Home >> Science & Nature >> Nature

A Description of Earthquake Faults

A fault is a fracture between two blocks of rock. It can be very small or very large, measured in the hundreds of miles. Some faults have a series of fractures between the blocks. Faults enable these blocks to move around relative to each other, and this activity can cause earthquakes. If the movement occurs rapidly, it causes an earthquake; if it occurs slowly it is called creep. Scientists classify faults based on the angle of the fault and the direction of slip along the fault.

Thrust Fault
- Thrust faults move deeper rocks over shallower ones. According to geologist Rob Butler, thrust faults are commonly found in areas where rocks have experienced horizontal shortening, places with down-slope failures at locations such as the toes of landslides. Thrust faults are in areas of compression where one plate is subducted under another plate; these occur in Japan and the Washington coast. A blind thrust fault is a thrust fault that does not rupture all the way up to the surface because it is buried under the uppermost layers of rock in the Earth's crust.
Normal Fault
- A normal fault is one in which the upper block drops down over the lower block. Normal faults occur in places where the lithosphere is stretching, pulling the sides apart. The lithosphere is the outermost part of the inside of the earth, including the crust and the upper mantle. Most active normal faults dip at angles steeper than 50 degrees. Sometimes normal faults are found in batches of faults that are all dipping in the same direction. These are called "domino faults." In addition to causing earthquakes, normal faults are responsible for bringing metamorphic rocks to the Earth's surface.
Reverse Fault
- Reverse faults are just the opposite of normal faults. They occur when the upper block moves up instead of down. Reverse faults come from compressional forces that push the side together.
  
  Normal faults and reverse faults are known as "dip-slip faults" because their motion goes either up or down along the dip direction.
Strike-Slip Fault
- In a strike-slip fault, the two blocks slide past one another. They move sideways rather than up or down along the dip direction. The U.S. Geological Survey says that strike-slip faults are vertical fractures where the blocks have moved horizontally. There are two specific types of strike-slip faults: right-lateral and left-lateral. The displacement of the far block is to the left in a left-lateral strike-slip fault, and to the right in a right-lateral strike-slip fault. The San Andreas Fault is an example of a right-lateral fault.

Nature