Watch out, Northern California. A new geological survey done by researchers at the U.S. Geological Survey and San Francisco State University warns that four specific areas of the San Andreas fault are overdue for large earthquakes.

The region running under the faults is largely urban and major earthquakes there would affect over 15 million people, as well as damage canals and dams involved with the state's major water supply.

Researchers discovered this risk after studying a geological phenomenon called fault creep. Fault creep is a slow and gradual release of tension on fault lines that helps keep the slipping and sliding of the Earth in check. The more fault creep an area has, the more stress released. In other words, fault creep is generally a good thing.

However, less fault creep means that the stress and tension at the fault lines continues to build, with the fault locked in place, which eventually leads to a big release in the form of a large earthquake. And that is, unfortunately, the case with these four faults.

"The extent of fault creep, and therefore locking, controls the size and timing of large earthquakes on the Northern San Andreas Fault system," says James Lienkaemper, a research geophysicist at U.S. Geological Survey (USGS). "The extent of creep on some fault sections is not yet well determined, making our first priority to study the urban sections of the San Andreas, which is directly beneath millions of Bay Area residents."

The particular fault lines at risk are the Green Valley fault, which lies between Fairfield and Napa; the Calaveras and Hayward faults in the east San Francisco Bay Area; and the Rodgers Creek fault, just north of San Francisco.

According to the new study, these four locked fault lines have not had a major earthquake since 1868. This fault occurred along the Hayward fault, which measured at a 6.8 magnitude. Researchers believe this particular fault is approaching its next major earthquake soon, based on previous studies.

The other three faults are also close to or past their times for major earthquakes. Based on what researchers discovered about fault creep, the next earthquakes on these lines will be at least 6.8 magnitude.

"The San Andreas Fault and its two other large branches, the Hayward and Northern Calaveras, have been quiet for decades," says Lienkaemper. "This study offers a good reminder to prepare today for the next major earthquake."

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