LEARN MORE ABOUT FAULTS

California's Earthquakes are caused by the movement between two tectonic plates, the Pacific Plate, and the North American Plate. Did you know that San Diego and Santa Barbara actually lie on the Pacific Plate, and not on the North American Plate as does most of the rest of the United States? The Western portion of California is an area which experiences frequent and strong earthquakes due to the interactions between the two tectonic plates. Earthquakes are vibrations in the earth produced by a rapid release of energy, such as a sudden slip on a fault.

The Pacific Plate is moving northwest relative to the North American Plate at a rate of 45 mm/year. This movement occurs along faults, mainly the San Andreas Fault. The San Andreas Fault system is the major fault system found in California. Fault systems running parallel to the San Andreas include the San Jacinto, Elsinore and Imperial.

Faults are breaks in rock masses along which movement has occurred. A fault zone can be identified by a band of crushed rock. The length of a fault may vary from a few centimeters to thousands of kilometers.

The Pacific Plate moves Northwest relative to the North American Plate at approximately the same rate that your fingernails grow. If you never cut your fingernails and you lived to be 100 years old, your fingernails would be approximately ten and a half feet long!
45 mm/year* may not seem like very much movement, but in
YEARS
100 years
1000 years
10000 years
1 Million years
 DISTANCE
4.5 m
45 m
450 m
45,000 m

or
or
or
or

10.5 ft
105 ft
1,050 ft
105,000 ft

So, in 10 million years, San Diego will be located approximately where we now find Santa Barbara.

* Faults do not necessarily slip the entire 45 mm/year, but can instead store up a portion of the 45 mm each year. This slip can be released in the form of a larger, infrequent earthquake.

Click here for an animation of possible future movement along the San Andreas Fault.

Strike Slip or Lateral Faults

Strike Slip or Lateral Faults are characterized by the horizontal movement of rocks on one side of a fault sliding past rocks on the other side. Many strike-slip faults are associated with plate boundaries and are called Transform faults. The San Andreas Fault is a Transform fault since it is associated with the boundary between the Pacific and the North American Plates.

Right Lateral Fault

The block on the opposite side of the fault moves to your right, as you face the fault. The San Andreas Fault is an example of a right lateral fault. Click here to see an animation of the fault movement.

Left Lateral Fault

The block on the opposite side of the fault moves to your left, as you face the fault. Click here to see an animation of the fault movement.

The Big Bend is a region of the San Andreas Fault where the fault is bent in a westerly directly, complicating the movement. The motion along this portion of the fault is not completely right lateral, but has a strong compressional component.

Evidence of Lateral Movement along a Fault

Stream Bed Displacement

Movement along faults causes intersecting features to be offset. In this case, a stream that crossed the fault has been offset. The block on the right side of the image is being displaced to the right relative to the block on the left. This is a right lateral fault. This is an aerial view of the San Andreas fault, west of Taft, California.

Fault Scarps or Fractures in the Earth's Surface

Click on image to zoom in.

This is an image of the North Anatolian fault (looking towards the south) in Turkey. It was taken from the Space Shuttle as it orbited 126 nauticle miles above the earth.

Instrument Recordings

Seismographs are instruments that magnify and record the motions of the Earth's surface caused by seismic waves. The amplitude of these seismic waves are translated into the Richter magnitude scale by seismologists.

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