Published March 31, 2025
BY OLIVIA FORT
For centuries, Southern Californians have lived atop a restless tectonic boundary, where the grinding of the Pacific and North American plates along the San Andreas Fault builds seemingly invisible tension that scientists warn could one day erupt into a catastrophic event known as “The Big One.”
So will it happen? And if so, what will happen?
Southern California rests uneasily on the San Andreas Fault zone, a geological boundary infamous for its role in shaping the region’s history and topology. Spanning more than 800 miles, this fault marks the divide between the Pacific and North American tectonic plates, where, according to research conducted by the National Park Service and the U.S. Geographical Survey, grinding movements generate immense pressure over time.
While hundreds of large earthquakes have shaken California over the years, The Big One is centuries overdue, with the USGS believing it will happen before 2032. (graphic by OLIVIA FORT)
“Each one of these faults carries the potential to rupture,” explains Frank Vernon, a research geophysicist at the Scripps Institute of Oceanography. “These ruptures could span hundreds of kilometers, like the San Francisco earthquake in 1906 or even longer.”
Historical records provide insight into the destructive power of this fault system. The 1857 Fort Tejon earthquake was one of the largest in Southern California’s history, producing violent shaking and extensive damage over hundreds of miles. The infamous 1906 San Francisco earthquake, by comparison, ruptured 300 miles of the fault and reshaped the region’s landscape in devastating ways, according to the U.S. Geological Survey.
“The 1906 earthquake represented an important milestone in the scientific understanding of earthquakes,” says Dr. William Ellsworth, a professor of geophysics at Stanford University. “It led to the creation of a physical model of an earthquake that we still use today.”
The devastation caused by “The Big One” could be unprecedented, with far-reaching consequences for Southern California’s infrastructure, economy and residents. Mark Benthien, the director of Public Education and Preparedness at the Statewide California Earthquake Center, emphasizes the importance of acknowledging the risks posed by California’s faults.
“Everyone should take steps to be prepared to survive and recover from large earthquakes, which can happen on many faults in California,” Benthien explains.
The magnitude scale measures the energy released during an earthquake, assigning a numerical value to indicate its size. It works logarithmically, meaning each whole number step represents roughly 32 times more energy, allowing scientists to quantify and compare earthquakes of varying intensities.
Lessons from past earthquakes reveal the vulnerability of Southern California’s infrastructure. The 1994 Northridge earthquake, with a magnitude of 6.7, caused widespread destruction, including collapsed freeways, severely damaged buildings and disruptions to essential services, with total damages exceeding $20 billion. By comparison, “The Big One,” projected at a magnitude of up to 8.0, would affect a larger area and pose even greater challenges.
A more accurate example of this would be the 2023 earthquake in Turkey and Syria, with a magnitude of 7.8, which caused over 50,000 fatalities and widespread devastation, including collapsed buildings and disrupted infrastructure. This tragedy offers a grim comparison to what Southern California could face if “The Big One” strikes.
“The San Andreas fault is our largest and thus can have the largest earthquakes, up to about a magnitude of 8,” says Benthien. “But other faults are directly beneath population centers, and even if smaller, earthquakes on these faults will cause major losses.”
Scientists with the U.S. Geographical Survey and California Integrated Seismic Network actively monitor the San Andreas Fault using advanced tools like GPS sensor networks and seismometers to track stress buildup. While earthquakes cannot be precisely predicted, early warning systems like the MyShake app provide critical seconds of notice to help mitigate risks and save lives.
“Little ruptures help us know things are happening, but they don’t prevent ‘The Big One,’” said Vernon, whose research has included running seismic arrays along the San Jacinto fault zone in order to further understand what the structure of the fault is.
Simple steps like securing furniture to prevent it from falling during shaking can reduce injuries. Keeping a fire extinguisher handy, taking safety training, organizing disaster supplies and important documents and practicing protective actions can help save lives during an earthquake.
“Risk reduction begins with recognizing the hazard and defining steps to mitigate it,” explains Ellsworth.
While Southern California waits, the fault lines remain silent but tense. The question is no longer if “The Big One” will strike, but when—and how Californians will respond when it does.
“We don’t know what could happen today. It could happen right now,” says Vernon. “Or it could not happen for another 100 years.”
