Compressional and tensional stress in rock. When the rock reaches its elastic limit, it snaps!, releasing energy in the form of vibration.
Primary - The initial vibration. Usually one big jar.
Secondary - This wave does the real damage. It is the side-to-side shaking that no building can fully withstand.
This scale measures intensity of earthquake waves. Utilizes a seismograph.
Modern technology still has no way of predicting earthquakes, but they can prevent damage done to buildings and cities using new building techniques.
This gives more drunken teens a good reason to get their parents to let them go on summer vacation in California.