What is a tsunami?
A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean, such as:• earthquakes
• landslide
• volcanic eruptions
• explosions
• meteorites
These disturbances can either be from below (e.g. underwater earthquakes with large vertical displacements, submarine landslides) or from above (e.g. meteorite impacts).
Tsunami is a Japanese word with the English translation: "harbour wave". In the past, tsunamis have been referred to as "tidal waves" or "seismic sea waves". The term "tidal wave" is misleading; even though a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. (Tides result from the gravitational influences of the moon, sun, and planets.) The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.
Tsunamis are also often confused with storm surges, even though they are quite different phenomena. A storm surge is a rapid rise in coastal sea-level caused by a significant meteorological event - these are often associated with tropical cyclones.
The physics of a tsunami
Tsunamis can have wavelengths ranging from 10 to 500 km and wave periods of up to an hour. As a result of their long wavelengths, tsunamis act as shallow-water waves. A wave becomes a shallow-water wave when the wavelength is very large compared to the water depth. Shallow-water waves move at a speed, c, that is dependent upon the water depth and is given by the formula:
where g is the acceleration due to gravity (= 9.8 m/s2) and H is the depth of water.
In the deep ocean, the typical water depth is around 4000 m, so a tsunami will therefore travel at around 200 m/s, or more than 700 km/h.
For tsunamis that are generated by underwater earthquakes, the amplitude (i.e wave height) of the tsunami is determined by the amount by which the sea-floor is displaced. Similarly, the wavelength and period of the tsunami are determined by the size and shape of the underwater disturbance.
As well as travelling at high speeds, tsunamis can also travel large distances with limited energy losses. As the tsunami propagates across the ocean, the wave crests can undergo refraction (bending), which is caused by segments of the wave moving at different speeds as the water depth along the wave crest varies.
What happens to a tsunami as it approaches land?
As a tsunami leaves the deep water of the open-ocean and travels into the shallower water near the coast, it transforms. If you read the "The physics of a tsunami" section, you will know that a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. 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, its height grows. This is called shoaling. Because of this shoaling effect, a tsunami that is unnoticeable at sea, may grow to be several metres or more in height near the coast.
The increase of the tsunami's waveheight as it enters shallow water is given by:
where hs and hd are waveheights in shallow and deep water and Hs and Hd are the depths of the shallow and deep water. So a tsunami with a height of 1 m in the open ocean where the water depth is 4000m would have a waveheight of 4 to 5 m in water of depth 10 m.
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. Depending on whether the first part of the tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide. Local bathymetry may also cause the tsunami to appear as a series of breaking waves.
Tsunamis have great erosion potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of metres inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of tens of metres.
How are tsunamis measured or observed?
In the deep ocean, a tsunami has a small amplitude (less than 1 metre) but very long wavelength (hundreds of kilometres). This means that the slope, or steepness of the wave is very small, so it is practically undetectable to the human eye. However, there are ocean observing instruments that are able to detect tsunamis.
Tide Gauges
Tide gauges measure the height of the sea-surface and are primarily used for measuring tide levels. Most of the tide gauges operated by the Bureau of Meteorology's National Tidal Centre are SEAFRAME stations (Sea Level Fine Resolution Acoustic Measuring Equipment). These consist of an acoustic sensor connected to a vertical tube open at the lower end which is in the water. The acoustic sensor emits a sound pulse which travels from the top of the tube down to the water surface, and is then reflected back up the tube. The distance to the water level can then be calculated using the travel time of the pulse. This system filters out small-scale effects like wind-waves and has the capacity to measure sea-level changes within 1mm accuracy.
The tide gauge at Cocos Island observed the tsunami on December 26th 2004 as it passed by the island, as shown in these observations made during December.
Satellites
Satellite altimeters measure the height of the ocean surface directly by the use of electro-magnetic pulses. These are sent down to the ocean surface from the satellite and the height of the ocean surface can be determined by knowing the speed of the pulse, the location of the satellite and measuring the time that the pulse takes to return to the satellite. One problem with this kind of satellite data is that it can be very sparse - some satellites only pass over a particular location about once a month, so you would be lucky to spot a tsunami since they travel so quickly. However, during the Indian Ocean tsunami of December 26th 2004, the Jason satellite altimeter happened to be in the right place at the right time.
The picture below shows the height of the sea surface (in blue) measured by the Jason satellite two hours after the initial earthquake hit the region southeast of Sumatra (shown in red) on December 26, 2004. The data were taken by a radar altimeter on board the satellite along a track traversing the Indian Ocean when the tsunami waves had just filled the entire Bay of Bengal. The data shown are the differences in sea surface height from previous observations made along the same track 20-30 days before the earthquake, showing the signals of the tsunami.
Picture courtesy of NASA/JPL-Caltech
The DART System
In 1995 the National Oceanic and Atmospheric Administration (NOAA) began developing the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. An array of stations is currently deployed in the Pacific Ocean. These stations give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder which detects the passage of a tsunami. (The pressure of the water column is related to the height of the sea-surface) . The data is then transmitted to a surface buoy via sonar. The surface buoy then radios the information to the Pacific Tsunami Warning Center (PTWC) via satellite. The bottom pressure recorder lasts for two years while the surface buoy is replaced every year. The system has considerably improved the forecasting and warning of tsunamis in the Pacific.
The Indian Ocean tsunami of 26th December 2004
An undersea earthquake in the Indian Ocean on 26th December 2004 produced a tsunami that caused one of the biggest natural disasters in modern history. Over 200,000 people are known to have lost their lives.
The waves devastated the shores of parts of Indonesia, Sri Lanka, India, Thailand and other countries with waves reported up to 15 m high, reaching as far as Somalia on the east coast of Africa, 4500 km west of the epicentre. Refraction and diffraction of the waves meant that the impact of the tsunami was noticed around the world and sea-level monitoring stations in places such as Brazil and Queensland also felt the effect of the tsunami.
This animation (10.4Mb) was produced by scientists in the Bureau of Meteorology's National Tidal Centre. A numerical model was used to replicate the generation and propagation of the tsunami and it shows how the waves propagated around the world's ocean basins.
The earthquake took place at about 1am UTC (8am local time) in the Indian Ocean off the western coast of northern Sumatra. With a magnitude of 9.0 on the Richter scale, it was the largest since the 1964 earthquake off Alaska and equal fourth largest since 1900, when accurate global seismographic record-keeping began.
The epicentre of the earthquake was located about 250 km south-southeast of the Indonesian city of Banda Aceh. It was a rare megathrust earthquake and occurred on the interface of the India and Burma tectonic plates. This was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. A megathrust earthquake is where one tectonic plate slips beneath another, causing vertical motion of the plates. This large vertical displacement of the sea-floor generated the devastating tsunami, which caused damage over such a large area around the Indian Ocean.
The earthquake was also unusually large in geographical extent. An estimated 1200 km of faultline slipped about 15 m along the subduction zone over a period of several minutes. Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the waves was in an east-west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had very few casualties despite being a populous low-lying country.
Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines. (See this travel time map). The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.
On its arrival on shore, the height of the tsunami varied greatly, depending on its distance and direction from the epicentre and other factors such as the local bathymetry. Reports have the height ranging form 2-3 m at the African coast (Kenya) up to 10-15 m at Sumatra, the region closest to the epicentre.
Warnings and predictions
See also: Tsunami warning system
Tsunami warning sign
Drawbacks can serve as a brief warning. People who observe drawback (many survivors report an accompanying sucking sound), can survive only if they immediately run for high ground or seek the upper floors of nearby buildings. In 2004, ten-year old Tilly Smith of Surrey, England, was onMaikhao beach in Phuket, Thailand with her parents and sister, and having learned about tsunamis recently in school, told her family that a tsunami might be imminent. Her parents warned others minutes before the wave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney.
In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other east-facing coasts that it reached. This was because the wave moved downwards on the eastern side of the fault line and upwards on the western side. The western pulse hit coastal Africa and other western areas.
A tsunami cannot be precisely predicted, even if the magnitude and location of an earthquake is known. Geologists, oceanographers, and seismologists analyse each earthquake and based on many factors may or may not issue a tsunami warning. However, there are some warning signs of an impending tsunami, and automated systems can provide warnings immediately after an earthquake in time to save lives. One of the most successful systems uses bottom pressure sensors, attached to buoys, which constantly monitor the pressure of the overlying water column.
Regions with a high tsunami risk typically use tsunami warning systems to warn the population before the wave reaches land. On the west coast of the United States, which is prone to Pacific Ocean tsunami, warning signs indicate evacuation routes. In Japan, the community is well-educated about earthquakes and tsunamis, and along the Japanese shorelines the tsunami warning signs are reminders of the natural hazards together with a network of warning sirens, typically at the top of the cliff of surroundings hills.[30]
The Pacific Tsunami Warning System is based in Honolulu, Hawaiʻi. It monitors Pacific Ocean seismic activity. A sufficiently large earthquake magnitude and other information triggers a tsunami warning. While the subduction zones around the Pacific are seismically active, not all earthquakes generate tsunami. Computers assist in analysing the tsunami risk of every earthquake that occurs in the Pacific Ocean and the adjoining land masses.
सुनामी
सुनामी चेतावनी
सुनामी आने से पहले
सुनामी के दौरान
सुनामी आने के बाद
न्यूजीलैंड का पूरा समुद्री तट सुनामी के खतरे में है। सुनामी समुद्री किनारों पर तीव्रता से बाढ़ ला सकती है, जिसके कारण सम्प त्ति का विनाशकारी नुकसान, चोटें और जनहानि हो सकती है।
सुनामी लहरों की श्रृंखला से मिलकर बनी एक प्राकृतिक घटना है जो तब उत्पिन्नी होती हैं जब समुद्र या झील में जल की व्याकपक मात्रा तेजी से विस्थाीपित होती है। सुनामी विशाल पनडुब्बी या तटीय भूकंप, पानी के नीचे भूस्खसलन जो भूकम्पल या ज्वाालामुखीय गतिविधि के कारण भी हो सकता है, बड़ी तटीय चट्टान या झील के किनारे के भूस्खंलन, या समुद्र की तलहटी के नीचे या इसके समीप ज्वालामुखीय उदगार आदि के द्वारा भी उत्पोन्नय हो सकती हैं।
सुनामी के तीन प्रकार
सुनामी तीन प्रकार की होती हैं। आप किस प्रकार की सुनामी का सामना करेंगे, यह सुनामी उत्प न्न होने वाले स्थामन से आप की दूरी पर निर्भर करता है।
• दूरस्था सुनामी दूर लम्बेक रास्ते में उत्प न्नु होती है, जैसे कि चिली में प्रशांत के उस पार से। ऐसी अवस्था में हमारे पास न्यूसजीलैंड के लिए चेतावनी का तीन घंटे से अधिक समय होगा।
• क्षेत्रीय सुनामी अपने गंतव्यह से दूर एक से तीन घंटे की यात्रा समय के बीच उत्प न्न, होती हैं। न्यू जीलैंड के उत्तर करमाडेक गर्त में पानी के नीचे ज्वा लामुखी उदगार क्षेत्रीय सुनामी उत्पीन्नर कर सकता है।
• स्थानीय सुनामी न्यूजीलैंड के बहुत ही करीब उत्पन्न होती हैं। इस प्रकार की सुनामी बहुत खतरनाक है क्योंीकि हमारे पास चेतावनी के केवल कुछ मिनट ही हो सकते हैं।
सुनामी चेतावनी
सम्भाीवित सुनामी के बारे में चेतावनी संदेश और संकेत अनेक स्रोतों से आ सकते हैं - प्राकृतिक, आधिकारिक या गैरआधिकारिक।
प्राकृतिक चेतावनी
स्थानीय स्रोत आधारित सुनामी के लिए, जो मिनटों में पहुंच सकती है, आधिकरिक चेतावनी के लिए समय नहीं होगा। ऐसे में प्राकृतिक चेतावनी के संकेतों को पहचानना और जल्दीे से कार्य करना महत्वचपूर्ण है।
अधिक जानकारी के लिए दायीं ओर के पैनल को देखें
आधिकारिक चेतावनी
आधिकारिक चेतावनियां केवल दूर की और क्षेत्रीय स्रोत आधारित सुनामी के लिए ही संभव हैं। सरकारी चेतावनियां नागरिक प्रतिरक्षा एवं आपदा प्रबंधन मंत्रालय द्वारा राष्ट्री य मीडिया, स्थालनीय प्राधिकारियों एवं अन्यप प्रमुख जिम्मेेदार एजेंसियों को प्रसारित की जाती हैं। आपकी स्थानीय काउंसिल स्थाानीय मीडिया, साइरन और संभवत: अन्यक स्थाएनीय व्य वस्थाेओं के द्वारा भी चेतावनियों को जारी कर सकती है।
गैरआधिकारिक या अनौपचारिक चेतावनी
आप दोस्तों, अन्या सामान्य जनों, अंतर्राष्ट्री य मीडिया और इंटरनेट से चेतावनियों को प्राप्त कर सकते हैं। चेतावनी को तभी सत्या।पित करें यदि आप बहुत जल्दी से ऐसा कर सकें। यदि आधिकारिक चेतावनी मिल सके, तो अनौपचारिक चेतावनियों के बजाय उस पर ज्यापदा भरोसा करें।
सुनामी आने से पहले
सुनामी के आने से पहले की तैयारी आपके घर और कारोबार के नुकसान को कम करने में और आपको जीवित रहने में मदद करेगी।
• यदि आप एक तटीय क्षेत्र में रहते हैं, तो अपने सुनामी के जोखिम और स्थाआनीय चेतावनी व्यदवस्थाबओं के बारे में अपनी काउंसिल से जानकारी लें।
• यदि आप विकलांग या विशिष्टं आवश्यबकताओं वाले व्य क्ति हैं, तो अपने आपको किसी चेतावनी व आपातकालीन प्रसारणों के बारे में सतर्क रखने के लिए अपने सहयोगी नेटवर्क में व्य वस्था करें।
• घरेलू आपातकालीन योजना बनाएं और गेटअवे किट तैयार रखें।
• पता करें कि नजदीक में कहां पर जमीन ऊंची है और आप वहां कैसे पहुंच पाएंगे। आप जितना कर सकें उतना ऊंचाई पर या तट से दूर आंतरिक क्षेत्र में पहुंचने की योजना बनाएं। जब आप घर पर हो, या आप काम पर हों या तट के निकट छुट्टियां मना रहें हो तो इन सब हालात के लिए अपने बचाव मार्ग की योजना बनाएं।
सुनामी के दौरान
• यदि संभव हो तो अपनी गेटअवे किट को अपने साथ ले लें। अपनी किट या सामान उठाने के लिए जोखिम वाले क्षेत्रों में न जाएं।
• अपने पालतू पशुओं को साथ ले लें यदि आप सुरक्षित रूप से ऐसा कर सकें।
• तुरंत जितना हो सके नजदीक की सबसे ऊंची भूमि की ओर जाएं, या तट से दूर आंतरिक क्षेत्र में पहुंचें। यदि स्थामन खाली करके बाहर जाने नक्शा मौजूद है, तो दिखाए गए मार्गों का अनुसरण करें।
• यदि संभव हो तो पैदल या साइकिल से जाएं और यदि बहुत जरूरी हो तभी गाड़ी चलाएं। यदि ड्राइविंग करते हुए आप स्थांन खाली कराए जाने वाले क्षेत्र से सुरक्षित बाहर निकल जाते हैं तो अपने पीछे आने वालों को जगह देने के लिए चलना जारी रखें।
• यदि आप सुनामी से बच कर भाग नहीं सकें, तो किसी मजबूत इमारत की ऊपरी मंजिल पर या छत पर या पेड़ के ऊपर चढ़ जाएं, या तैरती वस्तु को पक,ड़ ले और सहायता पहुंचने तक लगातार मजबूती से पकड़े रहें।
• नौकाएं तट की अपेक्षा आमतौर पर 20 मीटर से अधिक गहरे पानी में अधिक सुरक्षित होती हैं। नाव को समुद्र में ले जायें जब इसके लिए समय हो और ऐसा करना सुरक्षित हो।
• सुनामी देखने के लिए कभी भी किनारे पर मत जाएं। जोखिम वाले क्षेत्रों से तब तक दूर रहें जब तक खतरा पूरी तरह टलने की आधिकारिक चेतावनी न जारी कर दी जाए।
• अपने स्थावनीय रेडियो केन्द्रों को सुनें जहां आपातस्थिति प्रबंधन कर्मचारी, आपके समुदाय और परिस्थिति के लिए सबसे उपयुक्तण सलाह जारी करेंगे।
सुनामी आने के बाद
• नागरिक प्रतिरक्षा सलाह के लिए लगातार रेडियो को सुने और तब तक खाली किए गये क्षेत्र में वापस न जाएं जब तक खतरा पूरी तरह टलने की आधिकारिक चेतावनी न जारी कर दी जाए।
• सावधान रहें क्योंाकि एक से अधिक लहरें हो सकती हैं और यह 24 घंटे या अधिक समय तक सुरक्षित नहीं भी हो सकता है। पहली लहर के बाद में आने वाली लहरें और बड़ी हो सकती हैं।
• यदि चोट लगी हो तो अपनी जांच करें और जरूरी हो तो प्राथमिक चिकित्साख प्राप्तट करें। दूसरों की मदद करें यदि आप ऐसा कर सकें।
• इसके दृश्यी देखने मत जाएं।
• जब घरों या भवनों में पुन: प्रवेश करें, तो अत्यलन्त् सावधानी बरतें क्यों कि बाढ़ का पानी भवनों को क्षतिग्रस्तन कर चुका हो सकता है। उपयोगी सुविधाओं की टूटी लाईनों की जांच करें और समुचित प्राधिकारियों को इसकी रिपोर्ट करें।
• यदि आपकी संपत्ति नष्टक हो गई हो, तो बीमा उद्देश्योंो के लिए इसका विवरण लिखें और फोटो खींच लें। यदि आपकी सम्पभत्ति किराए की है, तो जितनी जल्दीई संभव हो सके अपने मकान-मालिक से सम्पतर्क करें और अपनी संबंधित बीमा कंपनी से सम्पीर्क करें।
