p1+earthquakes



Th e fastest wave, and therefore the first to arrive at a given location, is called the P wave. The P wave, or compressional wave, alternately compresses and expands material in the same direction it is traveling.
 * What causes earthquakes? 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, sink beneath each other in others or spread apart from each other 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. **
 * Earthquakes may also be felt a long distance from the fault plane where they occur. This is due to a wave action that ripples out from the epicenter of a quake. There are two types of waves. Body waves are generated and move rapidly underground, yet it can break out onto the surface where it becomes a surface wave. The surface wave, not unnaturally, is what causes most of the damage in an earthquake. The body wave travels faster than the surface wave and will often be felt a second or more before the arrival of the destructive surface wave. As waves radiate outward from their source, they weaken. Yet the character of the wave can be changed by the types of soil and rock that it travels through, sandy soils increase the damage potential of a quake **

An **earthquake** (also known as a **quake**, **tremor** or **temblor**) is the result of a sudden release of [|energy] in the [|Earth's] [|crust] that creates [|seismic waves]. The **seismicity** or **seismic activity** of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured with a [|seismometer] ; a device which also [|records] is known as a //seismograph.// The [|moment magnitude] (or the related and mostly obsolete [|Richter] magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly [|imperceptible] and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified [|Mercalli scale].
 * [[image:http://www.642weather.com/weather/wxblog/wp-content/uploads/2009/07/earthquakes1.gif width="436" height="299"]] **

=Earthquakes Around the World=

the dots rersents the locations of earthquackes.