Faults

Faults are discontinuities or fractures in a volume of rock due to different plate movements that stretch, shear, or compress that rock. Faults are commonly found at the three types of plate boundaries: convergent, divergent, and transform – and it is these faults that are responsible for earthquakes. There are three main types of faults: normal, reverse, and strike-slip, and they each create different types of earthquakes. As you will see, these fault structures are very similar to the plate boundary structures in our tutorial on plate boundaries.

Normal Fault:

Anatomy of a normal and reverse fault.
Credit: Wikipedia User Cferrero

Normal fault.
Retrieved from mamontoff.org

Normal faults occur when two masses are being pulled apart and one slides down the other. This is analogous to the divergent plate boundaries that form mid-ocean ridges. Earthquakes with these types of faults can be frequent, but they are weaker than the earthquakes caused by reverse or strike-slip faults because the diverging motion prevents large amounts of stress from building up.

Reverse Fault:

A reverse fault.
Retrieved from San Dieguito Union High School District page on Faults

Reverse faults occur when two masses converge and one subducts under the other. Extremely long reverse faults at subduction zones between plates can cause the largest earthquakes in the world due to the enormous stress that builds up as one plate is “thrust” under the other. Earthquakes occur when this stress is released. The largest of these earthquakes are called megathrust earthquakes.

A reverse fault in the Grands Causses near Bédarieux, France. The right side moves up while the left side moves down.
Credit: Wikimedia User Xhienne

Strike-Slip Fault:

Anatomy of a normal and reverse fault.
Credit: Wikipedia User Cferrero

Strike slip faults are faults where masses of rock slide laterally past each other. Fast moving strike-slip faults can cause frequent earthquakes, and the earthquakes that do occur are often very shallow and can therefore be extremely intense. The great San Francisco Earthquake of 1906 not only had a moment magnitude of 7.8 but occurred at a depth of 5 miles, and as a result, 80% of San Francisco was destroyed, with much of the damage due to fires throughout the city initiated by the earthquake that roared on for several days. This earthquake occurred along the most famous transform/strike-slip fault in the world, the San Andreas Fault.

San Francisco on fire in 1906.
Credit: Library of Congress

The San Andreas Fault along the Carrizo Plain. Credit: USGS

Not all tectonic earthquakes occur on fault zones, but the vast majority do. As the pictures below show, most of the earthquakes are centered across convergent, divergent, or transform plate boundaries, where the vast majority of faults are found. These “interplate” earthquakes contrast with “intraplate” earthquakes, which occur in the interior of plates and are relatively rare. A significant number of volcanic earthquakes are centered on “hot spots,” which are locations of volcanic activity not affiliated with plate tectonics (Hawaii and Yellowstone are two examples of hot spots in the United States).

Credit: NASA

To learn more about tectonic plate boundaries – the driving force behind the formation of faults, click here.

Written by Charlie Phillips – charlie.weathertogether.net. Last updated 12/1/2017