.
The full and complete system with special sensors and detector buoys (1,100 km off the Andaman coast near the Nicobar Islands), was finished by the end of 2005 and will be installed by late November 2006 by a joint Thai - USA team.
The buoys will
determine precisely whether a tsunami is imminent, and give about 1 hour's notice. This is a US-made network system formally known as the Deep Sea Tsunami Detection Equipment.
In the meantime, and in parallel to the ongoing effort, Thailand will also rely on earthquake measurement data to guage the threat of possible tsunamis.
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Whilst it is almost impossible to predict exact dates, location and magnitude of earthquakes, it is much easier to predict tsunamis and their extent as a result of such earthquakes.
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A tsunami is a wave train, or series of waves, generated in a body of water by an impulsive disturbance that vertically displaces the sea water. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of meteorites, can generate tsunamis.
In particular, relatively shallow earthquakes (less than 30 km) beneath the sea can generate tsunamis.
Needless to say, tsunamis can savagely attack coastlines.
Tsunamis are vastly different to the typical (wind-generated) waves, which we see at a coastal beach. Technically they are characterized as shallow-water waves, but with long periods and wave lengths. The wind-generated waves 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 of the order of one hour.
A shallow-water wave has a noteable ratio between the water depth and its wave length.
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. This implies that if the typical water depth in the ocean is about 4000 m, a tsunami travels at 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.
Thus even after they have travelled vast distances, tsunamis reach distant coastlines with almost as much power as they had when they left the immediate earthquake centre.
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As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms.
A tsunami travels at a speed that is related to the water depth. Thus, as the water depth decreases, the tsunami slows. However the tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant.
Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows.
Because of this shoaling effect, a tsunami, almost imperceptible at sea (being only 30cm above the surface), may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide.
So as a tsunami approaches shore, it begins to slow but grow in height.
Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence.
Despite these losses, tsunamis still reach the coast with tremendous amounts of energy.
Tsunamis may reach a maximum vertical height onshore above sea level, often called a runup height, of 10, 20, and even 30 meters.
Because tsunamis have very long wavelengths they come ashore more like a long lasting flood wave rather than the breaking surf usually seen at the beach.
The diagram illustrates the difference between tsunamis and wind waves when they come ashore.
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Let's look at it another way. If you throw a stone in a pond you create a series of concentric ripples. A tsunami is just like those ripples, except the disturbance that sets them in motion is of a much greater magnitude.
However the above analogy is limited because the pond waves loose energy quickly and disperse.
The worst tsunami waves are different. Not only do they have enormous energy, but they are "long waves" in the sense that the wavelength is many times greater than the ocean depth. All such waves travel at nearly the same speed, so that the energy doesn't spread so much, and they can survive over thousands of miles.
The usual tsunami-maker is the buckling of the seafloor caused by an earthquake, several kilometers beneath the seabed.
In the instant after the quake, the shape of the sea surface mirrors the contours of the now buckled seafloor below. But, just as quickly, gravity acts to return the sea surface to its original shape. As the rumpled sea flattens out, ripples race outward. A tsunami is born.
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World's worst tsunamis
* The 2004 Indian Ocean tsunami was the world's most deadly, killing about 226,000 people, with a wave height of about 100 feet (in Indonesia).
* The world's biggest tsunami, caused by a magnitude 8 quake that caused a massive landslide, hit the remote Lituya Bay in Alaska on
July 9, 1958. As the wave swept through Lituya Bay, it was forced to rise up, reaching an estimated height of 1,720 feet on the
other side of the bay, becoming a mega-tsunami. The sparsely populated bay was devastated, but damage was localized.
* The Krakatau island volcanic eruption of 1883 generated giant waves reaching heights of 125 feet, killing some 30,000 people.
It was the most violent volcanic eruption in modern history.
Sources: Singapore-based Earth Observatory; School of Earth Sciences, University of Melbourne; Australia Pacific Tsunami
Warning Center, Hawaii; Tsunami Research Center, USC
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Can you help? Volunteers required
The Tsunami Volunteer Centre in KhaoLak Nature Resort, PhangNga is looking for a large number of volunteers - committed down-to-earth people who can tolerate tough conditions.
Skills include the following:-
IT support
Web Site designers/developers
Language translators (written) - Thai/English
Language interpreters Thai/English
Volunteer coordinators
Special projects staff
Fund-raising staff
Please visit the volunteer web site for further information
* www.tsunamivolunteer.net *
Email: info@TsunamiVolunteer.net
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Relief and Cleanup Operations (January 2005)
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The relief operation has been quite heroic and the cleanup operation is proceeding rapidly. The entire area is very busy with activity.
All along highway 4 and the Pakarang peninsular, new electricity poles were erected within days of the disaster. Feeder cables reach the temporary accomodation, constructed by the Royal Thai army.
There are endless numbers of pickup trucks (belonging to residents, visitors, Thais, and foreigners) arriving in Khaolak and its environs, and bringing basic commodities such as rice, bottled water and cooking pots to the relief operations
drop off points. Many of the foreigners travel up from Phuket. So many concerned people have been helping with their own small relief efforts.
There have been numerous visits to the region from overseas aid foundations, and a great many dignatories from around the world including two USA ex-presidents.
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Heavy machinery (diggers and excavators) are in action everywhere. Most damaged hotel structures have already been cleared, and a good deal of the rubble and broken cars have been taken away;
but there are still some neat piles, scattering the landscape, awaiting clearance. Recycling is an important part
of the operation. For example, in addition to metal, the destroyed concrete remains of buildings will be recycled by smashing them into small
fragments, which will then be used for foundations of future construction.
An ackwardly beached Marine Police boat, well over one kilometer inland at the edge of the jungle to the east of highway 4 at Bang Niang, is becoming a
bit of an icon; as if there is not enough other evidence of the force of the tsunami waves.
All hotel information on this website has been updated as to current status. Although there was severe damage to many hotels, a few are now re-opening. Please refer to our Accomodation section.
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