FEARS are growing for the "Big One" as experts claim to have found proof a major earthquake AND a massive Tsunami is due to strike California.
Researchers say the Cascadia Subduction Zone generates a major event every 200 to 530 years - and last experienced a quake in the 1700s.
The study, led by scientists at the University of Texas, reckon an earthquake is more likely to strike off the coast of Washington and northern Oregon - although a large enough movement would impact the surrounding areas.
Experts say this is because seismic data shows the sediment there is more compact, with a small amount of water sitting in the pore spaces between the grains, leaving the plates more susceptible to a build-up of stress.
These conditions allow the sediment to stick and move together in the event of an earthquake, which means they're also able to move huge amounts of seawater - resulting in a tsunami.
Shuoshuo Han, who led the study, said: "We observed very compact sediments offshore of Washington and northern Oregon that could support earthquake rupture over a long distance and close to the trench, which increases both earthquake and tsunami hazards."
'MEGATHRUST FAULT': WHAT IS THE CASCADIA SUBDUCTION ZONE?
The Cascadia Subduction Zone (CSZ) is a 1,000km "megathrust" fault line that stretches from Northern Vancouver Island to Cape Mendocino California.
It separates the Juan de Fuca in the Pacific Ocean and North America plates, which have been pushing against each other for more than 300 years.
Great Subduction Zone earthquakes are the largest earthquakes in the world.
The CSZ has produced magnitude 9 or greater earthquakes in the past, and undoubtedly will in the future.
- Source: Pacific Northwest Seismic Network
Subduction zones are areas where one tectonic plate dives or "subducts" beneath another plate.
The thick sediment layer on the top is scraped off as it moves and piled up on the top plate, forming a thick wedge of material, while the rest of the sediment travels down with the bottom plate.
Nathan Bangs, a senior research scientist at UTIG and study co-author, said: "That combination of both storing more stress and the ability for it to propagate farther is important for both generating large earthquakes and for propagating to very shallow depths."
The possibility of an earthquake generating in shallow depths is what causes large tsunamis, such as the one that struck Tohoku in Japan in 2011.
Penn State University professor Demian Saffer, who co-wrote the study, said: "The results are consistent with existing constraints on earthquake behaviour, offer an explanation for differences in structural style along the margin, and may provide clues about the propensity for shallow earthquake slip in different regions."
It comes after a 3.2 magnitude earthquake hit southern California earlier this month.
The epicentre was in Los Angeles - home to Hollywood with a population of 4 million people - which sits on the Ring Of Fire fault line along with other Californian cities like San Francisco.
Experts say California is due another major apocalyptic tremor — which they have already dubbed "The Big One".