Californians are being told to brace for 'the big one' - a massive earthquake that typically occurs every 400 to 600 years and could leave thousands dead or homeless.
An earthquake scientist has added to claims the dreaded event is overdue, warning the San Andreas fault is 'locked, loaded and ready to roll'.
The fault is the longest in California and one of the state's most dangerous.
Thomas Jordan, director of the Southern California Earthquake Centre, said the fault has been 'too quiet' since 1857.
This is when the last big quake to strike a southern section rippled from Monterey County to the San Gabriel Mountains near Los Angeles with a magnitude of 7.9 on the Richter scale.
Speaking at the National Earthquake Conference in Long Beach, Mr Jordan said: 'The springs on the San Andreas system have been wound very, very tight and the southern San Andreas fault, in particular, looks like it's locked, loaded and ready to go.'
He also said other sections of the 810 mile-long (1,304km) fault are overdue for a quake too, the Los Angeles Times reported.
The fault is the longest in California and one of the state's most dangerous. This image shows layers of earth pushed into vertical positions by the collision of the Pacific and North American tectonic plates along the southern San Andreas Fault
In San Bernardino County, the fault hasn't moved much since an earthquake in 1812, with a southeast section near Salton Sea has been quiet since around 1690.
While scientists say the Pacific plate moving northwest of the North American plate should move 16ft (4.8 metres) every 100 years to relieve stress, this hasn't happened at San Andreas so stress has been building at points along the fault for more than a century.
WHAT IS THE 'BIG ONE'?
The 'Big One' is a hypothetical earthquake of magnitude 8 or greater that is expected to happen along the San Andreas fault.
Such a quake is expected to produce devastation to human civilization within about 50-100 miles of the quake zone, especially in urban areas like Palm Springs, Los Angeles and San Francisco.
He said it is important the state prepares to be rocked by a quake as strong as 8 on the Richter scale and praised Los Angles' plans to reinforce older concrete buildings and the city's aqueduct and telecommunications networks.
While the fault doesn't run under the city, a mega quake is expected to rock it, according to simulations.
A report by the US Geological Survey in 2008 warned a magnitude 7.8 earthquake could result in 1,800 deaths, 50,000 injuries and $200 billion (£138 billion) of damage.
Such devastation could be brought about by a strong quake in just two minutes, striking in the Coachella Valley, for example, which could also shake areas where sediments trap waves, such as east Los Angeles.
The damage caused by a similar strength quake was last seen in 1857, when an earthquake originating in Parkfield in Monterey County travelled south along the fault for 185 miles (300km), and then east from LA.
A report by the US Geological Survey in 2008 warned a magnitude 7.8 earthquake could result in 1,800 deaths, 50,000 injuries and $200 billion of damage. Here, scientists and politicians take part in an earthquake drill in an elementary school by hiding under a table
This haunting photograph shows people walking through rubble in San Francisco on 18 April 1906. Many people are worried that the city and LA, for example, would look like this again due to a massive quake
PLANS FOR THE 'BIG ONE'
Federal, state and military officials have been working together to draft plans to be followed when the 'Big One' happens.
These contingency plans reflect deep anxiety about the potential gravity of the looming disaster: upward of 14,000 people dead in the worst-case scenarios, 30,000 injured, thousands left homeless and the region's economy setback for years, if not decades.
As a response, what planners envision is a deployment of civilian and military personnel and equipment that would eclipse the response to any natural disaster that has occurred so far in the US.
There would be waves of cargo planes, helicopters and ships, as well as tens of thousands of soldiers, emergency officials, mortuary teams, police officers, firefighters, engineers, medical personnel and other specialists.
'The response will be orders of magnitude larger than Hurricane Katrina or Super Storm Sandy,' said Lt. Col. Clayton Braun of the Washington State Army National Guard.
It was so powerful, lasting between one and three minutes, that it liquefied soil, as well as destroying buildings.
Experts at the Southern California Earthquake Centre used a supercomputer in 2010 to simulate a magnitude 8 quake also starting in Monterey County.
It matched reports of devastation more than a century ago, heading to the Mexican border.
It predicted such a large quake would hit LA and the San Fernando Valley hard because of soft soil in these valleys trapping waves.
In March, a geophysicist warned the long-overdue earthquake set to hit southern California could be far worse than expected.
Julian Lozos, an assistant geophysics professor at California State University, claimed there is a strong chance this quake will coincide with one along the adjacent San Jacinto fault line, which runs through more heavily-populated cities.
If true, it would mean authorities have dramatically underestimated how many people will be affected by the natural disaster.
Evidence uncovered by the expert suggests the terrifying scenario occurred in 1812, devastating the region between San Diego and San Buenvaventura.
If that happened once, Dr Lozos said, there is a strong chance it can happen again.
'Looking at old earthquakes in general is really a good way to figure out what faults are capable of doing,' Dr Lozos wrote in his paper published on Saturday in the journal Science Advances.
Previously, geologists thought a quake shook San Jacinto shortly before the devastating San Andreas quake on 8 December, 1812.
Julian Lozos, an assistant geophysics professor at California State University, claimed there is a strong chance this quake will coincide with one along the adjacent San Jacinto fault line, which runs through more heavily-populated cities. Both fault lines are shown above
Federal, state and military officials have been working together to draft plans to be followed when the 'Big One' happens. This photograph shows cracked buildings in the thermal fields of the southern San Andreas Fault
However, Dr Lozos concluded the rupture along the San Jacinto fault line was in fact the starting point.
He analysed historical data on the 1812 San Andreas rupture, testing four different scenarios of where it could have started.
He believes it started in Mystic Lake, ripping north up the San Jacinto fault line until it jumped up into the dry creek that runs parallel.
Though the findings cannot be proved definitively, it presents a new element for scientists to acknowledge as they attempt to advise the West Coast on how to brace for the quake.
Indeed, University of California professor Lisa Grant Ludwig said the region is not prepared for such a situation.
'In southern California, much of our infrastructure was built to withstand a rupture of either the San Andreas or San Jacinto faults, but not both at the same time,' she explained.
In San Bernardino County, the fault hasn't moved much since an earthquake in 1812, with a southeast section near Salton Sea has been quiet since around 1690. This image shows swirled shapes of earth layers that were deformed by the collision of the Pacific and North American tectonic plates north of Salton Sea