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Earthquake If you are indoors when shaking starts: “DROP, COVER AND HOLD ON.” If you are not near a strong table or desk, drop to the floor against an interior wall and cover your head and neck with your arms. Avoid windows, hanging objects, mirrors, tall furniture, large appliances and cabinets filled with heavy objects. Do not try to run out of the structure during strong shaking.

If you are downtown, it is safer to remain inside a building after an earthquake unless there is a fire or gas leak. There are no open areas in downtown San Francisco far enough from glass or other falling debris to be considered safe refuge sites. Glass from high-rise buildings does not always fall straight down; it can catch a wind current and travel great distances. If you are in bed, stay there and cover your head with a pillow. Do not use elevators. If you use a wheelchair, lock the wheels and cover your head.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

When two oceanic tectonic plates converge, one is subducted or pulled under, the other. This convergent activity allows for magma from the earth's core to erupt at the point of contact. The cooled erupted lava and debris build up over time. When the pile of debris and lava build up high enough, land is formed above sea level. This land is called an island. Often, these island volcanoes stay active, continuously growing.

The Ring of Fire, also known as the Circum-Pacific Belt, is a string of islands with active volcanoes in the Pacific Ocean. This geographical region is the most active volcanic and seismic zone in the world, and corresponds with oceanic-continental convergence of multiple tectonic plates.

An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called

the epicente

How are earthquakes recorded? Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. (figure 4) The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. The difference in position between the shaking part of the seismograph and the motionless part is what is recorded.

Aftershocks, those who live through major earthquakes often say, are worse than the main shock in their own way. At least the main shock took them by surprise and was over fairly soon, in less than a minute usually. But with aftershocks, people are stressed already, dealing with disrupted lives and cities. They expect aftershocks at any minute, day or night. When a building is damaged by the main shock, aftershocks can take it down—maybe when you're inside cleaning it up.

Changes in the Air Some volcanoes erupt quietly; others violently shoot gas, steam and ash into the air. After a volcano erupts, gases are released into the atmosphere. These gases include water vapor as well as carbon dioxide, sulfur dioxide, hydrogen sulfide, carbon monoxide, hydrogen gas and methane. All of these gases can form acid rain in high concentrations. Incorporated in the mass of gases expelled from a volcano exist tiny rock particles called tephra. Tephra particles can adhere to gas particles and be carried for great distances according to the U.S. Geological Survey. The immediate effect of high concentrations of gas in the air result in closure of the airspace above and around the volcano to air traffic. Over time, upper level winds can carry volcanic gases around the world.Ash particles impede air traffic as well as create immense breathing difficulties for people living near an erupting volcano. After a volcano, these 2-mm or smaller ash particles fly through the air, coating everything near the volcano. Ash particles can form a layer just like snow but it's much harder to remove. Combined with water, the ash becomes a mud-like mass. The Federal Aviation Administration monitors volcano eruptions to prevent planes from traveling in areas with possible ash particles in the air due to the dangers of loss of visibility and possible engine failure. Ash particle clouds also rise into the atmosphere for transportation with the prevailing winds, making their existence dangerous to any aircraft flying in the vicinity. Changes to the Landscape Every volcano makes a change to the landscape of the earth after an eruption. Volcanoes build new earth with every eruption, spewing hot lava from deep inside the earth onto the much cooler surface. After a volcano erupts, the magma flows down the sides of the volcano, reaching a point where it cools enough to stop flowing. As the flow progresses, everything in the lava's path is consumed. Plants, animals, buildings, roads, and trees are burned completely. Lava flows cool over time and form new rock. Wind, rain, and water after lava flows with weathering to break down lava and turn it into soil. When a volcano expels high volumes of ash, this coats the surface of the land around a volcano. This creates a dangerous condition of unstable earth with a thick layer of ash coating the sides of mountains and hillsides. Lahars are violent mudslides that result of waterlogged ash rushing down the sides of a mountain. These dangerous mudslides carry ash, rock, and debris, obliterating everything in its path. Changes to the Water Rivers and streams near erupting volcanoes suffer the effects of ash, hot gases and lava. After a volcano erupts, the debris expelled by the volcano affects waterways in a number of ways. Streams and rivers form a very basic part of the hydrologic cycle to disperse water through a drainage basin toward the sea. Volcanoes disrupt this process by blocking stream and river bed flows, re-routing courses, and contaminating the water supply with dangerous gases and particles. Fine ash can be carried away easily by water, but it takes time to restore a waterway after an eruption. Effects on Plants and Animals Like humans, animals can survive with little permanent effect resulting from ash deposition--as long as the ash isn't too hot and doesn't cover their source of food. The greatest threat from a volcano comes from damage to the environment. Ash quickly contaminates water, a necessity for any animal. Without access to an untainted water supply, many wild animals move to safer areas. Fish are extremely susceptible to a change in water quality. Volcanic eruptions often result in complete kills in streams and rivers near volcanoes. Ash contaminates the water, and burned or dead trees don't provide enough shade along these rivers to keep water temperatures down. High sediment content in the water prevents proper feeding, movement and reproduction. Birds naturally have difficulty flying in areas with ash clouds. Hot gases are deadly to birds as well. Plants and trees suffer from the effects of heat generated by the eruption and deposition of ash and sediments. However, unburned trees recover and usually continue growing after ash is washed off the tree itself. According to the U.S. Geological Survey, the increased amount of erosion possible with ash and tephra depositions creates slower re-vegetation of any damaged areas. Tephra contains potassium and phosphorus that are valuable nutrients for rich, fertile soil. Weathering helps release these nutrients into the soil, creating the prime environment for renewed growth of native plants and trees.

The Earth's tectonic plates courtesy of the U.S. Geological Survey

Collission or convergent boundaries. Where plates collide. Plate edges may be either oceanic crust or continental crust. So when plates collide, we have only three possibilities: oceanic-oceanic, oceanic-continental, or continental-continental collisions. If oceanic crust collide with continental crust, the denser oceanic crust is subducted under the less dense continental crust (as at the Ring of Fire). If continental crust collide with continental crust they push each other up in a mountain range (like the Himalayas).

PARIS - THE magnitude 7.0 earthquake that struck Haiti in January, killing a quarter of a million people, also unleashed a string of tsunamis on the country's western coast, scientists reported on Sunday. Several waves measuring up to 75cm were caused by a combination of earth movement and coastal landslides. They included banks of sediment on the sea bed, accumulated at river deltas, which were displaced by the shock and unleashed the waves as they moved, the study says. Waves were reported west, north and south of the epicentre, which was within a few kilometres of the surface on the Enriquillo-Plaintain Garden fault, on the boundaries of two microplates in the Caribbean. Such 'slide-generated' tsunami are rare, but their risk is underestimated, according to the paper, published online in the journal Nature Geoscience. 'Even modest, predominantly strike-slip earthquakes can cause potentially catastrophic slide-generated tsunami,' it says. A strike-slip earthquake occurs when one side of the fault moves along horizontally and in parallel to the other side of the fault, rather than down or up. Vertical displacement, especially of the seabed, is best known for creating tsunamis. The study was led by a team led by Matthew Hornbach of the University of Texas at Austin. The January 12 quake was in tectonic terms a surprisingly complex affair, according to new research. Two-thirds of the movement was strike-slip, and a third was a thrust, or upward, movement. The quake inflicted huge damage to the capital, Port-au-Prince, injuring 300,000 people and leaving 1.5 million homeless. -- AFP

Causes of earthquakes If seen broadly we can say that earthquakes are caused due to two major reasons. The first reason is the eruption of volcanoes, which are sudden, and as is known volcanoes are seat of inner disturbance and can effect the plates which is the second cause of earthquakes. Earthquakes are caused due to disturbance in the movement of plates, which again can be caused due to various reasons like under crust waves or cracks in the plates. Plate Tectonic Theory The outer layer of the earth is divided into many sections known as plates, which are floating on the molten magma beneath the earth’s crust. Now the movement of these plates is determined by the convection current in the molten magma. The heat makes these plates rise and vice versa. Therefore after intervals there are plates that get submerged in the molten magma and there plates that rise upwards and at times even new crust is formed from the molten magma which in turn forms a new plate until it connects itself with the already existing ones. At times these plates and can be pushed up to form mountains and hills and the movement is so slow that it is really hard to comprehend that there is any movement at all. The movement and the results come out to be visible suddenly. Now these plates are the bases on which the continents stand and when these plates move the continents also move. Most of the earthquakes occur on the edges of the plates where a plate is under one or across. This movement disrupts the balance and position of all plates, which leads to tremors, which are called earthquakes.