San Andreas Fault: Everything you need to know about the California fracture zone and the next ‘Big One’
The massive fault line could trigger California’s next catastrophic earthquake, but how likely is the so-called ‘Big One’?
Several studies have reported that the number of natural disasters, such as the July 2025 earthquake and subsequent tsunami that affected parts of the West Coast of the United States, has increased significantly in the last few decades, fueled by climate change.
The San Andreas Fault: key facts
The San Andreas Fault is California’s longest and best-know fault, running from the Salton Sea in Southern California to off the coast of Mendocino in Northern California. It extends for approximately 746 miles (1,200 kilometers) and is split into three sections:
The northern segment stretches from Cape Mendocino to Hollister and includes the area around the San Francisco Bay.
The central segment runs from Hollister to Parkfield and is characterized by a phenomenon called aseismic creep, where the fault slips continuously without causing earthquakes. There has never been a large earthquake on this section in recorded history.
The southern segment extends from Parkfield to the Salton Sea. It is known for producing large earthquakes, such as the 1857 Fort Tejon earthquake, and is a significant threat to highly-populated areas like Los Angeles.
How do earthquakes happen?
Earthquakes occur as a result of tectonic plates, which are large slabs of the Earth’s crust, coming into some kind of contact with one another. The contact itself doesn’t cause the quake but does create stress on the crust. When the stress is released in a sudden fashion, a potentially destructive earthquake occurs.
California’s worst earthquakes
The most devastating tremors in California were the Fort Tejon earthquake in 1857 and the San Francisco earthquake in 1906, which both reached a magnitude of 7.9 on the Richter scale but had vastly different outcomes in terms of numbers of casualties.
Only two people were killed as a consequence of the Fort Tejon quake, which occurred before the San Andreas Fault had been discovered in 1895.
However, more than 3,000 people are thought to have perished due to the San Francisco earthquake and subsequent fires, which destroyed more than 80% of the city. It is the deadliest earthquake in the history of the United States.
When will the next big earthquake on the San Andreas Fault occur?
There is no real way of knowing when the next earthquake will hit California, with scientists looking to past quakes in an attempt to offer a rough estimate.
According to the U.S. Geological Survey, there is a 72% chance of a magnitude 6.7 earthquake happening in the San Francisco Bay Area before 2043, and a 60% probability of a similar event in the Los Angeles region before the same year.
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