The Big One

The "Big One" is a hypothetical devastating earthquake that is predicted to occur in California similar to or worse than the 1906 San Francisco earthquake.

## San Andreas Fault

The **San Andreas Fault** is a continental transform fault that extends roughly 1,200 kilometers (750 mi) through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate

* Its motion is right-lateral strike-slip (horizontal).
* The slip rate along the fault ranges from 20-35 mm/yr.

The fault divides into three segments, each with different characteristics and a different degree of earthquake risk.

### Northern

The northern segment of the fault runs from Hollister, through the Santa Cruz Mountains, epicenter of the 1989 Loma Prieta earthquake, then up the San Francisco Peninsula then offshore at Daly City near Mussel Rock.

* This is the approximate location of the epicenter of the 1906 San Francisco earthquake.

### Central

The central segment of the San Andreas 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, where the fault slips continuously without causing earthquakes.

### Southern

The southern segment, which stretches from Parkfield in Monterey County all the way to the Salton Sea, is capable of an 8.1-magnitude earthquake. At its closest, this fault passes about 35 miles (56 km) 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.

## The Big One

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 (1857) and northern (1906) segments of the fault, while the southern section has not seen any similar rupture for at least 300 years.

* A massive earthquake on that southern section of the San Andreas fault would result in major damage to the Palm Springs-Indio metropolitan area and other cities in San Bernardino, Riverside and Imperial counties in California, and Mexicali Municipality in Baja California.

* It would be strongly felt (and potentially cause significant damage) throughout much of Southern California, including densely populated areas of Los Angeles County, Ventura County, Orange County, San Diego County, Ensenada Municipality and Tijuana Municipality, Baja California, San Luis Rio Colorado in Sonora and Yuma, Arizona.

* 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 soil liquefaction).

The U.S. Geological Survey most recent forecast, known as UCERF3 (Uniform California Earthquake Rupture Forecast 3), released in November 2013, estimated that 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 Cascadia subduction zone (which stretches from Vancouver Island 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

See also: List of earthquakes in California
The San Andreas Fault has had some notable earthquakes in historic times:

* 1857 Fort Tejon earthquake: About 350 kilometers (220 mi) 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. Two deaths were reported. Its moment magnitude was 7.9.
* 1906 San Francisco earthquake: About 430 kilometers (270 mi) were ruptured in Northern California. The epicenter was near San Francisco. At least 3,000 people died in the earthquake and subsequent fires. The magnitude was estimated to be 7.8.
* 1957 San Francisco earthquake: A magnitude 5.7 quake with an epicenter on the San Andreas fault in the ocean west of San Francisco and Daly City.
* 1989 Loma Prieta earthquake: About 40 kilometers (25 mi) 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 was about 6.9. This quake occurred on October 17, 1989, at approximately 5:04 pm PDT.
* 2004 Parkfield earthquake: 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.