Scientists say they have found a potential zone in the San Andreas fault that could be the starting point for the next big earthquake to rock California.

That earthquake is often referred to as “The Big One”, but no one is quite sure when it’s going to occur. What we do know, though, is that it’s going to be pretty catastrophic.

In this study, published in the journal Lithosphere, a team led by Dr Susanne Jänecke from Utah State University examined the southern 30 kilometers (19 miles) of the fault. The entire thing spans about 1,300 kilometers (800 miles).

They found that this region was a “highly faulted” volume of rock, between about 1 and 4 kilometers (0.6 and 2.5 miles) wide. It was shaped as a sheared ladder-like structure, known as a Durmid structure, extending about 3 to 5 kilometers (1.9 to 3.1 miles) into Earth.

They said this fault structure extends from the main part of the fault in the northeast to a newly identified East Shoreline fault (ESF) to the southwest, which was previously unknown. This parallels the main San Andreas fault (mSAF) for about 100 kilometers (62 miles).

“It is not clear how past earthquakes interacted with this well-organized multi-fault structure, and, notes Jänecke, this makes future behavior difficult to predict,” a statement from the Geological Society of America said. “The mSAF was the only active fault considered by the geoscience community in this crucial area prior to our detailed study.”

Ladder-like faults such as this can be enormous, at least 25 kilometers (16 miles) wide and 150 kilometers (93 miles) long. When they fail, they can do so in “piecemeal fashion”, with cascading ruptures as they interact. The team note that such complicated faults are poorly understood, and need to be researched more to see what sort of risk they pose.

Earlier this week another study also found that “slow earthquakes” could pose a problem. Published in Nature Geoscience, it suggested that parts of the fault were moving a few centimeters every year, hinting at “stick-and-slip movements” that release energy over a period of months.

The risk of an earthquake in California is ever-present, and while studies such as these help better predict when one might happen, there is still much we don’t know.

"Based on current time-independent models, there's a 75% chance for an earthquake of magnitude 7 or larger in both northern and southern California within next 30 years," said Mostafa Khoshmanesh from Arizona State University, lead author on the Nature Geoscience paper, in a statement.