By Tom James

SEATTLE (Reuters) – The Big One may be overdue to hit California but scientists near Los Angeles have found a new risk for the area during a major earthquake: abrupt sinking of land, potentially below sea level.

The last known major quake on the San Andreas fault occurred in 1857, but three quakes over the last 2,000 years on nearby faults made ground just outside Los Angeles city limits sink as much as three feet, according to a study published Monday in the journal Scientific Reports.

Seismologists estimate the 800 mile-long San Andreas, which runs most of the length of the state, should see a large quake roughly every 150 years.

Scientists from California State University Fullerton and the United States Geological Survey found evidence the older quakes caused part of the coastline south of Long Beach to drop by one-and-a-half to three feet.

Today that could result in the area ending up at or below sea level, said Cal State Fullerton professor Matt Kirby, who worked with the paper’s lead author, graduate student Robert Leeper.

“It’s something that would happen relatively instantaneously,” Kirby said. “Probably today if it happened, you would see seawater rushing in.”

The study was limited to a roughly two-square-mile area inside the Seal Beach National Wildlife Refuge, near the Newport-Inglewood and Rose Canyon faults. Kirby acknowledged that the exact frequency of events on the faults is unclear, as is the risk that another quake will occur in the near future.

The smallest of the historic earthquakes was likely more intense than the strongest on record in the area, the magnitude 6.3 Long Beach earthquake of 1933, which killed 120 people and caused the inflation-adjusted equivalent of nearly a billion dollars in damage.

Today, the survey site is sandwiched by the cities of Huntington Beach and Long Beach, home to over 600,000 people, while nearby Los Angeles County has a population of 10 million.

Seismologist John Vidale, head of the University of Washington-based Pacific Northwest Seismic Network, said after reviewing the study he was skeptical such powerful quakes could occur very frequently in the area.

Kirby noted that the team could only collect soil core samples within the relatively undisturbed refuge, and that taking deeper samples would shed light on the seismic record even further back, potentially giving scientists more examples of similar quakes to work from.

(The story was refiled to correct the second paragraph to clarify timing of last known major quake on the San Andreas fault)

(Reporting by Tom James; Editing by Patrick Enright and James Dalgleish)