A major quake, known as the "Big One," could strike parts of Southern California at anytime, U. S. researchers announced on Sunday.

The 340-mile (544-kilometer) southern section of the San Andreas fault -- running from Monterey County to the Salton Sea -- is long overdue for a major quake, said researchers at University of California (UC) in Irvine and Arizona State University.

The fault could slip at anytime, triggering a massive, magnitude 8.1 quake, the researchers said in a report.

That will be significantly stronger and longer than the southern San Andreas' last major quake in 1857, the report said.

The 7.9 magnitude quake in 1875 ruptured 200 miles (320 kilometers) of fault between Monterey and San Bernardino counties.

The latest study contradicted with previous researches which suggested that the section of the fault through the Carrizo Plain, located approximately 100 miles (160 kilometers) northwest of Los Angeles, would remain dormant for at least another century.

"The next earthquake could be sooner than later," said Lisa Grant Ludwig of UC Irvine, co-author of the study.

He said in remarks published by the Los Angeles Times on Sunday that an earthquake running the length of the southern section is not assured, but "a possibility that should be considered and previously was not."

U.S. Geological Survey seismologist Lucy Jones, who was not involved in the study, said it was possible that the entire southern San Andreas fault could rupture.

Thomas Jordan, director of the Southern California Earthquake Center, calculated that would produce a magnitude-8.1 quake. Jones said that figure sounds about right.

The northern section of the fault, which begins north of Parkfield and ends in San Benito County, tends to move at a constant creep. Because stress is relieved regularly, large quakes don't occur there, The Times reported.