What is an Earthquake?
Earthquakes are a natural event. An earthquake occurs because of geologic forces inside the Earth. These forces build up slowly and eventually because so strong that they cause rocks to break underground. When this happens, tremendous energy is released suddenly in the form of motion that spreads out in all directions from the break, causing the ground to shake and move. This sudden release of energy and movement is what makes an earthquake so destructive.
The Ripple Affect
If you threw a stone into a calm pond, it creates a short splash and then ripples spread outward for a much linger time. Something similar happens in the Earth when there is an earthquake. When rocks slip past each other along a fault, the initial movement is over very quickly. However, the energy that is released causes ripples, called surface waves that spread outward. These waves gradually get weaker the farther they travel, until they eventually die out.
Earthquake Frequency
Different-sized earthquakes occur with greater or lesser frequency depending on the magnitude...There are approximately 10,000 earthquakes for magnitude 4 or greater each year… and there are about ten earthquakes of magnitude 7 or greater each year.