The Shaking Ground
An earthquake is a sudden, violent shaking of the ground, caused by the movement of rock beneath the Earth's surface. Most earthquakes occur along faults, which are cracks in the Earth's crust where tectonic plates meet.
What Causes Earthquakes?
The most widely accepted explanation for how earthquakes are generated is the elastic rebound theory.
1.Stress Buildup: Tectonic plates are always moving, but at the fault line, friction can cause the rocks to get 'stuck'. As the surrounding plates continue to move, the stuck rock bends and deforms, storing up elastic potential energy like a stretched rubber band.
2.Rupture: Eventually, the stress becomes so great that it overcomes the friction. The rock suddenly breaks and snaps back to its original, unstressed shape.
3.Energy Release: This sudden 'rebound' releases all the stored energy in the form of vibrations called seismic waves. These waves travel outwards from the break, causing the ground to shake.
Focus and Epicenter
The focus (or hypocenter) is the point inside the Earth where the rock first breaks and the earthquake begins.
The epicenter is the point on the Earth's surface directly above the focus. The shaking is usually strongest at the epicenter.
Seismic Waves
An earthquake produces different types of seismic waves. The two main types that travel through the Earth's interior are body waves:
P-waves (Primary waves):
They are compressional waves, meaning they push and pull the rock in the same direction that the wave is traveling (like a Slinky being pushed).
They are the fastest seismic waves and are the first to be detected by seismographs.
They can travel through solids, liquids, and gases.
S-waves (Secondary waves):
They are shear waves, meaning they move the rock up and down or side-to-side, perpendicular to the direction the wave is traveling.
They are slower than P-waves and arrive second.
They can only travel through solids. They cannot travel through liquids. The fact that S-waves do not pass through the Earth's outer core is the primary evidence that it is liquid.
Measuring Earthquakes
Scientists use two main scales to measure earthquakes:
The Mercalli Scale: This scale measures the intensity of an earthquake. It describes the effects of the earthquake at a specific location, based on what people felt and how much damage was done. It uses Roman numerals (I - XII) and is a qualitative measure.
The Richter Scale: This scale measures the magnitude of an earthquake. It is a measure of the total amount of energy released at the earthquake's source (the focus). It is a logarithmic scale, which means that for each whole number you go up, the ground shaking increases by 10 times. A magnitude 7 earthquake is 10 times stronger than a magnitude 6.