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Tsunamis

Tsunamis are ocean waves produced by earthquakes or underwatelandslides.

The word is Japanese and means 'harbor wave,' because of the devastating effects these waves have had on low-lying Japanese coastal communities. Tsunamis are often incorrectly referred to as tidal waves, but a tsunami is actually a series of waves that can travel at speeds averaging 450 (and up to 600) miles per hour in the open ocean. In the open ocean, tsunamis would not be felt by ships because the wavelength would be hundreds of miles long, with an amplitude of only a few feet. This would also make them unnoticeable from the air. As the waves approach the coast, their speed decreases and their amplitude increases. Unusual wave heights have been known to be over 100 feet high. However, waves that are 10 to 20 feet high can be very destructive and cause many deaths or injuries.

Tsunamis are most often generated by earthquake-induced movement of the ocean floor. Landslides, volcanic eruptions, and even meteorites can also generate a tsunami. If a major earthquake is felt, a tsunami could reach the beach in a few minutes, even before a warning is issued. Areas at greatest risk are less than 25 feet above sea level and within one mile of the shoreline. Most deaths caused by a tsunami are because of drowning. Associated risks include flooding, contamination of drinking water, fires from ruptured tanks or gas lines, and the loss of vital community infrastructure (police, fire, and medical facilities).

From an initial tsunami generating source area, waves travel outward in all directions much like the ripples caused by throwing a rock into a pond. As these waves approach coastal areas, the time between successive wave crests varies from 5 to 90 minutes. The first wave is usually not the largest in the series of waves, nor is it the most significant. Furthermore, one coastal community may experience no damaging waves while another, not that far away, may experience destructive deadly waves. Depending on a number of factors, some low-lying areas could experience severe inland inundation of water and debris of more than 1,000 feet.

Tsunami are a threat to people and property in coastal and low-lying estuarine areas. The waves travel quickly, rapidly flooding and damaging coastal communities, picking up debris as they go. A fast moving wave over 10 metres high can quickly destroy homes and communities. Tsunami also create seiching in harbours and confined estuaries.

Tsunami waves can travel inland along river beds as continuous single standing waves. This puts smaller inland communities at risk and contaminates rivers with saltwater.

A tsunami can threaten 'lifeline' services such as water, power, telecommunication and transportation networks. Find out more about managing Lifelines to deal with unexpected emergencies or natural hazards.

How To Prepare for a Tsunami if You Live in a Tsunami Risk Area
Learn whether tsunamis have occurred in your area or could occur in your area by contacting your local emergency management office, state geological survey, National Weather Service (NWS) office, or American Red Cross chapter. Find out your area’s flooding elevation.

If you are in an area at risk from tsunamis, you should:
Find out if your home, school, workplace, or other frequently visited locations are in tsunami hazard areas.
Know the height of your street above sea level and the distance of your street from the coast or other high-risk waters. Evacuation orders may be based on these numbers. Also find out the height above sea level and the distance from the coast of outbuildings that house animals, as well as pastures or corrals.
Plan evacuation routes from your home, school, workplace, or any other place you could be where tsunamis present a risk.
If possible, pick areas 100 feet (30 meters) above sea level or go as far as two miles (3 kilometers) inland, away from the coastline. If you cannot get this high or far, go as high or far as you can. Every foot inland or upward may make a difference. You should be able to reach your safe location on foot within 15 minutes. After a disaster, roads may become impassable or blocked. Be prepared to evacuate by foot if necessary. Footpaths normally lead uphill and inland, while many roads parallel coastlines. Follow posted tsunami evacuation routes; these will lead to safety. Local emergency management officials can advise you on the best route to safety and likely shelter locations.
If your children’s school is in an identified inundation zone, find out what the school evacuation plan is. Find out if the plan requires you to pick your children up from school or from another location. Telephone lines during a tsunami watch or warning may be overloaded and routes to and from schools may be jammed.
Practice your evacuation routes. Familiarity may save your life. Be able to follow your escape route at night and during inclement weather. Practicing your plan makes the appropriate response more of a reaction, requiring less thinking during an actual emergency situation.
Use a NOAA Weather Radio or stay tuned to a local radio or television station to keep informed of local watches and warnings.
Talk to your insurance agent. Homeowners' policies do not cover flooding from a tsunami. Ask about the National Flood Insurance Program (NFIP). NFIP covers tsunami damage, but your community must participate in the program.
Discuss tsunamis with your family. Everyone should know what to do in a tsunami situation. Discussing tsunamis ahead of time will help reduce fear and save precious time in an emergency. Review flood safety and preparedness measures with your family.
If you are visiting an area at risk from tsunamis, check with the hotel, motel, or campground operators for tsunami evacuation information and find out what the warning system is for tsunamis. It is important to know designated escape routes before a warning is issued.

Earthquakes

What is the Definition of an Earthquake?

An earthquake is the sudden, sometimes violent movement of the earth's surface from the release of energy in the earth's crust.
The word earthquake is also widely used to indicate the source region itself.
Earthquakes occur where the stress resulting from the differential motion of these plates exceeds the strength of the crust.
The highest stress (and possible weakest zones) are most often found at the boundaries of the tectonic plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called interplate earthquakes; the less frequent events that occur in the interior of the lithospheric plates are called intraplate earthquakes. Earthquakes also occur in volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes.

What Causes Most Earthquakes?

The crust of the earth when it is subject to tectonic forces, bends slightly. But, because the crust is rigid, when the stress or pressure exceeds the strength of the rocks, the crust breaks and snaps into a new position. Vibrations called seismic waves are generated and travel both through the earth and along its surface. These seismic waves cause the movement we call earthquakes.
Earthquake Prone Zones in India

Damages caused by earthquake

The effects of an earthquake are strongest in a broad zone surrounding the epicenter. Surface ground cracking associated with faults that reach the surface often occurs, with horizontal and vertical displacements of several yards common. Such movement does not have to occur during a major earthquake; slight periodic movements called fault creep can be accompanied by microearthquakes too small to be felt. The extent of earthquake vibration and subsequent damage to a region is partly dependent on characteristics of the ground. For example, earthquake vibrations last longer and are of greater wave amplitudes in unconsolidated surface material, such as poorly compacted fill or river deposits; bedrock areas receive fewer effects. The worst damage occurs in densely populated urban areas where structures are not built to withstand intense shaking. There, L waves can produce destructive vibrations in buildings and break water and gas lines, starting uncontrollable fires.

Damage and loss of life sustained during an earthquake result from falling structures and flying glass and objects. Flexible structures built on bedrock are generally more resistant to earthquake damage than rigid structures built on loose soil. In certain areas, an earthquake can trigger mudslides, which slip down mountain slopes and can bury habitations below. A submarine earthquake can cause a tsunami, a series of damaging waves that ripple outward from the earthquake epicenter and inundate coastal cities.

Safety rules to be followed during an earthquake

Prepare a Home Earthquake Plan
Choose a safe place in every room--under a sturdy table or desk or against an inside wall where nothing can fall on you.
Practice DROP, COVER, AND HOLD ON at least twice a year. Drop under a sturdy desk or table, hold on, and protect your eyes by pressing your face against your arm. If there's no table or desk nearby, sit on the floor against an interior wall away from windows, bookcases, or tall furniture that could fall on you. Teach children to DROP, COVER, AND HOLD ON!
Choose an out-of-town family contact.
Consult a professional to find out additional ways you can protect your home, such as bolting the house to its foundation and other structural mitigation techniques.
Take a first aid class from your local Red Cross chapter. Keep your training current.
Get training in how to use a fire extinguisher from your local fire department.
Inform babysitters and caregivers of your plan.

Eliminate Hazards, Including--
Bolting bookcases, china cabinets, and other tall furniture to wall studs.
Installing strong latches on cupboards.
Strapping the water heater to wall studs.

Prepare a Disaster Supplies Kit For Home and Car, Including--
First aid kit and essential medications.
Canned food and can opener.
At least three gallons of water per person.
Protective clothing, rainwear, and bedding or sleeping bags.
Battery-powered radio, flashlight, and extra batteries.
Special items for infant, elderly, or disabled family members.
Written instructions for how to turn off gas, electricity, and water if authorities advise you to do so. (Remember, you'll need a professional to turn natural gas service back on.)
Keeping essentials, such as a flashlight and sturdy shoes, by your bedside.

Know What to Do When the Shaking Begins
DROP, COVER, AND HOLD ON! Move only a few steps to a nearby safe place. Stay indoors until the shaking stops and you're sure it's safe to exit. Stay away from windows. In a high-rise building, expect the fire alarms and sprinklers to go off during a quake.
If you are in bed, hold on and stay there, protecting your head with a pillow.
If you are outdoors, find a clear spot away from buildings, trees, and power lines. Drop to the ground.
If you are in a car, slow down and drive to a clear place (as described above). Stay in the car until the shaking stops.

Identify What to Do After the Shaking Stops
Check yourself for injuries. Protect yourself from further danger by putting on long pants, a long-sleeved shirt, sturdy shoes, and work gloves.
Check others for injuries. Give first aid for serious injuries.
Look for and extinguish small fires. Eliminate fire hazards. Turn off the gas if you smell gas or think it's leaking. (Remember, only a professional should turn it back on.)
Listen to the radio for instructions.
Expect aftershocks. Each time you feel one, DROP, COVER, AND HOLD ON!
Inspect your home for damage. Get everyone out if your home is unsafe.
Use the telephone only to report life-threatening emergencies.

Steps to prevent earthquake hazadr from becoming a disaster
Earthquake resistant building

Reinforced Hollow Concrete Block Masonry (RHCBM) elements are designed both as load bearing walls for gravity loads and also as shear walls for lateral seismic loads, to safely with stand earthquakes. This structural system of construction is known as shear wall – diaphragm concept, which gives three-dimensional structural integrity for the buildings.
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Structural Features:
Each masonry element is vertically reinforced with steel bars and concrete/grout fill, at regular intervals, through the continuous vertical cavities of hollow blocks.
Similarly, each masonry element is horizontally reinforced with steel bars and concrete/grout fill at plinth, sill, lintel and roof levels, as continuous RC bands, using U-shaped concrete blocks in the masonry course, at respective levels.
Grid of reinforcement can be built into each masonry element without the requirement of any extra shuttering and it reduces the scope for corrosion of reinforcement.
As the reinforcement bars in both vertical and horizontal directions can be continued into the roof slab, and the lateral walls respectively, the structural integrity in all the three dimensions is achieved.
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Training
Training should be given to students and teachers on how to react during an earthquake. These training camps should be organised in between public places so that they can attract public's attraction.

Case Study

The 31 Dec 1881 Mw=7.9 Car Nicobar earthquake

This earthquake caused minor damage in the Andaman Island Penal colony and generated a tsunami that was observed throughout the Bay of Bengal but not along the Burmese coast. The tsunami did no damage around the Bay of Bengal where tide gauges recorded a maximum amplitude of 0.8 m (Oldham, 1884). An analysis of five tide gauge records reveals that the earthquake was Mw=7.9±0.2 and occurred on an east-dipping thrust fault below and to the west of Car Nicobar, an island at 9¡N midway between the Andaman and Nicobar islands (Ortiz and Bilham, 2003). GPS measurements at Port Blair indicate oblique convergence of the plate boundary (Paul et al. 2001). The earthquake is believed to have occurred on the interface between the Indian and Andaman Plates and the inferred mechanism of westward slip of the hanging wall slip is consistent with slip partitioning between the dipping subduction zone, and the strike-slip West Andaman fault east of Car Nicobar.

A feature of this earthquake is the inferred presence of a region of minor slip NE of the main rupture zone. This may have been a secondary earthquake triggered by the mainshock. Its timing would have to have occurred within a few minutes of the mainshock for it to have produced the sea wave observed at Port Blair. Local populations were concentrated in only two islands and therefore there is no corroboration of this inferred northern region of submarine faulting which occurred between them. It is probable that offshore corals may be of use in reconstructing an extended history of earthquakes in the Andaman-Nicobar islands. The island of Car Nicobar is believed to have been raised and tilted during the 1881 event. Deformation models that do not include this uplift result in an inappropriate estimate of the observed tsunami run-up on the island (Ortiz and Bilham, 2003).

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