Where Do Earthquakes Happen?

Earthquakes occur all the time all over the world, both along plate edges and along faults.

Along Plate Edges

Most earthquakes occur along the edge of the oceanic and continental plates. The earth's crust (the outer layer of the planet) is made up of several pieces, called plates. The plates under the oceans are called oceanic plates and the rest are continental plates. The plates are moved around by the motion of a deeper part of the earth (the mantle) that lies underneath the crust. These plates are always bumping into each other, pulling away from each other, or past each other. The plates usually move at about the same speed that your fingernails grow. Earthquakes usually occur where two plates are running into each other or sliding past each other.



Figure 1 - An image of the world's plates and their boundaries. Notice that many plate boundaries do not coincide with coastlines.

Along Faults

Earthquakes can also occur far from the edges of plates, along faults. Faults are cracks in the earth where sections of a plate (or two plates) are moving in different directions. Faults are caused by all that bumping and sliding the plates do. They are more common near the edges of the plates.

Types of Faults

Normal faults are the cracks where one block of rock is sliding downward and away from another block of rock. These faults usually occur in areas where a plate is very slowly splitting apart or where two plates are pulling away from each other. A normal fault is defined by the hanging wall moving down relative to the footwall, which is moving up.



Figure 2 - A normal fault. The 'footwall' is on the 'upthrown' side of the fault, moving upwards. The 'hanging wall' is on the 'downthrown' side of the fault, moving downwards.

Reverse faults are cracks formed where one plate is pushing into another plate. They also occur where a plate is folding up because it's being compressed by another plate pushing against it. At these faults, one block of rock is sliding underneath another block or one block is being pushed up over the other. A reverse fault is defined by the hanging wall moving up relative to the footwall, which is moving down.



Figure 3 - A reverse fault. This time, the 'footwall' is on the 'downthrown' side of the fault, moving downwards, and the 'hanging wall' is on the 'upthrown' side of the fault, moving upwards. When the hanging wall is on the upthrown side, it 'hangs' over the footwall.

Strike-slip faults are the cracks between two plates that are sliding past each other. You can find these kinds of faults in California. The San Andreas fault is a strike-slip fault. It's the most famous California fault and has caused a lot of powerful earthquakes.

     

Figure 4 - Two strike-slip faults. (left), A left-lateral strike-slip fault. No matter which side of the fault you are on, the other side is moving to the left. (right), A right-lateral strike-slip fault. No matter which side of the fault you are on, the other side is moving to the right.

Figure 1 used with permission from USGS.
<http://geology.er.usgs.gov/eastern/plates.html>.
Figures 2 through 4 used with permission from USGS.
<http://wrgis.wr.usgs.gov/docs/parks/deform/7modelsa.html>.
All other content 2007 Michigan Technological University. Permission granted for reproduction for non-commercial uses.