72. Earthquake

An earthquake is the result from the sudden release of stored energy in the Earth's crust that creates seismic waves. Earthquakes are accordingly measured with a seismometer, commonly known as a seismograph. At the Earth's surface, earthquakes may manifest themselves by a shaking or displacement of the ground. Sometimes, they cause tsunamis, which may lead to loss of life and destruction of property. An earthquake is caused by tectonic plates getting stuck and putting a strain on the ground. The strain becomes so great that rocks give way by breaking and sliding along fault planes. Earthquakes occur from the deformation of outer, brittle portions of "tectonic plates", the earth's outermost layer of crust and upper mantle. Due to the heating and cooling of the rock below these plates, the resulting convection causes the adjacently overlying plates to move, and, under great stresses, deform.

The rates of plate movements range from about 2 to 12 centimeters per year. Sometimes, tremendous energy can build up within a single, or between neighboring plates. If the accumulated stress exceeds the strength of the rocks making up these brittle zones, the rocks can break suddenly, releasing the stored energy as an earthquake. An earthquake begins when the plates push against earth other and the pressure builds. Cracks start to appear in walls and roads. The ground may shiver (tremor). Where the rocks snap seismic waves (body waves) make the ground move. The first ones (primary) travel through rock, volcanic lava, water and even air. They take about 20 minutes to go from one side of the earth to the other. The next waves (secondary) move only through solid rock. Body waves that reach the surface make the ground move like waves on the ocean. These are the most deadly.

The quake starts with a violent jolt at the epicenter followed by more tremors. Aftershocks may occur as everything is setting into a new position. Movement of the earth's crusts along the plate margins otherwise known as plate tectonics causes up heaving & instability in land/ocean masses. Plate Tectonics is the movement of the earths crust. The earth’s plates are constantly moving and push together and often they get into a "traffic jam". As plates move in different directions, pressure builds up over time. When the pressure overcomes the tendency of the plates to press together, there is a sudden shift resulting in an earthquake. This sudden release of energy creates seismic waves.

During an earthquake poorly designed structures can collapse and cause a lot of damage. The ground shakes, and cracks in the earth begin to form. Earthquakes are recorded with a seismograph. Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. A quake measuring 3 to 5 would be considered minor or light; 5 to 7 is moderate to strong; 7 to 8 is major, and 8 or more is great. On average, a magnitude 8 quake strikes somewhere every year and some 10,000 people die worldwide annually in earthquakes. Collapsing buildings claim by far the majority of lives, but the destruction is often compounded by mud slides, fires, floods, or tsunamis. Smaller temblors that usually occur in the days following a large earthquake can complicate rescue efforts and cause further death and destruction.