Four faults directly underneath California's densely populated San Francisco Bay Area show signs of being highly stressed and could rupture in a major earthquake at any time, according to a study published earlier this week.

Led by scientists from the U.S. Geological Survey and published in the Bulletin of the Seismological Society of America, the study says four faults -- the Calaveras, Green Valley, Hayward and Rodgers Creek Faults -- have built up enough seismic strain to unleash a quake on the scale of the 6.9-magnitude Loma Prieta earthquake back in 1989, which left more than 60 people dead and caused some $6 billion in damage.

What makes these four faults so dangerous isn't their size -- they are small parts of the more than 800-mile-long San Andreas Fault system -- but where they lie. The Hayward Fault, for example, stretches from Suisun Bay to San Jose, including much of Silicon Valley.

"The Hayward Fault is just right in the heart of where people live, and the most buildings and the most infrastructure," James Lienkaemper, a research geophysicist at the U.S. Geological Survey's Earthquake Science Center and the study's lead author, told LiveScience.

"But it's not just one fault, it's the whole shopping basket," he added. "If you are in the middle of the Bay Area, you are near a whole lot of faults, and I'm concerned about all of them."

The scientists involved in the study have been measuring tiny shifts in the position of the faults at the surface over the past several years, recording the degree to which each fault moves each year.

(MORE: Strongest Earthquake in Decades Strikes San Francisco Bay Area)

This movement, which the scientists call creep, may be as little as a few millimeters a year, but over time it can be enough to cause big cracks in roads and move street curbs. Creep relieves stress on a fault at the surface, but deep underground, faults can remain locked for hundreds of years until they finally break apart in an earthquake.

“The extent of fault creep controls the size and timing of large earthquakes, and measuring that creep rate helps tell us how much strain is building up on the faults underground -- although it can’t tell us when a fault will rupture in a quake,” Lienkaemper told the San Francisco Chronicle.

Lienkaemper and his study co-authors used their measurements of each fault's creep over the years, plus the amount of time that had elapsed since its last earthquake and other modeling data, to come up with the future earthquake potential for each.

What they found was that the Hayward Fault has enough stored energy to produce a 6.8-magnitude quake, while the Rodgers Creek and Green Valley faults each have stored enough to trigger a 7.1 quake.

"A large earthquake on the Hayward Fault would affect the entire San Francisco Bay Area -- over 5 million people," Stanford University Professor Greg Beroza, who was not involved with the study, told NBC News.

"Damage would likely be most severe near the fault in the East Bay, but there are areas removed from the fault, including the entire periphery of San Francisco Bay, and the Sacramento Delta, that would be very strongly shaken as well."

See more at LiveScience, or read the full study at the Bulletin of the Seismological Society of America.

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The road outside the main post office in San Francisco, Calif. which has been ripped up by the force of the 1906 earthquake. (Hulton Archive/Getty Images)