The Big One

The "Big One" is a hypothetical devastating that is predicted to occur in. The term has been variously used in common parlance over the years, but generally refers to a severely destructive earthquake, similar to or worse than the. In 2015, the U.S. Geological Survey predicted a 7% chance of a magnitude 8.0 earthquake in the next 30 years. Another U.S Geological Survey study predicted that a magnitude 7.8 earthquake along the southern in  could cause about 1800 deaths and $213 billion in damage.

San Andreas Fault
The San Andreas Fault is a continental that extends roughly 1200 km through. It forms the boundary between the  and the, and its motion is  (horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk. The slip rate along the fault ranges from 20 to 35 mm/yr.

The southern segment, which stretches from in  all the way to the, is capable of an 8.1-magnitude earthquake. At its closest, this fault passes about 35 mi to the northeast of Los Angeles. Such a large earthquake on this southern segment would kill thousands of people in Los Angeles, San Bernardino, Riverside, and surrounding areas, and cause hundreds of billions of dollars in damage.

A study published in 2006 in the journal Nature found that the San Andreas fault has reached a sufficient stress level for an earthquake of magnitude greater than 7.0 on the to occur. This study also found that the risk of a large earthquake may be increasing more rapidly than scientists had previously believed. Moreover, the risk is currently concentrated on the southern section of the fault, i.e. the region around Los Angeles, because massive earthquakes have occurred relatively recently on the central and northern  segments of the fault, while the southern section has not seen any similar rupture for at least 300 years. According to this study, a massive earthquake on that southern section of the San Andreas fault would result in major damage to the - metropolitan area and other cities in, and  counties in California, and  in. It would be strongly felt (and potentially cause significant damage) throughout much of, including densely populated areas of , , , , and , ,  in  and ,. Older buildings would be especially prone to damage or collapse, as would buildings built on unconsolidated gravel or in coastal areas where water tables are high (and thus subject to ). The paper 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."

Nevertheless, in the 18 years since that publication there has not been a substantial quake in the Los Angeles area, and two major reports issued by the (USGS) have made variable predictions as to the risk of future seismic events. The ability to predict major earthquakes with sufficient precision to warrant increased precautions has remained elusive.

The most recent forecast, known as  (Uniform California Earthquake Rupture Forecast 3), released in November 2013, estimated that an earthquake of magnitude 6.7 M or greater (i.e. equal to or greater than the ) occurs about once every 6.7 years statewide. The same report also estimated there is a 7% probability that an earthquake of magnitude 8.0 or greater will occur in the next 30 years somewhere along the San Andreas Fault. A different USGS study in 2008 tried to assess the physical, social and economic consequences of a major earthquake in southern California. That study predicted that a magnitude 7.8 earthquake along the southern San Andreas Fault could cause about 1,800 deaths and $213 billion in damage.

Cascadia connection
A 2008 paper, studying past earthquakes along the Pacific coastal zone, found a correlation in time between seismic events on the northern San Andreas Fault and the southern part of the (which stretches from  to northern California). Scientists believe quakes on the Cascadia subduction zone may have triggered most of the major quakes on the northern San Andreas within the past 3,000 years. The evidence also shows the rupture direction going from north to south in each of these time-correlated events. However the 1906 San Francisco earthquake seems to have been the exception to this correlation because the plate movement was moved mostly from south to north and it was not preceded by a major quake in the Cascadia zone.

Earthquakes
The San Andreas Fault has had some notable earthquakes in historic times:


 * : About 350 km were ruptured in central and southern California. Though it is known as the earthquake, the epicenter is thought to have been located far to the north, just south of . Two deaths were reported. Its  was 7.9.
 * : About 430 km were ruptured in Northern California. The epicenter was near . At least 3,000 people died in the earthquake and subsequent fires. The magnitude was estimated to be 7.8.
 * : A magnitude 5.7 quake with an epicenter on the San Andreas fault in the ocean west of San Francisco and Daly City.
 * : About 40 km were ruptured (although the rupture did not reach the surface) near, causing 63 deaths and moderate damage in certain vulnerable locations in the San Francisco Bay Area. was about 6.9. This quake occurred on October 17, 1989, at approximately 5:04 pm PDT.
 * : On September 28, 2004, at 10:15 am PDT, a magnitude 6.0 earthquake struck the Parkfield area. It was felt across the state, including the San Francisco Bay Area.