The San Andreas Fault is a geological fault, known as a right-lateral strike-slip fault, that spans a length of roughly 800 miles (1287 kilometers) through California. The San Andreas Fault marks a transform boundary between the Pacific Plate and the North American Plate. This fault is famous for producing large and devastating earthquakes.
On September 28, 2004 at 10:15 am, a magnitude 6.0 earthquake struck at Parkfield, California on the San Andreas Fault. This earthquake was originally expected to strike in 1993 based on then recently discovered theories of earthquake prediction. Eleven extra years elapsed before this prediction was finally fulfilled. Despite the extra time, the earthquake was no larger than originally expected.
The San Andreas Fault can be divided into 3 segments.
The southern segment begins in near the Salton Sea and runs northward before it begins a slow bend to the west when it meets the San Bernardino Mountains. Here, it runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Pass and continues to run northwest along the northern base of the San Gabriel Mountains. These mountains are a result of movement along the San Andreas Fault and are commonly called the Transverse Range.
After crossing through Frazier Park, the fault begins to bend northeast. This area is referred to as the "Big Bend" and is thought to be where the fault locks up in Southern California as the plates try to move past each other. This section of the fault has a recurrance interval of roughly 140 - 160 years.
The central segment of the fault runs in a northwestern direction from Parkfield to Hollister. While the southern section of the fault and the parts through Parkfield experience earthquakes, the rest of the central section of the fault exhibits a phenomenon called aseismic creep . This results in the fault being able to move without the need of earthquakes.
The northern segment of the fault runs from Hollister, through the San Francisco Peninsula where it briefly goes offshore, then follows the coast of California fairly closely before it makes a sharp turn west and goes offshore near Eureka, California.
The small town of Parkfield, California lies along the San Andreas Fault. Seismologists discovered that this section of the fault consistantly produces magnitude 6.0 earthquakes about every 22 years. Following earthquakes in 1857, 1881, 1901, 1922, 1934 and 1966, scientists predicted an earthquake to hit Parkfield in 1993. This quake eventually struck in 2004 (see Parkfield earthquake). Because of this frequent activity and prediction, Parkfield has become one of the most popular spots in the world to try to capture and record large earthquakes.
In 2004, work began just north of Parkfield on the San Andreas Fault Observatory at Depth (SAFOD). The goal of SAFOD is to drill a hole nearly 2.5 miles (4 kilometers) into the Earth's crust and into the San Andreas Fault. An array of sensors will be installed to capture and record earthquakes that happen near this area.
Other research that monitors slip rates along the fault has shown that Los Angeles and San Francisco (which rest on opposite sides of the San Andreas Fault) move towards one another at a rate of a 1/4 inch (0.6 cm) a year.
The San Andreas Fault has had three notable earthquakes in historic times:
- The Parkfield Experiment http://quake.wr.usgs.gov/research/parkfield/index.html
- San Andreas Fault Observatory at Depth http://www.icdp-online.de/sites/sanandreas/index/index.html
- Southern California Earthquake Data Center: San Andreas Fault http://www.data.scec.org/fault_index/sanandre.html
- USGS: The San Andreas Fault http://pubs.usgs.gov/gip/earthq3/
Last updated: 02-07-2005 13:12:27
Last updated: 05-03-2005 17:50:55