One block may move up while the other moves down, or one may move horizontally in one direction and the other in the opposite direction. There are over a million quakes annually, including those too small to be felt. The following table shows the average frequency of different magnitudes. 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.
The surface of the earth is divided like a jigsaw puzzle into giant pieces called tectonic or crustal plates. These giant pieces move slowly over partially melted rock known as the mantle . As they move, they slide along each other, move into each other, move away from each other, or one slips under another. California, Alaska, Japan, South America, and the Philippines are all on plate boundaries. Alaska has more earthquakes per year than the combined total of the rest of the United States.