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© Estelle Chaussard/UC Berkeley
The red line indicates the newly discovered link between the southern end of the Hayward Fault and the Calaveras Fault, once thought to be independent systems.
Two California fault lines — the Calaveras Fault and the Hayward Fault — are actually connected, new research shows, meaning an earthquake resulting from the pair could be much more destructive than originally thought, with a potential for a magnitude 7 quake or greater.

Seismologists from the University of California Berkeley used two decades of satellite data to look at "ground deformation" and "fault creep," a news release explained. Creep describes the constant, slow movement of a fault line as opposed to a fault line that's still between quakes, like San Andreas, according to the U.S. Geological Survey. "The Hayward Fault is actively moving, year-by-year," the USGS noted.

However, as Estelle Chaussard, lead researcher from UC Berkeley, explained to weather.com, though the ground is continuously moving, this movement is not producing any earthquakes. Until, that is, there's a rupture.

Suddenly, a much larger earthquake in California's Bay Area is possible, given the newly discovered link between the two faults. The researchers found that the Hayward Fault creep reached about 10 miles farther south than originally believed, meeting up with the Calaveras Fault. This hidden extension remained that way partially because vegetation covered the ground above it, making it harder to detect than in places where buildings or sidewalks gave away the below-the-surface activity.

Though the newly found creep happens outside of the area's urban centers, an earthquake along these fault lines will certainly be felt in nearby cities. "We thought we could have a magnitude 6.9 earthquake. Now that the two fault are connected, we could have magnitude 7+ earthquakes, 7.3 or even larger depending on where the rupture goes," Chaussard said. "Going from a magnitude 6.9 to a magnitude 7.3, that's 2.5 times the amount of energy released. We have to expect that we could have larger shaking earthquakes than we previously thought."

Next steps for this research include digging into the data, to further refine what Chaussard called "magnitude estimates" for specific spots along the fault line. That will help determine what, if any, adjustments needs to be made to area structures to keep people safe.

"The big question we were trying to address was whether the faults were linked or independent," she said. Now we know there's a direct connection.

The work was accepted into the Geophysical Research Letters journal on Thursday, Apr. 2, 2015.