what is an earthquake? how it is caused?
Answers
An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.
In California there are two plates - the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. The primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone.
The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great earthquakes when it finally releases.
Earthquake:
Earthquake is the sudden shaking of the aurface of the Earth. It is caused by the disturbance observed deep inside the Earth. Earthquakes may lead to destruction of life and property. Earthquakes also cause tsunamis and landslides. To understand how earthquake is caused, let us study the internal structure of the Earth.
Structure of the Earth
The Earth has three main layers core, mantle and crust. The aurface of the Earth where we live is known as the crust. The oceanic and continental crusts are the two parts that form the crust. The continental crust is generally older than the oceanic crust and consists of a number of moving plates that either collide with each other or pull apart from each other. Just below next layer is mantle. It is softer and denser than crust. The crust and the upper layer of the mantle are together known as the lithosphere. The lithosphere is divided into several large and small plates of rocks called the tectonic plates. These tectonic plates keep moving and sliding past each other and cause disasters like earthquakes.
How earthquakes occur:
The tectonic plates of the Earth are in a constant motion and are similar to the pieces of a jigsaw puzzle. When these plates move, they sometimes collide or slide past each other which causes the earthquake. The surface where they slip is called the fault plane (or the fault), Since the surfaces of these tectonic plates are rough, sometimes they get stuck while trying to move past each other. When the edges release themselves, the energy which got confined radiates outwards from the faults in the form of seismic waves. The place beneath the surface of the Earth where the earthquake starts is called the focus of the earthquake and the point on the surface of the Earth directly above the focus is called the epicentre of the earthquake. The intensity of an earthquake is maximum at the epicenter. In India, the areas most threatened by earthquake are Kashmir, western and central Himalayas, Rann of Kutch, Rajasthan and Indo-Gangetic plain.
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