Group items tagged

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.

Fasten all shelves securely to the walls. Use brackets to attach wall units, bookcases, and other tall furniture to wall studs. Standard steel brackets are fine and easy to apply. Place large, heavy objects on lower shelves or on the floor. They might fall during an earthquake and the less distance they have to fall, the better. You can also screw objects onto things, such as a desk.

Prepare disaster supplies in advance

Flashlight with extra batteries (preferably one for each individual) Portable, battery-operated radio with extra batteries Purchase and use "self powered radios" and "self powered flashlights". Batteries will be hard to find, if you can find them. Some models will charge cell phones, but the cell phone towers may be damaged, even destroyed. Purchase and use "glow sticks". These are safer than candles, since you will be dealing with ruptured gas lines, flammable, explosive gas. First aid kit and manual Emergency food and water (some sources say you should have up to 2 weeks worth of extra food on hand in case rescue workers cannot reach your location) Nonelectric can opener Essential medicines Cash and credit cards Sturdy shoes for each family member.

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

The short answer is that earthquakes are caused by faulting, a sudden lateral or vertical movement of rock along a rupture (break) surface.     Here's the longer answer: The surface of the Earth is in continuous slow motion. This is plate tectonics--the motion of immense rigid plates at the surface of the Earth in response to flow of rock within the Earth. The plates cover the entire surface of the globe. Since they are all moving they rub against each other in some places (like the San Andreas Fault in California), sink beneath each other in others (like the Peru-Chile Trench along the western border of South America), or spread apart from each other (like the Mid-Atlantic Ridge). At such places the motion isn't smooth--the plates are stuck together at the edges but the rest of each plate is continuing to move, so the rocks along the edges are distorted (what we call "strain"). As the motion continues, the strain builds up to the point where the rock cannot withstand any more bending. With a lurch, the rock breaks and the two sides move. An earthquake is the shaking that radiates out from the breaking rock.

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.

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.

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).