Ventura Fault
The Ventura Fault has a 'staircase' structure running close to surface level, which makes it capable of larger and more dangerous earthquakes Gareth J. Funning and Scott T. Marshall

The Ventura-Pitas Point fault system, which lies underneath the city of Ventura, has been found to stretch for miles not far beneath the surface of southern California, suggesting it is capable of generating large earthquakes.

The fault runs beneath the city of Goleta, and then underneath Santa Barbara, before running offshore and then after a 60-mile stretch it runs back under land at Ventura. The fault is thought to be capable of generating tsunamis as well as earthquakes.

The shape of the fault has not been definitively mapped, and two competing theories exist for the shape of the fault, which is directly related to how much damage it could do. The first is that the fault is deep and heads sharply down to a depth of about 13 miles. This structure would not be so dangerous, as seismic activity many miles below the surface would be likely to cause less damage.

The second is that the fault is shallow, with a flat section between two more gentle downward slopes, in a "staircase" arrangement. Presence of the fault closer to the surface would be more likely to lead to greater disturbances at surface level, and earthquakes of up to magnitude 8.

Modelling research suggests that the latter and more dangerous shape of the fault is the more likely one, according to a paper published in the journal Geophysical Research Letters.

The researchers used computer models of the crust to see what happened when the region was squeezed by tectonic plate movements. They found that the staircase arrangement was the most likely shape of the fault.

"Our models confirm that the Ventura-Pitas Point fault is a major fault, that lies flat under much of the coast between Ventura and Santa Barbara," said study author Gareth Funning of the University of California, Riverside, in a statement.

San Andreas Fault
The nearby San Andreas Fault is capable of generating devastating earthquakes. Scientists have now found that the Ventura Fault to the south is capable of earthquakes of magnitude 8 Leohotens/CC

"This means that a potential source of large earthquakes is just a few miles beneath the ground in those cities. We would expect very strong shaking if one occurred."

Scientists have repeatedly said that a large earthquake is overdue in California. The fault expected to generate "the Big One" is the San Andreas Fault, which runs to the north of the Ventura fault. This latest paper suggests that there are multiple faults capable of generating an earthquake of this scale.