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. Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles). Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.
Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of the earth. This motion is what causes earthquakes and volcanoes and has created most of the spectacular scenery around the world.
The so called Richter Scale was developed in 1935 by Charles Richter and Beno Gutenberg, who were both working at the California Institute of Technology The Richter magnitude test scale (local magnitude scale) assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismometer output. Measurements have no limits and can be either positive or negative. The Richter scale is not a physical device, but a mathematical formula. Another Richter scale is used for the Richter tuned harmonica, which is the most widely known type of harmonica. It has ten holes that offer the player a total of 19 notes. And yes, this has nothing to do whatsoever with earthquakes ;-)
Micro < 2.0
Very minor 2.0-2.9
Generally not felt, but recorded
Often felt, but rarely causes damage
Noticeable shaking of indoor items, rattling noises. Significant damage unlikely
Can cause major damage to poorly constructed buildings over small regions. At most slight damage to well-designed buildings
Can be destructive in areas up to about 100 miles across in populated areas
Can cause serious damage over larger areas
Can cause serious damage in areas several hundred miles across
Rarely, great 9.0 or greater
Devastating in areas several thousand miles across
Per month : Approximately 80,000 - Per day : Approximately 2,600 - Per minute : Approximately 2 - And, per second, one earthquake is felt approximately every 30 seconds. Of these only a relative few are capable of causing damage. Earthquakes are common natural events.