The Shaking Earth
An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves.
Cause of Earthquakes: Elastic Rebound Theory
Tectonic plates are constantly moving, but friction at plate boundaries (faults) can cause them to become locked.
As the plates continue to move, the rocks on either side of the fault bend and store elastic potential energy, like a stretched rubber band.
When the stress exceeds the strength of the rocks, the fault ruptures. The rocks suddenly snap back to their original, unstressed shape in a process called elastic rebound.
This sudden movement releases the stored energy in the form of seismic waves, which radiate outwards from the point of rupture.
Earthquake Terminology
Focus (or Hypocenter): The point within the Earth where the earthquake rupture begins.
Epicenter: The point on the Earth's surface directly above the focus.
Seismic Waves
Body Waves: Travel through the Earth's interior.
1.P-waves (Primary waves): These are longitudinal (compressional) waves. They are the fastest seismic waves and can travel through solids, liquids, and gases.
2.S-waves (Secondary waves): These are transverse (shear) waves. They are slower than P-waves and can only travel through solids. The fact that S-waves do not pass through the Earth's outer core is the primary evidence that it is liquid.
Surface Waves: Travel along the Earth's surface and are the most destructive.
Measuring Earthquakes
Mercalli Intensity Scale: Measures the effects of an earthquake at a particular location (what people felt, how much damage was done). It is a descriptive, qualitative scale.
Richter Magnitude Scale: A logarithmic scale that measures the magnitude of an earthquake, which is related to the amount of energy released. An increase of 1 on the Richter scale corresponds to a 10-fold increase in the amplitude of the seismic waves and roughly a 32-fold increase in the energy released.