What causes earthquakes?

An earthquake is caused by the sudden release of energy that results when rock in the Earth’s crust is under so much stress that it suddenly breaks. The split masses of rock then grind past one another as they are pushed by the forces that caused the stress. The area of contact between the grinding masses is called a fault. The direction of motion of each of the rock masses may be horizontal, vertical, or a combination of these motions (see figure 4). The force that causes the stress within the rock is a result of the movement of giant plates that make up the Earth’s outer layer (see figures 5 and 6).

Figure 4. If movement along a fault is horizontal (A), the fault is called a strike-slip fault. If movement is vertical (B), it is called a dip-slip fault. The fault shown in B is also called a normal fault. If the block in front was pushed up and over the block behind it, it would be called a reverse fault.
Images by Jeff Kirby.

Figure 5. A very simplified representation of the movement of three plates. The yellow arrows are meant to assist with visualizing the general direction of plate movement. The Pacific and North American plates are sliding past each other along the San Andreas fault. The red arrows show the convection currents that cause the African and North American plates to spread apart, allowing magma to move into the gap and create new crust.
Image by Jeff Kirby.

Plate tectonics

The outer layer of the Earth is divided into huge plates, like a cracked eggshell. Driven by convection that permits heat to escape from the Earth’s interior, the plates move at a rate of about 2 to 25 centimeters per year, carrying with them continental land masses and ocean floor alike.

Figure 6. View of the Earth showing approximate margins of the major plates. Most earthquakes are triggered when plates grind past each other laterally, as they do in California, or vertically, as in Alaska or South America. Earthquakes felt in Indiana are the result of stresses transmitted inward from the boundaries and perhaps the base of the North American plate. (Arrows indicate general direction of plate movement.) Modified from Simkin and others, 1994; earth image from NASA Web site.