An earthquake is a sudden movement of the Earth, caused by movement between tectonic plates. For hundreds of millions of years, the forces of plate tectonics have shaped the Earth as the huge plates that form the Earth's crust slowly move over, under, and past each other. Sometimes the movement is gradual. At other times, the plates are locked together, unable to release the accumulating energy. When the accumulated energy grows strong enough, the plates break free causing an earthquake. If the earthquake occurs in a populated area, it may cause many deaths and injuries, and extensive property damage.
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. People have known about earthquakes for thousands of years, of course, but they didn't know what caused them. In particular, people believed that the breaks in the Earth's surface--faults--which appear after earthquakes, were caused *by* the earthquakes rather than the cause *of* them. It was Bunjiro Koto, a geologist in Japan studying a 60-mile long fault whose two sides shifted about 15 feet in the great Japanese earthquake of 1871, who first suggested that earthquakes were caused by faults. Henry Reid, studying the great San Francisco earthquake of 1906, took the idea further. He said that an earthquake is the huge amount of energy released when accumulated strain causes a fault to rupture. He explained that rock twisted further and further out of shape by continuing forces over the centuries eventually yields in a wrenching snap as the two sides of the fault slip to a new position to relieve the strain. This is the idea of "elastic rebound" which is now central to all studies of fault rupture.
Earthquakes can cause severe and widespread damage to weak buildings or structures, or to those located on ground subject to fault breakage, strong shaking, or landsliding. The slip (movement) on the fault may break the surface of the Earth, offsetting roads and tearing apart buildings or pipelines built across the fault. Such damage can be spectacular, but it is limited to the vicinity of the fault.
Earthquakes have been recorded as early as 1177 B.C. in China. Of course earthquakes have been a part of myth and legend since the dawn of man. In Greek Mythology , Posseidon (Neptune in the Roman pantheon ) was "God of the Sea". Yet one of his powers was thought to be that of "earth shaker". As a tsunami is often the result of an earthquake, this was an appropriate power for a sea god.
In European history , the earliest recorded earthquake occurred in 580 B.C. In North America the great earthquakes of 1811-1812 occurred near New Madrid, Missouri. The magnitude of the quakes are not known, but they are estimated to have been about 8 on theRichter scale. There were actually three large quakes with aftershocks between and for months after. The quake was so wide-spread it was felt as far away as Boston.
Earthquakes occur because friction holds the rock masses on either side of the fault together, while plate motion slowly bends and stretches the rock, storing energy much like the energy stored in a stretched rubber band. An individual earthquake occurs when the stored energy
becomes greater than the friction, and the rock masses suddenly slip along the fault. Much of
the energy stored in the stretched rock is released in seismic waves, which travel out in all directions from the fault on which the earthquake occurre.
Our earth is made of many layers just like an onion. We can divide it into four main layers: the inner or solid core, the outer or liquid core, mantle, and crust. The inner core is composed mostly of iron and is extremely hot. This heat causes the outer core to remain in a liquid or molten form. Most of the earth's mass is in the mantle. But if pressure were to be applied on this layer, it deforms slowly. The crust is the thinnest layer and is rocky. It is also the coldest layer relatively. Because it is cold, the layer is also brittle and fractures easily during an earthquake.
Volcanoes are formed usually near plate boundaries, most commonly divergent/constructive plate boundaries, such as the Mid-Atlantic Ridge. Here the crust is moving apart and causing melting due to a decrease in pressure. The magma rises, filling the cracks in the crust and forms fissure volcanoes and sea mounts, which are submarine volcanoes. Volcanoes also form at convergent/destructive boundaries, like the the rim of the Pacific ocean known as the Ring of Fire, which contains about 68% of Earth's subaerial volcanoes. A volcano here is formed when a denser, colder oceanic plate, subducts under another plate, either a continental or warmer oceanic plate. The subducted plate will melt under the high temperatures in the mantle. This will form magma, which will in turn form cracks in the plates, which allows the magma to flow out from the mantle into the crust and sometime onto Earth's surface. The lava (when magma reaches the surface it is called lava) will then harden and form layers, slowly forming a volcano.
Basically, the process is identical to an earthquake on land. The Earth's crust is made up of Plates, which move against each other. Sometimes they get stuck, resulting in a build-up of pressure which eventually breaks the obstruction. It the sudden release of this pressure, that causes an Earthquake. The only difference with a Seaquake, is that this happens underwater, on the Seabed. The major problem with Seaquakes, are their secondary effects, the vibration and shockwaves can displace a large volume of water, which becomes a Tidal Wave or Tsunami .
A tsunami) is a series of waves, made in an ocean or other body of water by an earthquake , landslide, volcanic eruption , or meteorite impact . Tsunamis can cause huge destruction when they hit coastlines. Some people call tsunamis “tidal waves”, but these large waves really have little to do with tides, so the term “tidal wave” does not really suit them.
Tsunami waves are different from the waves you can usually find rolling into the coast of a lake or ocean. Those waves are made by wind offshore and are quite small compared with tsunami waves. A tsunami wave in the open ocean can be more than 100 km across. That’s roughly the length of 1000 American football fields! Tsunami waves are huge and can travel very quickly, at about 700 km/hr, but they are only about one meter high in the open ocean.