A look at how geologists are keeping an eye on California's 800 mile-long San Andreas Fault which many believe is overdue for a major earthquake in the region that could destroy some of the most valuable real-estate in the world. The San Andreas Fault is a continental transform fault that extends roughly 810 miles (1,300 km) through California in the United States. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics, and a different degree of earthquake risk. Although the most significant (Southern) segment only dates back about 5 million years, the oldest sections were formed by the subduction of a spreading ridge 30 million years ago. The fault was first identified in 1895 by Professor of geology Andrew Lawson from UC Berkeley who discovered the northern zone. It is named after a small lake which was formed in a valley between the two plates. Following the 1906 San Francisco Earthquake, Lawson concluded that the fault extended all the way into southern California. In 1953, geologist Thomas Dibblee astounded the scientific establishment with his conclusion that hundreds of miles of lateral movement could occur along the San Andreas Fault. The likelihood of a Megathrust earthquake ('the big one') is the subject of formal study at the University of California. A project called the San Andreas Fault Observatory at Depth (SAFOD) is drilling into the fault to improve recording and prediction of future quakes. The next "Big One" A study in 2006 concluded that the San Andreas fault has reached a sufficient stress level for the next "big one", or a M ≥ 7.0, to occur. It also concluded that the risk of a large earthquake may be increasing more rapidly than researchers had previously thought. The paper stated that, while the San Andreas Fault had experienced massive earthquakes in the central (1857) and northern (1906) segments, the southern section of the fault has not seen any similar rupture for at least 300 years. Such an event would result in substantial damage to Palm Springs and other cities in San Bernardino, Riverside and Imperial counties in California, and Mexicali municipality in Baja California. It would be felt throughout much of Southern California, including densely populated areas of metropolitan San Bernardino, Los Angeles, Orange County, San Diego, Ensenada and Tijuana, Baja California, San Luis Rio Colorado in Sonora and Yuma, Arizona. It concluded: "The information available suggests that the fault is ready for the next big earthquake but exactly when the triggering will happen and when the earthquake will occur we cannot tell [...] It could be tomorrow or it could be 10 years or more from now. As both the public and scientific community continue to speculate on the size of the next earthquake to strike California, predicting major earthquakes with sufficient precision to warrant taking increased precautions has long been sought but remains elusive. Nonetheless, the 2008 Uniform California Earthquake Rupture Forecast (UCERF) has estimated that the probability of an M ≥ 6.7 earthquake within the next 30 years on the northern and southern segments of the San Andreas fault is somewhere between 21% and 59%. Cascadia connection Recent studies of past earthquake indicate there is a correlation in time between seismic events on the northern San Andreas Fault and the southern part of the Cascadia subduction zone (which stretches from Vancouver Island to northern California). The San Andreas Fault has had some notable earthquakes in historic times: 1857 Fort Tejon earthquake: About 217 miles (349 km) were ruptured in central and southern California. Though it is known as the Fort Tejon earthquake, the epicenter is thought to have been located far to the north, just south of Parkfield. Only two deaths were reported. The magnitude was about 7.9. 1906 San Francisco earthquake: About 267 miles (430 km) were ruptured in Northern California. The epicenter was near San Francisco. At least 3000 people died in the earthquake and subsequent fires. This time the magnitude was estimated to be 7.8. 1989 Loma Prieta earthquake: About 25 miles (40 km) were ruptured (although the rupture did not reach the surface) near Santa Cruz, California, causing 63 deaths and moderate damage in certain vulnerable locations in the San Francisco Bay Area. Moment magnitude this time was about 6.9. The earthquake also postponed game 3 of the 1989 World Series at Candlestick Park. This quake occurred on October 17, 1989, at approximately 5:04 P.M. PDT. 2004 Parkfield earthquake: On September 28, 2004, at 10:15 A.M. PDT, a magnitude 6.0 earthquake struck California on the San Andreas Fault. It was felt across the state, including the San Francisco Bay Area.

• published: 12 Dec 2013
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