Tsunami and earthquake

Transcription

1 Tsunami Tsunami is a series of seven or eight huge waves, formed in a body of ocean water, by an undersea disturbance, which vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, under water explosions etc can generate these types of undersea disturbances. The tsunami waves travel in all directions from the area of disturbance, much like the ripples that happen after throwing a rock. This tsunami waves may travel in the open sea as fast as 450 miles per hour. As the waves approach shallow waters along the coast, then they grow to a great height as 40 meters or more and smash into the shore, which cause devastating property damage and loss of life in coastal regions and islands. The word tsunami comes from the Japanese words tsu (means harbour) and nami (means wave). The meaning and origin place of this term tsunami is very appropriate, considering about 80 percent of all tsunamis occur in the Pacific Ocean and Japan costal areas, though it can be formed at any large bodies of water, even at inland lakes. The Pacific Ocean is surrounded by an area called Ring of Fire, a highly active zone of volcano and earthquake, which are most favorable for tsunami formation. This Ring of Fire circles the Pacific Ocean, from Alaska down to the west coasts of North and South America and up along the east coast of Asia, including parts of China, Japan and Russia. In less than a day, the tsunamis can travel from one side of the Pacific to the other. The tsunami threat with many areas (Alaska, Philippines, Japan, U.S. West Coast etc) near Pacific can be immediate, as tsunamis from nearby earthquakes are taking only a few minutes to reach these areas. Tsunami behaves like tidal waves with much more height, but tsunami actually have nothing with tides, though tsunamis and tides both produce waves of water and the damage may be worse, if a tsunami hits at high tide time. The wavelength (from crest to crest) of normal wind waves is about 100 meters, where as the wavelength of tsunami wave is about 200 kilometers, in the deep sea. Due to this enormous wavelength, the wave oscillation at any given point takes 20 to 30 minutes to complete a cycle and has amplitude of only about 1 meter. For this reason, in the open sea, tsunamis would not be felt by ships. For the same reason, it will also be unnoticeable from the air. As the tsunami approaches on the shallow water of coastal areas, the wave is compressed due to wave shoaling. At the same time, speed of forward travel of the tsunami wave reduces to below 80 kilometers per hour; with reduced wavelength, which is less than 20 kilometers; but the amplitude of wave grows 0

2 enormously, producing a distinctly visible wave at costal areas. At costal areas, since the wavelength of tsunami-wave is still a few miles long, the tsunami may take minutes to rise up to full height, so it will be visible as a massive flood of rising ocean, rather than a wall of water. Open bays and coastlines adjacent to very deep water may form this tsunami further into a step-like wave, with a steep breaking front. Tsunamis are not rare, as at least 25 tsunamis are occurred in the last century. The December 26th, 2004 Indian Ocean Tsunami (also known as Asian Tsunami, Indonesian Tsunami, Boxing Day Tsunami etc) killed nearly 230,000 people in eleven countries and flooded coastal communities with waves up to 30 meters high. It was one of the deadliest natural disasters in recorded history of Indonesia, Sri Lanka, India and Thailand. Luckily Bangladesh escaped from the devastation of this tsunami, as it traveled in east-west directions. Tsunami and earthquake The outer layers of the Earth are divided into lithosphere and asthenosphere. The lithosphere is hard and rigid; on the other hand the asthenosphere is weaker, hotter and flows more easily. Lithosphere lies at depths up to 100 km and asthenosphere lies below the lithosphere, at depths between 100 and 200 km below the surface, but it can extend as deep as 400 km also. The lithosphere is broken up into eight major and many minor parts, these parts are called tectonic plates. These plates move in relation to one another, which is typically 50 to100mm annually. When the edges of the plates run into one another, the earthquakes occur. Such edges of plates are called fault lines or faults. Sometimes the forces along faults can build-up over long periods of time, so that when the rocks finally break, an earthquake occurs. The location where two different plates meet is called a plate boundary. So an earthquake is the result of a sudden release of energy in the Earth's crust, due to movements of tectonic plates, as they collide and/or slide under one another (subduction), at plate boundary areas; though large-scale volcanic activity, landslides, mines blasts, nuclear experiments etc also can cause earthquakes. For a tsunami to occur there must be some kind of vertical movement along the fault. This vertical movement must be capable of displacing huge amounts of water, thus causing waves. Due to an earthquake, sea floor may abruptly deforms also and vertically displaces such amount of water, from its equilibrium position. Ordinarily earthquakes under magnitude 7.5 on the Richter scale normally can not cause tsunamis, although some instances of this have been recorded. More over, earthquakes must need to occur underneath or near the ocean and also the 1

3 distances of earthquakes, water volume and water depth must be sufficient to generate tsunamis. All oceanic regions of the world can experience tsunamis due to earthquakes, but in the Pacific Ocean, there are much more frequent occurrences of large, destructive tsunamis, because many large earthquakes are available along the margins of the Pacific Ocean. December 26, 2004, Indian Ocean tsunami was subduction type earthquake and triggered a series of devastating tsunamis along the coasts of most landmasses bordering the Indian Ocean. This earthquake had a magnitude of between 9.1 and 9.3, as second largest earthquake ever recorded on a seismograph. It also had the longest duration of faulting between 8.3 and 10 minutes, ever observed, which caused the entire world to vibrate as much as 1 cm. Tsunamis caused by other sources Most of the tsunamis are generally caused by earthquakes, less commonly by submarine landslides, infrequently by submarine volcanic activities and very rarely (in other words theoretically) by extraterrestrial collision in the ocean. These occurrences have to be large enough, so that they able to displace a huge amount of sea water; otherwise may dissipate quickly and rarely affect coastlines due to distant from the source and small size. Landslides (or landslips) are a geological phenomenon, which includes a wide range of ground movement, caused due to icefalls, rock falls etc. Although the action of gravity is the primary driving force for a landslide to occur, there are some other contributing factors, including a triggering (caused by earthquake, explosion, bed weather etc) may also require. Landslides that occur undersea or have impact into sea water can generate tsunamis. The most notable example of a landslide-induced tsunami occurred at Southern France in 1980, where the movement of a significant amount of earth surface for the construction of an airport, triggered an underwater landslide, which resulted in destructive tsunami waves and hit the harbor of Thebes. Another known landslide induced tsunami was formed at the head of Lituya Bay in 1958, which produced a tsunami wave with an initial surge estimated at 524 m. Volcanic activities occur at undersea, have an ability to produce tsunami waves. Violent volcanic eruptions represent impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area; although this type of tsunami is relatively infrequent. Such 2

4 disturbances can also be caused by a volcanic explosion. More over volcanoes that have been erupting continuously for a long time may have empty magma chamber. If the roof of this chamber collapses, then a crater is formed, which can be up to one kilometer in diameter. Water gushes into this crater in a very short amount of time, causing a tsunami. One of the largest and most destructive such tsunamis ever recorded was generated in August 26, 1883, after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia. This explosion generated giant waves that reached 125 feet above sea-level, destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing about 36, 417 people. Extraterrestrial collisions occur at time of falling of celestial bodies (asteroids, meteors etc) from space, to earth surface. Tsunamis caused by this extraterrestrial collision are an extremely rare occurrence. This is still a theoretical matter, as no meteor/asteroid induced tsunami has been recorded in recent history. Experts realize that if these celestial bodies should strike the ocean, a large volume of water would unquestionably be displaced to cause a tsunami. They have calculated that if a moderately large asteroid of 5 to 6 km in diameter, strikes the middle of a large ocean basin, such as the Atlantic Ocean, then it will produce a tsunami, which will travel all the way to the Appalachian Mountains, in the upper two-thirds of the United States. Due to a such tsunami, both side cities of the Atlantic Ocean can be washed out. Signs of an approaching tsunami There is often no advance warning of an approaching tsunami. Unfortunately, even with the significant advances made in science and technology over the past hundred years or so; like a cyclone, there is still no longtime early warning for a tsunami, as most of the tsunami warnings are only few minutes or hours away from tsunami attack. So people must take responsibility to always be aware of their own surroundings regarding tsunamis. However, there are some warning signs of tsunami being developed and in use to reduce the damages from tsunami. Some examples of signs of an approaching tsunami are given as below, Earthquake: Since earthquake is the main cause of any tsunami, so any earthquake with considerable strength occurs near or under a body of water, can be a sign of approaching tsunami. Tsunamis travel at between 14 km/s and 0.28 km/s in open water, where as seismic waves (used to measure earthquakes, at seismometer) travel with a 3

5 typical speed of 4 km/s. This gives some time for a possible tsunami forecast to be made and warnings to be issued to threatened areas. So from seismometer readings (locations, time, magnitude etc), along with the knowledge regarding costal areas (shape, depth, size etc), experts can warn the people for tsunamis. Unfortunately this process sometimes produces many false alarms. So information from other sources (i.e. shore-based tide gauges, DART buoys etc) is also used, to verify the existence of a tsunami. Bottom pressure sensor: Since during tsunamis, height of sea level from seabed increases. So from the bottom pressure (undersea) can get the sea level heights, as well as tsunami warnings. For this reason, one of the most important systems is developed, which is bottom pressure sensor. These sensors are anchored and attached to buoys. Sensors on the equipment constantly monitor the pressure of the overlying water column, from this can get tsunami warning, as well as tsunami track. From this principle, in 1995 the US National Oceanic and Atmospheric Administration constructed the stations for Deep-ocean Assessment and Reporting of Tsunamis (DART). These stations give detailed information about tsunamis while stations are still far off shore. Each station consists of a sea-bed bottom pressure recorder (at a depth of about 6000 m) which detects the passage of a tsunami and transmits the data to a surface buoy via acoustic modem. The surface buoy then transmits the information to the PTWC, via the GOES satellite system. Some other organizations also give same information based on multisensor (both seismic wide spectrum, and pressure gauges), established on deep ocean platforms. This sea bottom acquired data is relayed via surface buoys to a control center, using Inmarsat mini-c transceivers. Tsunami alerts are then transmitted via Inmarsat, SafetyNET, NAVTEX, fax, voice, internet, mobile SMS etc to various places. Tsunami warning system : It is an international network system to detect tsunamis and issue warnings, consists of two equally important components: a network of sensors to detect tsunamis and a communication infrastructure to issue timely alarms. It also has two tsunami warning systems; one is international and the other is regional. The Tsunami warning system works on the combination of the principles of earthquake, bottom pressure sensor etc. Pacific tsunami warning center (PTWC), Indian Ocean tsunami warning system, North Eastern Atlantic, the Mediterranean and connected seas tsunami warning 4

6 system, Caribbean tsunami warning system etc are working under the umbrella of International tsunami warning system. Regional warning system uses seismic data about nearby earthquakes to determine a tsunami, if available. Such system is capable of issuing warnings to the general people (via public address systems and sirens) in less than 15 minutes. The Tsunami warning system has various communication methods (by mobile SMS, , fax, radio, telex, satellite etc) enabling emergency messages to be sent to the emergency services, armed forces, coast gourds, Govt. authorities etc. Receding of water from shoreline: Lowest part of any wave is called trough and highest part is called crest. If the leading edge of the tsunami wave is the trough, rather than a crest of the wave, then the water along the shoreline may recede dramatically, exposing areas that are normally always submerged, at half of the wave's period, before the tsunami wave's arrival. If the slope of the coastal seabed is shallow, this recession can exceed many hundreds of meters. This type of receding of water may have a sucking sound, as many survivors reported. So this abnormal receding can serve as an advance warning of the approaching tsunami. During the Indian Ocean tsunami of December 26, 2004, the sea also receded. Instead of running away from shore line, many people then went onto the exposed sea bed to investigate. So finally they were unable to escape to high ground and died. Behavior of animals: Some zoologists hypothesize that animals have ability to sense subsonic waves from an earthquake or a tsunami. However, the evidence of such ability is controversial and has not been established scientifically, as many animals in the same areas drowned, caused due to earthquake or tsunami. Some sources are telling, some animals (e.g., elephants), in Sri Lanka, in the 2004 Indian Ocean earthquake, may have heard the sounds of the tsunami, as it approached the coast, since these animals had some reactions, to move away from the approaching noise of tsunami. However if it is correct, then from careful observation and monitoring of the behavior of animals, may possible to get warnings of earthquake and tsunami. Pacific Tsunami Warning Center The Pacific Tsunami Warning Center (PTWC) is one of two tsunami warning centers in the United States and functioning as a part of International tsunami warning system program, which is an international network, working for protection from tsunami disasters. It is covering regionally (local) for the State of Hawaii and 5

7 internally mainly for Pacific Ocean areas, though it s work extents to much more areas including Indian Ocean, as after the Indian Ocean tsunami, PTWC has taken on additional areas of responsibility including the Indian Ocean, South China Sea, Caribbean Sea, and Puerto Rico & U.S. Virgin Islands (until regional capability is in place for these areas). PTWC was established in 1949, following the 1946 Aleutian Island tsunami, which caused 165 casualties in Hawaii and Alaska. This centre is located at Ewa Beach of USA PTWC uses seismic data as its starting point and then takes into account oceanographic, including tide gauge data, when calculating possible threats. The center then forecasts the future of the tsunami, issuing warnings to threaten areas, all around the Pacific basin. As tsunamis need more time (then seismic wave) to travel trans-oceanic distances, so PTWC can afford to make confirmed (without any false alarms) forecasts. PTWC normally issues 3 types of bulletins: 1. Tsunami Information Bulletin- At this time, though a threat exists, there is no evidence that a tsunami is making its way across the Pacific. 2. Tsunami Watch- PTWC has determined the earthquake may very likely have created a tsunami. The PTWC is also advising parties to be alert as PTWC awaits tide data to support tsunami generation. 3. Tsunami Warning- PTWC finds conditions serious enough to issue immediate concern to parts of the Pacific. The message will include approximate arrival times of tsunamis for various parts of the Pacific. Indian Ocean Tsunami Warning System The Indian Ocean Tsunami Warning System is a tsunami warning system, set up to provide warning to countries bordering the Indian Ocean. The system became completely active in late June 2006, under the leadership of UNESCO (United Nations Educational, Scientific and Cultural Organization), as the Intergovernmental Oceanographic Commission of UNESCO received a mandate from the international community to coordinate the establishment of the system, during the course of several international and regional meetings, including the World Conference on Disaster Reduction (Kobe, Japan, January 2005), and the Phuket Ministerial Meeting on Regional Cooperation on Tsunami Early Warning Arrangements (Phuket, Thailand, 28 and 29 January 2005). 6

8 Accordingly UNESCO did it and eighteen months after the tragic tsunami of December 2004, the UNESCO claimed that the entire Indian Ocean region has a warning system, capable of receiving and distributing tsunami advisories around the clock. There are now 26 national tsunami information centers in the Indian Ocean region, receiving data from 25 new seismographic stations and 3 deep-ocean sensors. But there is still a need for more infrastructures, particularly to ensure more effective warnings to be relayed, as well as smart evacuation methods, to avoid any damages. This need was confirmed in grave manner by the July 2006 tsunami, hit Indonesia s Java Island. As immediately after the July 2006 Java earthquake, the Indonesian government received tsunami warnings, but failed to relay the alert properly, to the citizens. Under this system, it has been suggested that in Muslim-dominated coastal areas, the loudspeakers fitted to mosques could be used to broadcast warnings, as an extra measure to warn the people. Preparations & precautions It is not possible to prevent a tsunami. But some actions can be taken as preparations & precautions for tsunami, to avoid and reduce loses. GENERAL PREPARATIONS Some protection walls can be constructed, as Japan has constructed a tsunami wall of up to 4.5 m high in front of populated coastal areas. Floodgates and channels can be arranged to redirect the water from incoming tsunami. Some natural factors can be arranged, such as tree cover on the shoreline. Tsunami signs to be posted in various locations, to give indication for safe evacuation passages. Plan many evacuation routes from home, workplace or any other places, as tsunami can attack any moment. Avoid building or living in buildings, within very short distances from coastline. These areas are more likely to experience damages from tsunamis. Consult with a professional for advice regarding constructions of building, road, wall etc, as improper constructions can make the situation more badly. A special designed and constructed tsunami shelters to be arranged within 10 min working distance from shoreline. Regular tsunami-drills to be carried out, to arrange conciseness about tsunamis. 7

9 Arrange to get immediate tsunami news, alerts and warnings, via mobile SMS. Make sure all family members know, how to respond to a tsunami. Teach family members how and when to turn off gas, electricity and water. Teach children how and when to call police or fire department and which radio station to listen for official information. An emergency kit (containing food, water, medicine, emergency clothing, flashlight, some cash money etc) should be ready. Develop an emergency communication plan, if family members are separated from one another during a tsunami (a real possibility during the day, when adults are at work and children are at school). Make sure everyone knows the name, address, and phone number of the contact person. Make a Tsunami disaster management plan, to protect people and property. Tsunami disaster management plan is available in most of the tsunami threatened areas; which is helping people regarding precautions & preparation for tsunamis. Specially in port, naval, coast guard, sea-beach etc areas, it becomes a binding, to arrange such plans. The Tsunami disaster management plan is covering all the required actions to be taken at any tsunami attacks. So all just need to follow the plan, under the supervision of authorized persons. Normally each Tsunami disaster management plan has 3 steps, Step -1: immediate actions after tsunami warnings. Step -2: evacuation actions. Step -3: recovery actions, after tsunami attack. At step -1, the followings to be followed, Establish control rooms, with proper communication equipment. Keep standby all responsible people. Al road and water transport to be kept standby with full fuel and proper communication equipment, including VHF (for Chittagong port area has to maintain VHF watch on channel-12) All inland, coaster and fishing vessels to be shifted to safe distance, so that these can not damage each other and/or the structures on coastal areas (for Chittagong port, all capable inland, coaster and fishing vessels can be shifted to 20º N Latitude line, at a minimum distance of 40 nautical moles from St. Martin island and all non-capable & small vessels to go near Shah Amanat Bridge. Also all foreign-going ships have to leave berths and proceed towards sea). All activates to be stopped (i.e. Chittagong Port Authority can stop cargo and maintenance works) 8

10 All lose equipment and machineries to be lashed properly. Special security measures to be taken (i.e. Chittagong Port Authority may issue security level-2). Emergency kit should be ready. Keep standby the medical team. At step -2, the followings to be followed, Maintain a good crowd management plan for evacuation. Evacuations to be done with small groups, under the leadership of authorized persons. Make sure that no panic is created. At step -3, the followings to be followed, Return the evacuees. Remove road blocks. Restoration of essential services (electricity, water supply, telephone etc) Survey the damages and arrange necessary repairs. Arrange for food, water etc for people, if require. Arrange special security measures. If the area is not under Tsunami disaster management plan, then have to face the situation very smartly with a very cool brain, keeping in mind that tsunamis can be caused by an underwater disturbance or an earthquake, people living along the coast should consider an earthquake or a sizable ground rumbling and/or noticeable rapid rise or fall, as a warning signal for tsunami. One more thing, should not assume that one wave means that the tsunami danger is over, as the next wave may be larger than the first one. At an area, outside Tsunami disaster management plan, following guidelines may help the people. ACTIONS BEFORE TSUNAMI If in coastline area (like sea beach etc) and there is any symptom of tsunami, then nobody should wait for tsunami warnings to issue, the immediate duty is to alert all and everybody should run to safest high land, which is at least 50m above sea level or go up to two miles inland away from the coastline. If can't get this high or far, then go as high as possible. If not in coastline area, but within threatened area and there is any symptom of tsunami; then people should listen to a radio or television to get 9

11 the latest emergency information and be ready to evacuate, if asked to do so. ACTIONS AFTER TSUNAMI Stay tuned to a battery-operated radio for the latest emergency information. Help injured or trapped persons. Stay out of damaged buildings. Return home only when authorities say it is safe. Enter your home with caution. Use a flashlight when entering damaged buildings. Check for electrical shorts and live wires. Open windows and doors to help dry the building. Check food supplies and test drinking water, as water and food that have come in contact with flood waters may be contaminated. Check for gas leaks. If any smell of gas with or without blowing or hissing noise, then open a window and quickly leave the place. Turn off the gas at the outside main valve and call the gas company. Check for sewage and water line s damages and arrange necessary repairs. Use the land phones and mobiles only for emergency calls, as others also require to make emergency calls. Land phones and mobiles frequently remain overloaded in disaster situations. Examine walls, floors, doors, staircases, and windows to make sure that the building is not in danger of collapsing. 10