The San Andreas fault is one of California’s most dangerous fault and its longest.
It has been quiet since 1857, and that’s too quiet for leading earthquake scientists, who say the fault is locked, loaded and ready to roll.
Although the Pacific plate is moving northwest relative to North America at about 16ft (5m) every 100 years the southern San Andreas fault has been quiet for more than a century. In other words, the fault has not relieved stress that has been building up for more than a century.
Other sections of the San Andreas fault also are far overdue for a big quake. Further southeast of the Cajon Pass, such as in San Bernardino County, the fault has not moved substantially since an earthquake in 1812, and further southeast toward the Salton Sea, it has been relatively quiet since about 1680 to 1690.
Even though the San Andreas fault does not go directly into Los Angeles — it is 30 miles away from downtown — the city is expected to be heavily shaken by a large earthquake on that fault.
Simulations of a possible magnitude-7.8 San Andreas fault that begins at the Salton Sea and spreads west toward the San Gabriel mountains show seismic shaking waves “bent into the Los Angeles area,” Jordan said. One video shows strong ground shaking stretching from northern San Diego County to Barstow.
Using the world’s largest supercomputer at the time, the Southern California Earthquake Center in 2010 unveiled a simulated magnitude-8 earthquake that begins in Monterey County, like in 1857, but travels even farther south, heading toward the Mexican border. The L.A. Basin and the San Fernando Valley will be hit hard because the shaking will be trapped by soft soils in the valley and basin.
Facts about a rupture of the southern San Andreas Fault
A 2008 U.S. Geological Survey report warned that a magnitude 7.8 earthquake on the southern San Andreas fault would cause more than 1,800 deaths, 50,000 injuries, $200 billion in damage and severe, long-lasting disruptions.
Such an earthquake could cause shaking for nearly two minutes, with the strongest shaking in the Coachella Valley, Inland Empire and Antelope Valley, but it also could send pockets of strong shaking into areas where sediments trap shaking waves, such as the San Gabriel Valley and East Los Angeles.
Earthquake history along the fault
The devastating potential of the fault became clear with a 1857 temblor, which had an estimated magnitude of 7.9. It became known as the Fort Tejon quake.
The quake then barreled south on the San Andreas for about 185 miles, through Fort Tejon near the northern edge of Los Angeles County, then east toward the Cajon Pass in San Bernardino County, near what is now the 15 Freeway.
The quake was so powerful that the soil liquefied, causing trees as far away as Stockton to sink. Trees were also uprooted west of Fort Tejon. The shaking lasted 1 to 3 minutes.