The San Andreas fault line is an 800-mile long tectonic boundary between the Pacific Plate and the North American Plate – two great fragments of the Earth’s crust.
The fault line extends about 10 miles into the Earth and is an intricate system of crushed and broken rock along the Californian coast.
According to the United States Geological Survey (USGS), the fault line extends from Northern California, through San Francisco and down and just north of Los Angeles.
The USGS said: “The San Andreas fault forms a continuous narrow break in the Earth's crust that extends from northern California southward to Cajon Pass near San Bernardino.
“Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates.
“In this stretch of the fault zone, the name ‘San Andreas’ generally is applied to the northeasternmost branch.”
Is the San Andreas fault line at risk of a major earthquake?
Geologists hypothesise the southern end of the San Andreas fault line could be the central point for a major earthquake striking the region.
A new study published in the peer-reviewed journal Lithosphere, examined the possibility of a magnitude 7.5 or stronger earthquake along a southern 30km stretch of the fault line.
Geologist Susanne Jänecke, Utah State University, constructed a detailed structural map of this fault line fragment to identify a “transpressional, 1–4-km-wide ladder-like structure” dubbed Durmid Hill.
The Durmid Hill was found to extend northwest from the San Andreas fault and east into a newly identified East Shoreline fault zone.
In the study, Dr Jänecke wrote: ‘The great width of the East Shoreline fault zone in Durmid Hill and the even larger with and spatial extent of the Durmid ladder structure imply that surface faulting hazards from a major earthquake rupture in this part of the San Andreas fault zone might be dispersed across a 40 square kilometres area, if both master faults and the intervening cross faults are activated at once.”
However, the geologist noted it is still unclear how this newly discovered structure interacted with past earthquakes along the fault line.
The uncertainty makes it hard to predict future earthquake behaviour, Dr Jänecke added, and until recently geologists only focused on the active San Andreas fault.
But the size and geometry of the East Shoreline fault zone and Durmid Hill both suggest the zones could fail in the event of a single earthquake.
The study noted: “If ladder-like strike-slip fault zone rupture in a piecemeal fashion they will have an especially unpredictable surface-faulting hazard.”
According to the USGS, large earthquakes along the San Andreas fault have occurred roughly every 150 years in the past 1,400 to 1,500 years.
The last major earthquake struck the fault in 1906 in the San Fransisco Bay Area but the USGS is not convinced the scale and intensity of future earthquakes can be predicted way ahead of impact.