San Andreas fault
Folded layers of sediment can be seen on this hillside directly above the San Andreas fault along Antelope Highway in Palmdale, California Reuters/Lucas Jackson

California's San Andreas fault has been furiously building up tectonic tension for so long that it's very close to finally shuddering into a massive earthquake, an expert has warned.

The southern part of the fault is "locked, loaded and ready to roll", according to Thomas Jordan, director of the Southern California Earthquake Center.

"The pressure has been building on that part of the fault without being relieved for more than a hundred years. The springs on the San Andreas system have been wound very, very tight," Jordan warned at the National Earthquake Conference in Long Beach, California.

The fault runs almost the entire length of California, close to 800 miles (1,290km), but it is the section close to Los Angeles that currently worries scientists the most.

The last big earthquake to strike the southern San Andreas was in 1857 when a magnitude 7.9 earthquake ruptured 185 miles between Monterey County and the San Gabriel Mountains near Los Angeles. It has been quiet since then — too quiet, worries Jordan. Scientists believe such fractures need to release some tension gradually over time or a major tremor is almost guaranteed.

A 2008 US Geological Survey report warned that a 7.8-magnitude quake on the southern San Andreas fault could cause more than 1,800 deaths and 50,000 injuries. Such a quake would cause shaking for a full two minutes.

Jordan said it's important that California focuses on becoming resilient to a potential huge earthquake, one as strong as a magnitude 8. He praised Los Angeles' plan to require earthquake retrofits on apartment and concrete buildings.

It is not the first time a scientist has warned that California is in for temblor trouble. In 2014 scientists warned of an impending rupture of the San Andreas fault in the San Francisco Bay area, again because there had been little tension release the previous years.

In March 2016 Standford scientists published a study on a magnitude-7.5 quake in 1812 that was caused by simultaneous ruptures on the San Jacinto and the San Andreas fault, that they warned could happen again.