What is an Earthquake?
An earthquake takes place when two blocks of the earth suddenly slip past one another. The surface where they slip is called a Fault place.
The location below the earth’s surface where the earthquake starts is called the hypo center, and the location directly above it on the surface of the earth is called the epicenter.
Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place before a larger earthquake takes place. The largest main earthquake is called the main shock. Main shocks are always followed by aftershocks that occur in the same place and can continue for weeks, months and even years after the main shock.
What causes earthquakes and where do they happen?
The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece. It is made up of many pieces covering the surface of the earth.
These pieces are called tectonic plates, and the edges of the plates are called the plate boundaries. These pieces keep moving around slowly, sliding past one another and bumping into each other.
These plate boundaries are made up of many faults, and most earthquakes around the world occur on these faults.
Why does earth shake during earthquake?
While the edges of faults are stuck together and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being accumulated.
When the force of the moving block finally overcomes the friction of the jagged edges of the fault and it breaks free, all that stored up energy is released. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us.
The energy radiates outward from the fault in all directions, in the form of seismic waves- like ripples on a pond.
How are earthquakes recorded?
Earthquakes are recorded by instruments called seismographs. The recording they make is called a seismogram. The seismograph has a base that sets firmly in the ground, and a heavy weight that hangs free.
When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead, the string that is hanging, absorbs all the movement. The difference in position, between the shaking part of the seismograph and the motionless part, is what is recorded.
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