1906 San Francisco Earthquake

The morning of April 18, 1906

At about 5:12 a.m., a Foreshock was felt in San Francisco, followed seconds later by violent shaking from the main earthquake. Its Epicentre was close to San Francisco, near the coast. Strong shaking in the city lasted roughly 45 seconds to a minute.

Because it was so early, many people were still asleep at home. Chimneys toppled, brick walls fell into the streets and buildings on filled land near the bay were badly damaged as soft ground shook harder and in places settled or flowed through Liquefaction. Damage extended well beyond San Francisco, with heavy destruction in places such as Santa Rosa, San Jose and the Stanford University campus.

A 470-kilometer rupture of the San Andreas Fault

The earthquake ruptured the northern part of the San Andreas Fault for roughly 470 km (about 296 miles), from near San Juan Bautista in the south to Cape Mendocino in the north. The San Andreas is a strike-slip Fault, where the Pacific Plate moves northwest relative to the North American Plate, so the ground on either side slid sideways past each other.

Along the fault, fences, roads and rows of trees were offset by several meters. Some of the largest offsets were measured in Marin County, north of San Francisco. Its estimated Magnitude of about 7.9 is based on modern analysis of historical records, since modern magnitude methods did not yet exist.

Fire: the greatest destroyer

Fires began almost immediately, started by broken chimneys, overturned stoves and damaged gas and electrical lines. The earthquake had broken water mains across the city, leaving firefighters with little water. Attempts to create firebreaks with explosives sometimes started new fires instead.

The fires burned for several days and destroyed a large part of the city, including the downtown and many neighborhoods. Much of the destruction in San Francisco was caused by fire rather than shaking. Roughly half or more of the city's residents were left homeless, and many lived in tents and temporary camps in parks for months.

The human toll

Early official counts listed only a few hundred deaths. Later research by historians found that many deaths, including those in poorer neighborhoods and Chinatown, were never officially recorded. Today, deaths are estimated at around 3,000 or more, which makes it the deadliest earthquake in U.S. history.

How 1906 changed earthquake science

Shortly after the earthquake, the State of California formed a commission led by geologist Andrew Lawson to investigate it. Its detailed findings, published in what is known as the Lawson report, documented the fault rupture, the pattern of damage and the effects of ground conditions. It remains one of the most important studies of a single earthquake.

Using survey measurements from before and after the earthquake, Harry Fielding Reid developed the elastic rebound theory. He proposed that the rocks on either side of a locked fault slowly bend as the plates move, storing strain like a stretched spring, until the fault suddenly slips and the rocks snap back. This idea is still the basic explanation for how most earthquakes happen. Our article on how earthquakes happen explains it further.

• Showed clearly that a large earthquake can rupture hundreds of kilometers of fault at once.
• Revealed that soft, filled ground near the bay shook much harder than nearby rock.
• Demonstrated how fire after an earthquake can cause more damage than the shaking.
• Laid foundations for the scientific study of earthquake hazards in California.

Could it happen again?

Yes. The San Andreas Fault and other faults in the Bay Area will produce large earthquakes in the future, though no one can say when. The region is far better prepared today, with stronger building codes, earthquake early warning and a much better understanding of hazards, but many older buildings remain vulnerable.

If you live in the region, our article on earthquake probability in California explains the long-term forecasts, and you can follow current activity on the EQRadar live map or look up historic earthquakes on the history page.

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