Earthquake:-
Any sudden disturbances below the earth's surfaces,may produce vibrations or shaking at the crust. When rocks break,the particles next to the break are set in motion. It is the movement of one rock mass against another that causes vibrations. Some of these vibrations reach the surface and are called earthquake.
The place of origin of an earthquake inside the earth is called its focus
The point on the earth's surface vertically above the focus is called epicentre.
Cause of earthquake
Earthquakes are usually caused when rock underground suddenly breaks along a fault. This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
How to measure earthquake...??
The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but that’s not something scientists can simply measure with a measuring tape since faults are many kilometers deep beneath the earth’s surface. So how do they measure an earthquake? They use the seismogram recordings made on the seismographs at the surface of the earth to determine how large the earthquake. A short wiggly line that doesn’t wiggle very much means a small earthquake, and a long wiggly line that wiggles a lot means a large earthquake. The length of the wiggle depends on the size of the fault, and the size of the wiggle depends on the amount of slip.
The size of the earthquake is called its
magnitude.
There is one magnitude for each earthquake. Scientists also talk about the
intensity of shaking from an earthquake, and this varies depending on where you are during the earthquake.
Before and after earthquake damage
World's largest earthquake
The world's largest earthquake with an instrumentally documented magnitude occurred on May 22, 1960 near Valdivia, in southern Chile. It was assigned a magnitude of 9.5 by the United States Geological Survey. It is referred to as the "Great Chilean Earthquake" and the "1960 Valdivia Earthquake."
The United States Geological Survey reports this event as the "largest earthquake of the 20th Century." Other earthquakes in recorded history may have been larger; however, this is the largest earthquake that has occurred since accurate estimates of magnitude became possible in the early 1900s.