Right now, most analyses focus on the segregation of the fault network into northern and southern sections. The last time the southern section ruptured was back in 1857, when a 360-kilometer-long (224-mile-long) section jutted forwards at a shallow depth. This registered as a 7.9M event, and it lasted for three minutes.
Not all of it ruptured, however. One section near the Salton Sea hasn’t experienced a massive event since the late 17th century, although it has recently been exhibiting some unexpected seismic activity.
The northern section last had a major rupture back in 1906, where a far less populated San Francisco took the brunt of the tremors, and 3,000 people lost their lives.
According to the USGS study, the Pacific Plate is trending northwards besides the North American Plate at a rate of around 5.1 centimeters (2 inches) per year. This migration is uneven, however, and evidence clearly shows that plate movement has not significantly occurred along at least a third of the boundary for around 150 years, perhaps longer.
When this stress is eventually released, it could trigger not just one section, but the entire fault to fracture. If the entire fault line – all 1,300 kilometers (800 miles) of it – were to rupture to the tune of an 8.0-9.0M event, it would devastate large swaths of San Francisco and Los Angeles and tens of other huge cities and towns. Such an event would kill thousands and displace millions.
Sadly, this is not just a hypothetical situation. This cataclysm is real, and it is on its way.
A probabilistic hazard map of the contiguous US. The redder the region, the more likely it is in the next 10 years to accelerate as a fraction of gravity due to seismic activity. The higher the fraction, the worse the damage. USGS