Peta Tsunami
Peta area yang kena imbas akibat Tsunami di Aceh, Indonesia 26 Desember 2004
Untuk mengetahui teknik monitoring tsunami yang paling mutahir sila lihat di http://www.envirtech.org/envirtech_tsunameter.htm
Wednesday, December 26, 2007
TSUNAMI
Peta area yang kena imbas akibat Tsunami di Aceh, Indonesia 26 Desember 2004
Untuk mengetahui teknik monitoring tsunami yang paling mutahir sila lihat di http://www.envirtech.org/envirtech_tsunameter.htm
TSUNAMI
Tsunami
Dari Wikipedia, the free encyclopedia, diterjemahkan bebas ke Bahasa Indonesia
Tsunami adalah satu rangkaian ombak/gelombang yang dihasilkan manakala serombongan air, seperti suatu samudra atau danau dengan cepat dipindahkan pada suatu skala yang sangat besar / raksasa. Gempabumi, tanah longsor, letusan vulkanik dan bintang jatuh/meteor yang besar berpotensi untuk menghasilkan suatu tsunami. Efek dari suatu tsunami dapat terbentang dari yang kecil tidak terasa sampai yang sangat berbahaya dan membinasakan segalanya, seperti yang baru-baru ini terjadi di Aceh Desember ,2004.
Terminologi tsunami berasal dari bahasa Jepang (tsu= pelabuhan) dan (nami = gelombang).Terminologi “Tsunami” diciptakan oleh nelayan yang kembali ke pelabuhan dan menemukan area melingkupi pelabuhan sudah rusak dilanda gelombang besar, walaupun mereka belum sadar akan adanya gelombang dari laut lepas. Suatu tsunami bukanlah kejadian dari laut dalam, tapi lebih sederhana dari itu yaitu mempunyai amplitude yang kecil, dan mempunyai panjang gelombang yang sangat panjang bahkan sampai ratusan kilometer. Oleh karena itu keberadaan gelombang tsunami tidak berapa terasa di laut lepas/laut dalam yang hanya membentuk gelombang kecil di samudera.
Tsunamis juga dikenal sebagai gelombang pasang surut sebab ketika mendekati daratan yang menerima karakteristik dari suatu gelombang pasang bergerak maju dengan sangat cepat dibandingkan jambul ombak yang dibentuk oleh angin di samudra, orang kebanyakan lebih mengenal jenis ombak ini dibandingkan gelombang yang dapat menghasilkan tsunami.
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Sebabakibat
Tsunami dapat dihasilkan oleh gangguan apapun yang dengan cepat memindahkan suatu massa air yang sangat besar, seperti suatu gempabumi, letusan vulkanik, batu bintang/meteor atau tanah longsor. Bagaimanapun juga, penyebab yang paling umum terjadi adalah dari gempabumi di bawah permukaan laut. Gempabumi kecil bisa saja menciptakan tsunami akibat dari adanya longsor di bawah permukaan laut/lantai samudera yang mampu untuk membangitkan tsunami.
Tsunami dapat terbentuk manakala lantai samudera berubah bentuk secara vertical dan memindahkan air yang berada di atasnya. Dengan adanya pergerakan secara vertical dari kulit bumi, kejadian ini biasa terjadi di daerah pertemuan lempeng yang disebut subduksi. Gempa bumi di daerah subduksi ini biasanya sangat efektif untuk menghasilkan gelombang tsunami dimana lempeng samudera slip di bawah lempeng kontinen, proses ini disebut juga dengan subduksi.
Tanah longsor di dalam laut dalam , kadang-kadang dicetuskan oleh gempabumi yang besar; seperti halnya bangunan yang roboh akibat letusan vulkanik, mungkin juga dapat mengganggu kolom air akibat dari sediment dan batuan yang bergerak di lantai samudera. Jika terjadi letusan gunungapi dari dalam laut dapat juga menyebabkan tsunami karena kolom air akan naik akibat dari letusan vulkanik yang cukup besar lalu membentuk suatu tsunami. Contoh seperti yang terjadi di Gunung Krakatau.
Sekitar era tahun 1950 an ditemukan tsunamis yang lebih besar dibandingkan sebelumnya percaya atau tidak mungkin ini disebabkan oleh tanah longsor, bahan peledak, aktifitas vulkanik dan peristiwa lainnya. Gejala ini dengan cepat memindahkan volume air yang besar, sebagai energi dari material yang terbawa atau melakukan ekspansi energi yang ditransfer ke air sehingga terjadi gerakan tanah. Tsunamis disebabkan oleh mekanisme ini, tidak sama dengan tsunamis di lautan lepas yang disebabkan oleh beberapa gempabumi, biasanya menghilang dengan cepat dan jarang sekali berpengaruh sampai ke pantai karena area yang terpengaruh sangat kecil.Peristiwa ini dapat memberi kenaikan pada gelombang kejut lokal yang bergerak cepat dan lebih besar ( solitons), Seperti gerakan tanah yang terjadi di Teluk Lituya memproduksi suatu gelombang dengan tinggi 50- 150 m dan mencapai area pegunungan yang jaraknya 524 m. Bagaimanapun juga , suatu tanah longsor yang besar dapat menghasilkan megatsunami yang mungkin berdampak pada samudera.
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Karakteristik
Walaupun sering dikenal sebagai " ombak pasang surut", suatu tsunami tidak kelihatan seperti kesan populer " suatu gelombang/lambaian normal hanya dengan energi yang lebih besar". Sebagai gantinya kelihatan seperti suatu pasang yang bergerak maju cepat yang memaksa mencari jalan dengan caranya sendiri menghadapi segala rintangan. Kebanyakan kerusakan adalah disebabkan oleh massa air sangat besar di belakang muka gelombang sehingga ketinggian permukaan laut terus naik dan menyebabkan banjir yang kuat ke dalam kawasan pantai. Berat beban air yang sangat bertenaga ini dapat menyapu bersih semua yang ada. Benda besar seperti Kapal laut, batu besar, bahkan mobil bisa terangkat dan terbawa gelombang tsunami beberapa km jauhnya dari tempat asalnya.
Tsunami bertindak dengan cara yang berbeda dari gelombang laut atau badai ombak yang khas; tsunami adalah gejala perpindahan secara menyeluruh massa air di samudra ( sering dengan kedalaman beberapa kilometre) bukannya hanya di permukaan saja , sehingga tsunami ini berisi energi tak terukur, menyebar dengan kecepatan tinggi dan dapat bepergian dengan jarak yang sangat jauh dengan efisiensi yang tinggi dan kerugian energi yang seditik/kecil. Suatu tsunami yang terjadi di Aceh dapat menyebabkan kerusakan juga di India, Thailand dan bahkan sampai ke pantai Afrika merusak dan meluluhlantakkan semuanya dan pusat gempanya berada pada ribuan kilometres dari asal nya, ada beberapa saat jeda waktu akibat dari pergerakan gelombang tsunami di Samudera untuk mencapai daerah yang jauh dari asalnya. Jadi energi linier per meter pada gelombang berkurang dengan adanya tenaga invers terhadap jarak dari pusat gempabumi/tsunami. Ini dalam bentuk 2 dimensi tapi sama dengan hukum inverse bujursangar dalam tiga dimensi.
Peristiwa tunggal tsunami mungkin melibatkan satu rangkaian ombak dengan tinggi yang bervariasi; bisa juga disebut suatu rangkaian kereta api. Di laut lepas , tsunami mempunyai perioda waktu yang sangat panjang ( waktu untuk puncak gelombang berikut untuk meloloskan gelombang sebelumnya ), dari beberapa menit ke jam, dan dengan panjang gelombang sampai beberapa ratus kilometer. Ini adalah sangat berbeda dari gelombang yang dihasilkan oleh angin yang khas ,membengkak pada [atas] samudra, yang mungkin mempunyai masa sekitar 10 detik dengan panjang gelombang 150 meter.
Tingginya nyata dari suatu gelombang tsunami adalah sering kurang dari satu meter. Ini kenyataannya sering diabaikan oleh orang yang berada di atas kapal laut. Energi dari tsunami yang melewati kolom air sampai ke dasar laut , tidak sama dengan gelombang permukaan, yang mana secara khas hanya menjangkau sampai kedalaman sekitar 10 meteran.
Gelombang tsunami bergerak di samudra dengan kecepatan 500 hingga 1,000 km/h. Ketika gelombang tsunami mendekati daratan, lautan dangkal kecepatan gelombangnya berkurang sehingga muka gelombang menjadi pendek dan tinggi dan jarak antara tinggi gelombang (amplituda) menjadi lebih kecil. Sedangkan seseorang yang sedang berada di laut dalam air mungkin akan tidak akan merasakan adanya tsunami, gelombang tsunami dapat mencapai tinggi 30 m atau lebih ketika mendekati garis pantai dan memampatkannya . Proses dapat disamakan suatu cambuk yang bentuknya meruncing. Sewaktu gelombang bergerak dari pegangan yang besar ke ujung cambuk yang kecil, energi yang sama besar tersimpan dalam material atau benda yang kecil (ujung cambuk) yang bergerak dengan sangat cepat dan merusak segalanya pada waktu dia menerima energi yang sangat besar.
Suatu gelombang/lambaian menjadi gelombang laut dangkal' jika perbandingan antara kedalaman air dan panjang gelombangnya sangat kecil, dan tsunami mempunyai panjang gelombang yang besar ( beratus-ratus kilometer), tsunami dapat bertindak sebagai gelombang laut dangkal di laut dalam. Gelombang laut dangkal bergerak dengan t kecepatan yang memadai;sama dengan akar dua produk dari percepatan gravitasi ( 9.8 m/s2) dan kedalaman air. Sebagai Contoh, di samudra Pasifik, di mana kedalaman air adalah sekitar 4000 m, suatu tsunami bergerak dengan kecepatan sekitar 200 m/s ( 720 km/jam) dengan kerugian energi yang sedikit/kecil, bahkan dapat mencapai jarak ribuan kilometer. Pada kedalaman air 40 m, kecepatannya menjadi 20 meter/detik ( sekitar 72 km/jam), yang mana lebih lambat dibandingkan dengan kecepatan di samudra terbuka akan tetapi gelombang/lambaian akan tetap sukar untuk mendahuluinya. Bagaimanapun suatu dugaan ada untuk percepatan. Yang Menghantar " punuk/gundukan" lebih awal yang disebut "perubahan terus menerus daya gerak" sepadan dengan kepadatan dikalikan dengan pangkat dua percepatan itu. Ini memberi tekanan temporer yang penuh sepanjang gempa sebagai sepadan dengan satu kali atau dua kali dan di samping tekanan hidrostatik itu. hanya sayangnya tidak ada bukti untuk ini.
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). They also need not be 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.
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Tsunami wave
Ocean waves are normally divided into three groups, characterized by depth:
- Deep water
- Intermediate water
- Shallow water
Even though a tsunami is generated in deep water (around 4000 m below mean sea level), tsunami waves are considered shallow-water waves. As the tsunami wave approaches the shallow waters of shore, its time period remains the same, but its wavelength decreases rapidly, thus causing the water to pile up to form tremendous crests, in an effect known as "shoaling".
Tsunamis form "solitary waves", or waves with crests but no troughs — more like sand dunes than sine waves. Tsunami waves are also called "N-waves", as they resemble the English letter "N".
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Signs of an approaching tsunami
The following have at various times been associated with a tsunami [1]:
- An earthquake may be felt.
- Large quantities of gas may bubble to the water surface and make the sea look as if it is boiling.
- The water in the waves may be unusually hot.
- The water may smell of rotten eggs (hydrogen sulfide) or of petrol or oil.
- The water may sting the skin.
- A thunderous boom may be heard followed by
- a roaring noise as of a jet plane
- or a noise akin to the periodic whop-whop of a helicopter,
- or a whistling sound.
- The sea may recede to a considerable distance.
- A flash of red light might be seen near the horizon.
- As the wave approaches, the top of the wave may glow red.
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Warnings and prevention
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Tsunamis cannot be prevented or precisely predicted, but there are some warning signs of an impending tsunami, and there are many systems being developed and in use to reduce the damage from tsunamis.
In instances where the leading edge of the tsunami wave is its trough, the sea will recede from the coast half of the wave's period before the wave's arrival. If the slope is shallow, this recession can exceed many hundreds of metres. People unaware of the danger may remain at the shore due to curiosity, or for collecting fish from the exposed sea bed.
In instances where the leading edge of the tsunami is its first peak, succeeding waves can lead to further flooding. Again, being educated about a tsunami is important, to realise that when the water level drops the first time, the danger is not yet over. In a low-lying coastal area, a strong earthquake is a major warning sign that a tsunami may be produced.
Regions with a high risk of tsunamis may use tsunami warning systems to detect tsunamis and warn the general population before the wave reaches land. In some communities on the west coast of the United States, which is prone to Pacific Ocean tsunamis, warning signs advise people where to run in the event of an incoming tsunami. Computer models can roughly predict tsunami arrival and impact based on information about the event that triggered it and the shape of the seafloor (bathymetry) and coastal land (topography).[2]
One of the early warnings comes from nearby animals. Many animals sense danger and flee to higher ground before the water arrives. The Lisbon quake is the first documented case of such a phenomenon in Europe. The phenomenon was also noted in Sri Lanka in the 2004 Indian Ocean earthquake ([3]). Some scientists speculate that animals may have an ability to sense subsonic Rayleigh waves from an earthquake minutes or hours before a tsunami strikes shore (Kenneally, [4]).
While it is not possible to prevent a tsunami, in some particularly tsunami-prone countries some measures have been taken to reduce the damage caused on shore. Japan has implemented an extensive programme of building tsunami walls of up to 4.5 m (13.5 ft) high in front of populated coastal areas. Other localities have built floodgates and channels to redirect the water from incoming tsunamis. However, their effectiveness has been questioned, as tsunamis are often higher than the barriers. For instance, the tsunami which hit the island of Hokkaido on July 12, 1993 created waves as much as 30 m (100 ft) tall - as high as a 10-story building. The port town of Aonae was completely surrounded by a tsunami wall, but the waves washed right over the wall and destroyed all the wood-framed structures in the area. The wall may have succeeded in slowing down and moderating the height of the tsunami but it did not prevent major destruction and loss of life.
The effects of a tsunami can be mitigated by natural factors such as tree cover on the shoreline. Some locations in the path of the 2004 Indian Ocean tsunami escaped almost unscathed as a result of the tsunami's energy being sapped by a belt of trees such as coconut palms and mangroves. In one striking example, the village of Naluvedapathy in India's Tamil Nadu region suffered minimal damage and few deaths as the wave broke up on a forest of 80,244 trees planted along the shoreline in 2002 in a bid to enter the Guinness Book of Records. [5] Environmentalists have suggested tree planting along stretches of sea coast which are prone to tsunami risks. While it would take some years for the trees to grow to a useful size, such plantations could offer a much cheaper and longer-lasting means of tsunami mitigation than the costly and environmentally destructive method of erecting artificial barriers.
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Past tsunamis
See also List of historic tsunamis by death toll.
Tsunamis occur most frequently in the Pacific Ocean, but are a global phenomenon; they are possible wherever large bodies of water are found, including inland lakes, where they can be caused by landslides. Very small tsunamis, non-destructive and undetectable without specialized equipment, occur frequently as a result of minor earthquakes and other events.
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6100 B.C. and before
In the North Atlantic Ocean (Norwegian Sea), the Storegga Slides were a major series of sudden underwater land movements over the course of tens of thousands of years, which caused tsunamis and megatsunamis across a wide area.
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Circa 3,500 B.C.
The Passage of the Red Sea in the Torah may have been a real Megatsunami event. The eruption of the Santorini volcano would have generated a Megatsunami estimated at 600-foot. It could have caused waters to recede temporarily and drowned the Egyptian army when the waters returned. See, Cause of the Passage. [6]
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1607 - Bristol Channel, England and Wales
In 2002 it was suggested that the Bristol Channel floods of 1607 in England and Wales, UK, may have been caused by a tsunami.
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1700 - Vancouver Island, Canada
January 26 - The Cascadia Earthquake, one of the largest earthquakes on record, ruptures the Cascadia Subduction Zone offshore from Vancouver Island to northern California, creating a tsunami logged in Japan and oral traditions of the Native Americans.
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1755 - Lisbon, Portugal
Tens of thousands of Portuguese who survived the great 1755 Lisbon earthquake were killed by a tsunami which followed a half hour later. Many townspeople fled to the waterfront, believing the area safe from fires and from falling debris from aftershocks. Before the great wall of water hit the harbour, waters retreated, revealing lost cargo and forgotten shipwrecks.
The earthquake, tsunami, and subsequent fires killed more than a third of Lisbon's pre-quake population of 275,000. Historical records of explorations by Vasco da Gama and other early navigators were lost, and countless buildings were destroyed (including most examples of Portugal's Manueline architecture). Europeans of the 18th century struggled to understand the disaster within religious and rational belief systems. Philosophers of the Enlightenment, notably Voltaire, wrote about the event. The philosophical concept of the sublime, as described by philosopher Immanuel Kant in the Observations on the Feeling of the Beautiful and Sublime, took inspiration in part from attempts to comprehend the enormity of the Lisbon quake and tsunami.
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1883 - Krakatoa explosive eruption
The island volcano of Krakatoa in Indonesia exploded with devastating fury in 1883, blowing its underground magma chamber partly empty so that much overlying land and seabed collapsed into it. A series of large tsunami waves was generated from the explosion, some reaching a height of over 40 metres above sea level. Tsunami waves were observed throughout the Indian Ocean, the Pacific Ocean, the American West Coast, South America, and even as far away as the English Channel. On the facing coasts of Java and Sumatra the sea flood went many miles inland and caused such vast loss of life that one area was never resettled but went back to the jungle and is now the Ujung Kulon nature reserve.
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1929 - Newfoundland tsunami
On November 18, 1929, an earthquake of magnitude 7.2 occurred beneath the Laurentian Slope on the Grand Banks. The quake was felt throughout the Atlantic Provinces of Canada and as far west as Ottawa, Ontario and as far south as Claymont, Delaware. The resulting tsunami measured over 7 metres in height and took about 2½ hours to reach the Burin Peninsula on the south coast of Newfoundland, where 28 people lost their lives in various communities.
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1946 - Pacific tsunami
The Aleutian Island earthquake tsunami that killed 165 people on Hawaii and Alaska resulted in the creation of a tsunami warning system, established in 1949 for Pacific Ocean area countries. The tsunami is locally known in Hawaii as the April Fools Day Tsunami in Hawaii due to people thinking the warnings were an April Fools prank.
Note: The Pacific Tsunami Warning Center was established to track these killer waves and provide warning.
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1960 - Chilean tsunami
The Great Chilean Earthquake, at magnitude 9.5 the strongest earthquake ever recorded. Its epicenter off the coast of South Central Chile, generated one of the most destructive tsunamis of the 20th century.
It spread across the entire Pacific Ocean, with waves measuring up to 25 metres high. The first tsunami arrived at Hilo, Hawaii approximately 14.8 hrs after it originated off the coast of South Central Chile.
The highest wave at Hilo Bay was measured at around 10.7 m (35 ft.). 61 lives were lost allegedly due to people's failure to heed warning sirens. When the tsunami hit Onagawa, Japan, almost 22 hours after the quake, the wave height was 3 m above high tide. The number of people killed by the earthquake and subsequent tsunami is estimated to be between 490 and 2,290.
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1963 - Vajont Dam disaster
The reservoir behind the Vajont Dam in northern Italy was struck by an enormous landslide. A tsunami was triggered which swept over the top of the dam (without bursting it) and into the valley below. Nearly 2,000 people were killed.
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1964 - Good Friday tsunami
After the magnitude 9.2 Good Friday Earthquake, tsunamis struck Alaska, British Columbia, California and coastal Pacific Northwest towns, killing 121 people. The tsunamis were up to 6 m tall, and killed 11 people as far away as Crescent City, California.
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1976 - Moro Gulf tsunami
On August 16, 1976 at 12:11 A.M., a devastating earthquake of 7.9 earthquake hit the island of Mindanao, Philippines. It created a tsunami that devastated more than 700 kms of coastline bordering Moro Gulf in the North Celebes Sea. An estimated number of victims for this tragedy was 5,000 dead, 2,200 missing or presumed dead, more than 9,500 injured and a total of 93,500 people were left homeless. It devastated the cities and provinces of Pagadian City, Zamboanga del Sur, Zamboanga City, Basilan, Sulu, Sultan Kudarat, Maguindanao, Cotabato City, Lanao del Sur and Lanao del Norte.
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1979 - Tumaco tsunami
A magnitude 7.9 earthquake occurred on December 12, 1979 at 7:59:4.3 (UTC) along the Pacific coast of Colombia and Ecuador. The earthquake and the resulting tsunami caused the destruction of at least six fishing villages and the death of hundreds of people in the Colombian province of Nariño. The earthquake was felt in Bogotá, Cali, Popayán, Buenaventura and several other cities and towns in Colombia and in Guayaquil, Esmeraldas, Quito and other parts of Ecuador. When the Tumaco Tsunami hit the coast, it caused great destruction in the city of Tumaco, as well as in the small towns of El Charco, San Juan, Mosquera and Salahonda on the Pacific Coast of Colombia. The total number of victims of this tragedy was 259 dead, 798 wounded and 95 missing presumed dead.
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1993 - Okushiri tsunami
A devastating tsunami occurred off the coast of Hokkaido in Japan as a result of an earthquake on July 12, 1993. As a result, 202 people on the small island of Okushiri lost their lives, and hundreds more were missing or injured.
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2004 - Indian Ocean tsunami
Animation of the 2004 Indonesian Tsunami from NOAA/PMEL Tsunami Research Program
The 2004 Indian Ocean earthquake, which 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.[7] 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.
Unlike in the Pacific Ocean, 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.
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Other tsunamis in South Asia
| Tsunamis in South Asia (Source: Amateur Seismic Centre, India)[8] | ||||||
|---|---|---|---|---|---|---|
| Date | Location | |||||
| 1524 | Near Dabhol, Maharashtra | |||||
| 02 April 1762 | Arakan Coast, Myanmar | |||||
| 16 June 1819 | Rann of Kachchh, Gujarat | |||||
| 31 October 1847 | Great Nicobar Island | |||||
| 31 December 1881 | Car Nicobar Island | |||||
| 26 August 1883 | Krakatoa volcanic eruption | |||||
| 28 November 1945 | Mekran coast, Balochistan | |||||
| 26 December 2004 | Banda Aceh, Indonesia; Tamil Nadu, Kerala, Andhra Pradesh, Andaman and Nicobar Islands, India; Sri Lanka; Thailand; Malaysia; Maldives; Somalia; Kenya; Tanzania | |||||
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Other historical tsunamis
Other tsunamis that have occurred include the following:
- circa 500 C.E.: Poompuhar, Tamil Nadu, India, Maldives
- 1541, Brazil. It happened in the first European settlement in Brazil, São Vicente. There is no record of deaths or injuries, but the town was almost completely destroyed.
- January 20, 1606/1607: along the coast of the Bristol Channel (main article) thousands of people were drowned, houses and villages swept away, farmland was inundated and flocks were destroyed by a flood that might have been a tsunami. The cause of the flood remains disputed, it is quite possible that it was caused by a combination of meteorological extremes and tidal peaks (discussion).
- January 26, 1700: the Cascadia Earthquake (estimated 9.0 magnitude) caused massive tsunamis across the Pacific Northwest
- One of the worst tsunami disasters engulfed whole villages along Sanriku, Japan, in 1896. A wave more than seven stories tall (about 20 m) drowned some 26,000 people.
- 1946: An earthquake in the Aleutian Islands sent a tsunami to Hawaii, killing 159 people (five died in Alaska).
- July 9, 1958: A huge landslip caused the highest ever reported tsunami which was 524 metres high. This happened in the fjord shaped Lituya Bay, Alaska, USA. It travelled at over 150 km/h.
- May 26, 1983: 104 people in western Japan were killed by a tsunami spawned from a nearby earthquake.
- 17 July, 1998: A Papua New Guinea tsunami killed approximately 2200 people [9]. A 7.1 magnitude earthquake 24 km offshore was followed within 11 minutes by a tsunami about 12 m tall. While the magnitude of the quake was not large enough to create these waves directly, it is believed the earthquake generated an undersea landslide, which in turn caused the tsunami. The villages of Arop and Warapu were destroyed.
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North American and Caribbean tsunamis
- 14 November 1840 - Great Swell on the Delaware River
- 18 November 1867 - Virgin Islands
- 17 November 1872 - Maine
- 11 October 1918 - Puerto Rico
- 18 November 1929 - Newfoundland
- 9 January 1926 - Maine
- 4 August 1946 - Dominican Republic
- 18 August 1946 - Dominican Republic
Possible Tsunamis
- 35 Million years ago - Chesapeake Bay impact crater, Chesapeake Bay
- 9 June 1913 - Longport, NJ
- 6 August 1923 - Rockaway Park, Queens, NY .
- 8 August 1924 - Coney Island, NY .
- 19 August 1931 - Atlantic City, NJ
- 21 September 1938 - Hurricane, NJ coast.
- 19 May 1964 - Northeast USA
- 4 July 1992 - Daytona Beach, FL
Source: NOAA National Weather Service Forecast Office
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European tsunamis
- 16 October 1979 23 people died when the coast of Nice, France, was hit by a tsunami.
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See also
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References
- Dudley, Walter C. & Lee, Min (1988: 1st edition) Tsunami! ISBN 0-8248-1125-9 link
- Kenneally, Christine (December 30, 2004). "Surviving the Tsunami". Slate. link
- Macey, Richard (January 1, 2005). "The Big Bang that Triggered A Tragedy", The Sydney Morning Herald, p 11 - quoting Dr Mark Leonard, seismologist at Geoscience Australia.
- Lambourne, Helen (March 27, 2005). "Tsunami: Anatomy of a disaster". BBC News. link
- abelard.org. tsunamis: tsunamis travel fast but not at infinite speed. Website, retrieved March 29, 2005. link
- What about the famous image of a great canyon of water? Could this have any basis in reality?
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External links
Wikimedia Commons has media related to:
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Articles and websites
- Tsunami Forums
- NOVA: Wave That Shook The World site and special report shot within days of the 2004 Indian Ocean tsunami.
- Biggest Tsunami Countdown - Description of the five largest historical tsunamis
- NOAA Tsunami — General description of tsunamis and the United States agency NOAA's role in Tsunami hazard assessment, preparedness, education, forecasts & warnings, response and research.
- Can HF Radar detect Tsunamis? University of Hamburg HF-Radar
- Development Gateway Tsunami Special
- The Higher Ground Project Stories of children who survived the tsunami
- The International Centre for Geohazards (ICG)
- ITIC tsunami FAQ
- NOAA PMEL Tsunami Research Program (United States)
- USGS: Surviving a tsunami (United States)
- ITSU Coordination Group for Pacific Tsunami Warning System
- Pacific Tsunami Museum
- Tsunamis and Earthquakes
- Tsunami Centers - United States National Weather Service
- Tsunami in Sumatra (Germany; in German)
- Science of Tsunami Hazards journal
- The International Centre for Geohazards (ICG)
- The Indian Ocean tsunami and what it tells us about tsunamis in general
- Tsunami: Magnitude of Terror
- General Tsunamis Resources
- Natural Disasters - Tsunami Great research site for kids
- Envirtech Tsunami Warning System based on seabed seismics and sea level gauges.
[edit]
Images and video
See also: Images and video, 2004 Indian Ocean earthquake
- Large Collection of Amateur Tsunami Videos with Thunbnail Images and Detailed Descriptions
- 5 Amateur Camcorder Video Streams of the December 26, 2004 tsunami that hit Sri Lanka, Thailand and Indonesia.
- 2004 Asian Tsunami Satellite Images (Before and After)
- Satellite Images of Tsunami Affected Areas High resolution satellite images showing the effects of the 2004 tsunami on the affected areas in Indonesia, Thailand and Nicobar island of India.
- Computer-generated animation of a tsunami
- Animation of 1960 tsunami originating outside coast of Chile
- The Survivors - A moving travelogue full of stunning images along the tsunami ravaged South-Western Coast of India
[edit]
Fiction
- Michael Crichton's State of Fear (2004) explores unintended consequences of human intervention with natural forces. [10]
diambil dari http://science.howstuffworks.com/tsunami.htm/printable:
| How Tsunamis Work | ||
| by Nathan Halabrin and Robert Valdes | ||
On December 26, 2004, a massive underwater earthquake off the coast of Indonesia's Sumatra Island rattled the Earth in its orbit. The quake measuring 9.0 on the Richter scale is the largest one since 1964. Dozens of aftershocks with magnitudes of 5.0 or higher occurred in the following days. But the most powerful and destructive aftermath of this devastating earthquake is the tsunami that it caused. The death toll has reached higher than 280,000, and many communities suffered devastating property damage.
 Photo courtesy DigitalGlobe The shore of Banda Aceh, Sumatra, before and after the 2004 tsunami |
 Photo courtesy DigitalGlobe Banda Aceh northern shore detail, 2004, before and after the tsunami |
The devastation of this tsunami overshadowed the devastation of any other tsunami we've seen in recent history, but scientifically, the course of events followed the same basic sequence of a typical tsunami. In this article, we'll look at what causes tsunamis, the physics that drives them and the effects of a tsunami strike. We will also examine scientists' worldwide efforts to monitor and predict tsunamis in order to avoid disasters like the one that occurred in the final days of 2004.
Anatomy of a Wave
The word "tsunami" is from the Japanese words tsu (harbor) and nami (waves). A tsunami is a wave or series of waves in the ocean that can be hundreds of miles long and have been known to reach heights of up to 34 ft (10.5 m). These "walls of water" travel as fast or faster than a commercial jet. The massive December 26, 2004 tsunami traveled 375 miles (600 km) in 75 minutes. That's 300 mph (480 kph). These walls of water are capable of inflicting massive damage along coastal lands.
 Photo courtesy DigitalGlobe Tsunami strikes Sri Lanka, December 26, 2004 |
In order to understand tsunamis, let's first look at waves in general. Most of us are familiar with waves from days at the beach or local water park wave pools. Waves are made up of a crest (the highest point of the wave) and a trough (the lowest point of the wave). Waves are measured in two ways:
- The wave height is the distance between the crest and trough.
- The wave length is the horizontal distance between two consecutive wave crests.
 Photo courtesy U.S. Navy Anatomy of a normal wave |
The frequency of waves is measured by the time it takes for two consecutive waves to cross the same point. This is called the wave period.
Tsunamis and normal waves have all of the same parts and are measured in the same ways, but there are many differences between the two. The chart below shows some of the differences.
vs. Typical Wind-generated Wave | ||
| Wave Feature | Wind-generated Wave | Tsunami Wave |
| Wave Speed | 5-60 mph (8-100 kph) | 500-600 mph (800-1,000 kph) |
| Wave Period (time required for two waves to pass a single point in space) | 5 to 20 seconds apart | 10 minutes to 2 hours apart |
| Wave Length (horizontal distance between two waves) | 300-600 feet apart (100-200 meters apart) | 60-300 miles apart (100-500 km apart) |
The primary differences are size, speed and source. Let's look at what creates a normal wave.
Waves in the ocean are created by a number of things (gravitational pull, underwater activity, atmospheric pressure), but the most common source for waves is the wind.
 Photo courtesy DigitalGlobe Banda Aceh flooding, 2004 |
 Photo courtesy DigitalGlobe Banda Aceh Grand Mosque, before and after the 2004 tsunami |
When the wind blows across a smooth water surface, the air molecules grab water molecules as they are carried across the water by the wind. The friction between the air and water stretches the water's surface, creating ripples in the water known as capillary waves. The capillary waves move in circles. This circular motion of water continues vertically underwater, though the power of this motion decreases in deeper water. As the wave travels, more and more water molecules are collected, increasing the size and momentum of the wave. The most important thing to know about waves is that they do not represent the movement of water, but instead show the movement of energy through water.
In normal waves, the wind is the source of that energy. The size and speed of wind waves is dependant on the strength of the wind.
In the next section, we'll look at what causes tsunamis.
The Birth of a Tsunami
The most common causes of tsunamis are underwater earthquakes. To understand underwater earthquakes, you must first understand plate tectonics. The theory of plate tectonics suggests that the lithosphere, or top layer of the Earth, is made up of a series of huge plates. These plates make up the continents and seafloor. They rest on an underlying viscous layer called the asthenosphere.
Think of a pie cut into eight slices. The pie crust would be the lithosphere and the hot, sticky pie filling underneath would be the asthenosphere. On the Earth, these plates are constantly in motion, moving along each other at a speed of 1 to 2 inches (2.5-5 cm) per year. The movement occurs most dramatically along fault lines (where the pie is cut). These motions are capable of producing earthquakes and volcanism, which, when they occur at the bottom of the ocean, are two possible sources of tsunamis.
 Formation of a tsunami |
When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one. This is called subduction. Underwater subduction often leaves enormous "handprints" in the form of deep ocean trenches along the seafloor.
In some cases of subduction, part of the seafloor connected to the lighter plate may "snap up" suddenly due to pressure from the sinking plate. This results in an earthquake. The focus of the earthquake is the point within the Earth where the rupture first occurs, rocks break and the first seismic waves are generated. The epicenter is the point on the seafloor directly above the focus.
When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes the water upward above normal sea level. This is the birth of a tsunami. The earthquake that generated the December 26, 2004, tsunami in the Indian Ocean was a 9.0 on the Richter scale -- one of the biggest in recorded history.
Hitting the Water
Once the water has been pushed upward, gravity acts on it, forcing the energy out horizontally along the surface of the water. It's sort of the same ripple effect you get from throwing a pebble in the water, but in reverse: The energy is generated by a force moving out of rather than into the water. The energy then moves through the depths of the water and away from the initial disturbance.
The tremendous force created by the seismic disturbance generates the tsunami's incredible speed. The actual speed of the tsunami is calculated by measuring the water depth at a point in time when the tsunami passes by. The speed is the square root of the product of acceleration of gravity and the quantity of water depth, or:
- t = square root (g x d)
t = tsunami speed in meters per second
g = acceleration of gravity (32 feet/10 meters per second/per second)
d = quantity of water depth
Photo courtesy NOAA
An animation of the 2004 Indian Ocean tsunami
A tsunami's ability to maintain speed is directly influenced by the depth of the water. A tsunami moves faster in deeper water and slower in shallower water. So unlike a normal wave, the driving energy of a tsunami moves through the water as opposed to on top of it. As a result, as a tsunami moves though deep water at hundreds of miles an hour, it is barely noticeable above the waterline. A tsunami is typically no more than 3 feet (1 meter) high until it gets close to shore.
Once a tsunami gets close to shore, it takes its more recognizable and deadly form.
Photo courtesy USGS
Simulation of a theoretical tsunami in the Pacific
A distant tsunami travels more than 600 miles (1,000 km) from the source area before it reaches land. Distant tsunamis are more likely to occur in the Pacific Ocean and are capable of traveling across the entire ocean in less than one day. Since distant tsunamis make such long trips with a relatively constant speed, experts can predict their arrival with a fair degree of accuracy. This makes it easy to warn and evacuate people that could be affected by the wave. A local tsunami travels toward nearby coastal lands within 60 miles (100 km) of the source. Local tsunamis are usually the result of submarine landslides and typically occur in a bay or harbor. Local tsunamis are particularly dangerous because they can reach land within a matter of minutes. This type of "sneak attack" makes it hard to warn the public about the tsunami's approach. |
Landfall
When a tsunami reaches land, it hits shallower water. The shallow water and coastal land acts to compress the energy traveling through the water. This starts the transformation of the tsunami.
The topography of the seafloor and shape of the shore begins to affect the tsunami's appearance and behavior. Also, as the velocity of the wave diminishes, the wave height increases considerably -- the compressed energy forces the water upward. A typical tsunami approaching land will slow down to speeds around 30 miles per hour (50 kph), and the wave heights can reach up to 90 feet (30 meters) above sea level. As the wave heights increase during this process, the wave lengths shorten considerably. (Think of squeezing an accordion.)
A witness on the beach will see a noticeable rise and fall of beach water when a tsunami is imminent. Sometimes, the coastal water will disappear completely as it is drawn into the tsunami. This amazing event is followed by the actual trough of the tsunami reaching shore. Tsunamis most often arrive as a series of strong and fast floods of water, not one single, enormous wave. However, a bore, which is a large vertical wave that arrives with a churning front, may appear. Bores are often followed by rapid floods of water, which make them particularly destructive. Other waves can follow anywhere from five to 90 minutes after the initial strike -- the tsunami wave train, after traveling as a series of waves over a long distance, is releasing itself onto land.
Tsunamis typically result in staggering body counts. This is especially true when they strike without warning. Tsunamis can level development and strip away coastlines, pulling everything in their path out to sea.
 Photo courtesy National Geophysical Data Center Tsunami breaking over pier in Hilo, Hawaii |
 Photo courtesy National Geophysical Data Center Wreckage of a political party clubhouse from a tsunami that hit the Aleutian Islands in 1946 |
The areas of greatest risk during a tsunami strike are within 1 mile (1.6 km) of the shoreline, due to the flooding and scattered debris, and less than 50 feet (15 m) above sea level, due to the height of the striking waves.
A tsunami is even capable of reaching sheltered areas due to varying land features and the underlying seascape. For instance, a protected bay area with a narrow inlet can give a tsunami a "funnel" to travel through, amplifying the destructive power of the waves. Also, a river channel can provide room for a tsunami bore to rush through, allowing it to flood tremendous tracts of land.
Until a tsunami strikes, it's difficult to predict how it will interact with the features of the affected land. The wrap-around effect occurs along island coastlines when multiple wave strikes hit different areas of surrounding land, resulting in different degrees of flooding. Harbor resonance is a chaotic and highly destructive tsunami side effect created when waves continuously reflect and bounce off of the edges of a harbor or bay. Harbor resonance can cause the amplification of circulating wave heights and even increase the duration of the wave activity within the area.
|
December 26, 2004
On December 26, 2004, the world's most powerful earthquake in more than 40 years struck deep under the Indian Ocean off the west coast of Sumatra, triggering a massive tsunami. One of the things that made this event particularly destructive is that the tsunamis struck relatively well-populated areas in the middle of the tourist-packed holiday season. Here is a timeline of the disaster:
12:59 am GMT - A massive 9.0 earthquake occurs in the Indian Ocean off Sumatra, Indonesia. The quake is so large it is felt in the neighboring countries of Thailand, Malaysia and Singapore. Huge buildings in the Thai capital of Bangkok shake under the force of the earthquake and its aftershocks. Bangkok is 1,242 miles (nearly 2,000 km) from where the earthquake took place.
1:07 am GMT - After the quake, stations in Australia alert the NOAA Pacific Tsunami Warning Center of the earthquake and the potential tsunami threat. There are widely conflicting reports from different sources about the size of the quake. Different reports put the earthquake at magnitudes varying between 6.6 and 8.9. At the same time, an Indonesian radio station reports the death of nine villagers as the result of a tidal wave.
 Photo courtesy DigitalGlobe Kalutara, Sri Lanka - receding waters |
 Photo courtesy DigitalGlobe Kalutara flooding close-up - receding waters |
2:27 am GMT - The massive waves hit Kalmunai, Sri Lanka.
2:30 am GMT - Kattankudy is hit. By now, almost the entire east coast of Sri Lanka is under 9 feet (2.7 meters) of water.
2:40 am GMT - Over the next 15 minutes, Batticaloa, Mullaitivu and Trincomalee, Sri Lanka, are hit. Yala, Thailand, is also struck by the tsunami. Though it has not yet been reported, more than 15,000 people have died.
2:57.am GMT - News wire services release the first report: "Earthquake sets off big waves." Reports of heavy damage and fatalities began to come in from Thailand's Phukit resort area. The official death toll is still nine. All reports are unclear, and the numbers are unconfirmed.
 Photo courtesy DigitalGlobe Beach damage in Kalutara |
3:00 am GMT - An AFP news correspondent in Colombo, Sri Lanka, gets a phone call from reporters in Trinco: "The sea is coming in." In the same moment, Valvettiturai and Hambantota are hit. Almost 7,000 people are washed out to sea.
3:15 am GMT - A Washington Post correspondent reports a tsunami hitting Weligama, Sri Lanka. The Sri Lankan provinces of Matara, Galle and Panadura are also hit. Another 5,000 people die.
3:20 am GMT - Loaded with European tourists, the Sri Lankan resort Rae of Kalutara is hit. Satellite imagery reveals the water reaching 500 yards (460 meters) in from the shore line.
3:30 am GMT - The AFP news correspondent in Colombo gets a call from Matara indicating that a second round of waves is coming. Waves hit the Indian coast. PTWC begins getting wire reports from the Internet about Sri Lankan casualties. Negombo, Sri Lanka, is hit.
3:46 am GMT - AFP news reports massive casualties and numbers of homeless in Sri Lanka.
4:11 am GMT - Rapidly rising water levels in India damage the coastline. Some small tremors are felt.
5:00 am GMT - PTWC advises the U.S. Pacific Command in Hawaii of the potential threat of more tsunamis in the western Indian Ocean.
5:13 am GMT - Sri Lanka deploys the military and asks India for help.
5:41 am GMT - The Prime Minister of Thailand orders the evacuation of three major provinces, including Phukit.
6:09 am GMT - In a few hours, the tsunami has all but crossed the ocean, flooding Male, the capitol of Maldives.
7:15 am GMT - The PTWC advises the U.S. State Department on the continuing threat of tsunamis in Madagascar and Africa. In the next few hours, organized relief efforts begin.
Tsunami Prediction
Scientists are constantly trying to learn new ways to predict the behavior of tsunamis. At this point, most data is gathered after a tsunami has already done its damage.
In a post-tsunami survey, a number of things are measured. Scientists are particularly interested in the inundation and run-up features after the waves strike land. Inundation is the maximum horizontal distance penetrated inland. Run-up is the maximum vertical distance above the sea level that the waves reached. Inundation and run-up are often determined by measuring the distance of killed vegetation, scattered debris along the land and eyewitness accounts of the incident.
 Photo courtesy National Geophysical Data Center Tsunami generated by earthquake of April 1, 1946, Aleutian Islands, Alaska. On the left, a building before the tsunami. On the right, the same building after the tsunami. |
 Photo courtesy National Geophysical Data Center Damage from tsunami generated by earthquake of May 22, 1960, coast of Chile |
Scientists have made great strides in monitoring and predicting the ongoing threat of tsunamis. One center continuously monitoring seismic events and changes in the tide level is the Pacific Tsunami Warning Center (PTWC). The PTWC is located in Ewa Beach, HI, and services the Hawaiian Islands and surrounding U.S. territories by working in conjunction with other regional centers. The West Coast & Alaska Tsunami Warning Center (ATWC) in Palmer, AK, serves the Aleutian Islands area along with British Columbia, Washington state, Oregon and California. This center is of particular importance because submarine earthquakes in this region have created waves that moved throughout the Pacific Ocean before striking elsewhere.
Tsunamis are detected by open-ocean buoys and coastal tide gauges, which report information to stations within the region. Tide stations measure minute changes in sea level, and seismograph stations record earthquake activity. A tsunami watch goes into effect if a center detects an earthquake at 7.5 or higher on the Richter scale. Civil defense agencies are then notified, and data from tidal gauge stations are closely monitored. If a threatening tsunami passes through and is noted by the gauge stations, a tsunami warning is issued to all potentially affected areas. Evacuation procedures in these areas are then implemented.
The Deep-Ocean Assessment and Reporting of Tsunamis (DART) uses unique pressure recorders that sit on the ocean bottom. These recorders are used to detect slight changes in the overlying water pressure. The DART system is capable of detecting a tsunami as small as a centimeter high above the sea level.
 Photo courtesy NOAA DART recording device |
The most serious problem facing humans is the fact that tsunami waves, once in motion, cannot be stopped. Scientists and civil agencies can only devote resources to predicting tsunamis and creating effective plans for protecting coastal areas from their ravages.
For more information on tsunamis and related topics, check out the links on the next page.
Lots More Information
Related HowStuffWorks Articles
- How Rip Currents Work
- How Earthquakes Work
- How Volcanoes Work
- Harrowing Survival Stories
- How Wave Pools Work
| More Great Links |  |
- International Tsunami Information Center
- National Oceanic and Atmospheric Administration
- Tsunami Physics
- Pacific Tsunami Warning Center
- Federal Emergency Management Agency: Tsunami Hazards
- Pacific Marine Environmental Laboratory: Tsunami Links
- Yahoo News: Quake rattled Earth orbit, changed map of Asia - December 28, 2004
- Tsunami Relief Organizations
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