The San Andreas may be the most well-known fault in California, but no one alive today experienced its most recent significant earthquake that ruptured 116 years ago.
The Great San Francisco Earthquake struck at 5:12 a.m. local time on April 18, 1906. The magnitude 7.9 quake caused shaking that lasted up to 60 seconds and sent vibrations all the way to southern Oregon, Los Angeles, and central Nevada. The earthquake and subsequent fires killed an estimated 3,000 people and left half of the city’s 400,000 residents homeless.
Researchers study older significant earthquakes like this one to better understand how frequently they strike on a fault. Although no one can predict when an earthquake will occur, scientists can better assess seismic hazards by calculating the probability of a ground-rupturing earthquake occurring in a specific area within a certain number of years.
Considering the historical average rate of large earthquakes on some faults in California, including the San Andreas, the odds that there hasn’t been a major earthquake in the last 116 years are very low.
In a new publication, USGS geologist Devin McPhillips studied sites along three faults in California with long histories of earthquakes to better understand what’s happening. “The wait between earthquakes might be longer than we previously thought because the evidence for older earthquakes isn’t always 100% accurate,” McPhillips said. “If some of these older earthquakes are misidentified, then the long wait we are currently experiencing might be more typical.”
It’s California’s fault
The San Andreas Fault first captured the world’s attention on that fateful April day 116 years ago when it ruptured along the northernmost 296 miles (477 km) of the 800 mile- (1300 km) long fault that marks where the Pacific plate slides by the North American plate. The relative movement along the fault varied from 2 to 32 feet (0.5 m to 9.7 m), which means if two people were standing on opposite sides of the fault, in some places one would slide away from each other by up to 32 feet from the other. In southern California, the last rupture of the San Andreas Fault occurred in 1857.
Geologists find that the total accumulated displacement from earthquakes (sudden fault movement) and creep (steady fault movement) is at least 350 miles (560 km) along the San Andreas fault since it came into being about 15-20 million years ago.
