Earthquake faults are caused by the Earth's crust moving upward, downward, and sideways. Normal faults occur when plates move apart. Reverse faults develop when plates smash together. The San Andreas Fault is a place where plates slip past each other.
Rock layers broken by a fault, USGS
What are faults
Faults are fractures in the crust where rocks move parallel to the break. Major faults like the San Andreas Fault develop where crustal plates meet. Convection currents inside the Earth are the driving force that create faults.
Faults in subduction zones
Subduction zone faults form where one plate is being forced beneath another plate. The fault line in these areas can be hundreds of kilometers long. When they break along this fault line it creates great earthquakes that can be over 9.0 on the moment magnitude scale.
Normal earthquake faults develop in areas where land is pulling apart or stretching. The rocks fracture and one block of land slips downward in relation to the block of land on the other side of the fault zone.
A reverse fault develops in areas where land masses are coming together. The land is compressed and one side of the fault moves upward.
If the angle between the two land masses is less than 45 degrees one section may slip over another without reaching the surface. Faults with less than a 45 degree angle are thrust faults.
San Andreas Fault
The San Andreas Fault is a strike slip fault. If you stand on one side of the San Andreas Fault it looks like the land has moved to the right. Scientists call this type of fault a right lateral strike slip fault. During the 1906 San Francisco earthquake a road at Tomales Bay was offset 21 feet.
The San Andreas Fault is also called a transform fault. This type of fault is found in the middle of the oceans where plates are moving apart. The transform fault connects two normal faults that are separated by the curvature of the Earth.
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