1906 San Francisco earthquake

At 5:12 a.m. on Wednesday, April 18, 1906, one of the most catastrophic natural disasters in American history struck the city of San Francisco. The earthquake hit the coast of Northern California with an estimated moment magnitude of 7.9 and a maximum Mercalli intensity of XI (Extreme). The earthquake ignited several fires around the city that burned for three days and destroyed nearly 500 city blocks. The earthquake and fires killed an estimated 3,000 people and left half of the city's 400,000 residents homeless. The event remains the deadliest earthquake in the history of the United States and a defining moment in the long history of San Francisco.

Geological Background and Fault Rupture

The earthquake originated along the San Andreas Fault, the great tectonic boundary running the length of California. The San Andreas Fault is a continental transform fault that forms part of the tectonic boundary between the Pacific plate and the North American plate. The earthquake ruptured the northernmost 296 miles (477 kilometers) of the San Andreas Fault, from northwest of San Juan Bautista to the triple junction at Cape Mendocino, confounding contemporary geologists with its large, horizontal displacements and great rupture length.

At 5:12 a.m. local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay Area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter in the Pacific Ocean just 2 miles west of San Francisco. The amount of horizontal slip, or relative movement along the fault, varied from 2 to 32 feet (0.5 m to 9.7 m). Violent shocks punctuated the strong shaking, which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada.

For years, the epicenter of the quake was assumed to be near the town of Olema, in the Point Reyes area of Marin County, due to local earth displacement measurements. In the 1960s, a seismologist at UC Berkeley proposed that the epicenter was more likely offshore of San Francisco, to the northwest of the Golden Gate. The most recent analyses support an offshore location for the epicenter, although significant uncertainty remains.

One important characteristic of the shaking intensity noted in Lawson's 1908 report was the clear correlation of intensity with underlying geologic conditions. Areas situated in sediment-filled valleys sustained stronger shaking than nearby bedrock sites, and the strongest shaking occurred in areas where ground reclaimed from San Francisco Bay failed in the earthquake. The 1906 earthquake ruptured the northernmost 296 miles (477 km) of the San Andreas Fault between San Juan Bautista and Cape Mendocino — by comparison, the 1989 Loma Prieta earthquake had a rupture length of only 25 miles.

The Fires and Immediate Destruction

San Francisco had experienced earthquakes in 1864, 1898, and 1900, but nothing like the 1906 event. Cable cars abruptly stopped, City Hall crumbled, and the Palace Hotel's glass roof splintered and littered the courtyard below. San Francisco's brick buildings and wooden Victorian structures were especially devastated.

Fires immediately broke out in San Francisco due to broken gas lines, fallen power lines, and lanterns that had toppled over during the earthquake. Water lines were ruptured as well, which made it difficult to put the fires out. The earthquake triggered a massive fire fueled by collapsing structures and exacerbated by the failure of the water system. The quake was followed by a massive fire that swept from the business section near Montgomery Street and the South of Market district toward Russian Hill, Chinatown, North Beach, and Telegraph Hill. The blaze continued for four days, until its smouldering ashes were ultimately extinguished by rain.

More than 500 blocks in the city centre — covering some 4 square miles (10 square km) — were leveled. The inferno destroyed some 28,000 buildings, and the total property value loss was estimated at $350 million. At 7 a.m., U.S. Army troops from Fort Mason reported to the Hall of Justice, and San Francisco Mayor E.E. Schmitz called for the enforcement of a dusk-to-dawn curfew and authorized soldiers to shoot to kill anyone found looting.

Casualties and the Displaced Population

The true human cost of the disaster was underreported for decades. Some early death estimates exceeded 500, with early counts ranging from 375 to over 500. However, hundreds of fatalities in Chinatown went ignored and unrecorded. The frequently quoted value of 700 deaths caused by the earthquake and fire is now believed to underestimate the total loss of life by a factor of 3 or 4. Most of the fatalities occurred in San Francisco, and 189 were reported elsewhere.

In 2005, the city's Board of Supervisors voted unanimously in support of a resolution written by novelist James Dalessandro and city historian Gladys Hansen to recognize the figure of 3,000+ as the official total. Most of the deaths occurred within San Francisco, but 189 were reported elsewhere in the Bay Area; nearby cities such as Santa Rosa and San Jose also suffered severe damage.

Between 227,000 and 300,000 people were left homeless out of a population of about 410,000; half of those who evacuated fled across the bay to Oakland and Berkeley. Aid poured in from around the country and the world, but those who survived faced weeks of difficulty and hardship. Survivors slept in tents in city parks and the Presidio, stood in long lines for food, and were required to do their cooking in the street to minimize the threat of additional fires.

Scientific Legacy

The 1906 earthquake transformed the scientific understanding of seismology and fault mechanics. The California earthquake of April 18, 1906, ranks as one of the most significant earthquakes of all time. Today, its importance comes more from the wealth of scientific knowledge derived from it than from its sheer size.

Analysis of the 1906 displacements and strain in the surrounding crust led Reid (1910) to formulate his elastic-rebound theory of the earthquake source, which remains today the principal model of the earthquake cycle. Professor H.F. Reid of Johns Hopkins University developed this "Theory of Elastic Rebound," which explains the series of events that ultimately leads to earthquakes. Plate motion folds and elastically distorts the crust until the accumulated strain in the crust is released, creating large seismic waves that shake the crust violently.

As a basic reference about the earthquake and the damage it caused, geologic observations of the fault rupture and shaking effects, and other consequences of the earthquake, the Lawson (1908) report remains the authoritative work, as well as arguably the most important study of a single earthquake. The report was led and edited by Professor Andrew Lawson of the University of California, Berkeley. Seismologists estimated the average speed of the 1906 rupture along the San Andreas Fault to the north of the epicenter to be 8,300 miles per hour (3.7 km/sec), and 6,300 miles per hour (2.8 km/sec) to the south.

California did not routinely map faults and study earthquakes until after the magnitude 7.7 Arvin-Tehachapi earthquake of 1952 — a gap that the findings of the 1906 event had long made urgent. The lessons of 1906 directly shaped modern seismic-zonation practices across California and informed earthquake preparedness standards applied nationwide.

Aftermath and Rebuilding

Despite the near-total destruction, San Francisco undertook a rapid and determined rebuilding effort. Much of the city was rebuilt to be earthquake- and fire-resistant. New plans for civic development made headway as the debris of the old city vanished. In 1915, San Francisco invited the world to see the results of its efforts at the Panama-Pacific International Exposition.

The aftermath spurred significant improvements in San Francisco's infrastructure and disaster response capabilities. The 1906 earthquake and fire underscored the urgent need for a dedicated water supply system for firefighting. The existing system had failed during the disaster, necessitating critical improvements. Congress responded to the disaster in several ways. The House and Senate Appropriations Committees enacted emergency appropriations for the city to pay for food, water, tents, blankets, and medical supplies in the weeks following the earthquake and fire. They also appropriated funds to reconstruct many of the public buildings that were damaged or destroyed.

Over the previous 60 years, the city had become the financial, trade, and cultural center of the West, operating the busiest port on the West Coast. It was the "gateway to the Pacific," through which growing U.S. economic and military power was projected into the Pacific and Asia. Though San Francisco rebuilt quickly, the disaster diverted trade, industry, and population growth south to Los Angeles, which during the 20th century became the largest and most important urban area in the West.

Between 1908 and 1913, Congress debated whether to make a water resource available or preserve a wilderness when the growing city of San Francisco proposed building a dam in the Hetch Hetchy Valley to provide a steady water supply — a direct outgrowth of the vulnerability to fire revealed by the 1906 disaster.