A magnitude 7.9 earthquake struck the coast of Northern California at 05:12 am Pacific Standard Time on Wednesday, April 18, 1906. With a maximum Mercalli intensity of XI (Extreme), it created high-intensity shaking from Eureka on the North Coast to the Salinas Valley, an agricultural region to the south of the San Francisco Bay Area.
Extensive fires soon broke out in San Francisco and lasted for several days. More than 3,000 people died and over 80% of the city was destroyed. The event is remembered as the deadliest earthquake in the history of the United States. The death toll remains the greatest loss of life from a natural disaster in California's history and high on the list of worst American disasters, natural or man-made. In 2005, footage of the event was selected for preservation in the United States National Film Registry by the Library of Congress as being "culturally, historically or aesthetically significant."
Tectonic setting 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 strike-slip fault is characterized by mainly lateral motion in a dextral sense, where the western (Pacific) plate moves northward relative to the eastern (North American) plate. This fault runs the length of California from the Salton Sea in the south to Cape Mendocino in the north, a distance of about . The maximum observed surface displacement was about 20 feet (6 m); geodetic measurements show displacements of up to 28 feet (8.5 m).1906 San Francisco Quake: How large was the offset?; USGS Earthquake Hazards Program — Northern California. Retrieved September 3, 2016
Earthquake
A strong foreshock preceded the main shock by about 20 to 25 seconds, and the strong shaking of the main shock lasted about 42 seconds. The 1906 earthquake preceded the development of the Richter scale, but the most widely accepted estimate on the modern moment magnitude scale is 7.9, with proposed values from 7.7 to 8.3. Severe deformations in the Earth's crust took place both before and after the earthquake's impact. Accumulated strain on the faults in the system was relieved during the earthquake, which caused damage along a segment of the San Andreas plate boundary. The 1906 rupture propagated both northward and southward for a total of ,1906 Earthquake: How long was the 1906 Crack?; USGS Earthquake Hazards Program – Northern California. Retrieved September 3, 2006 with shaking felt from Oregon to Los Angeles, and as far inland as central Nevada.Christine Gibson; "Our 10 Greatest Natural Disasters," American Heritage, Aug./Sept. 2006.
Before the 1906 quake there were decades of minor earthquakes – more than at any other time in the historical record for northern California. Previously interpreted as precursory activity to the 1906 earthquake, they have been found to have a strong seasonal pattern and are now believed to be caused by large seasonal sediment loads of in coastal bays that overlie faults, erosion sediments left over from hydraulic mining in the later years of the California gold rush.
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. An offshore epicenter is supported by the occurrence of a local tsunami recorded by a tide gauge at the San Francisco Presidio; the wave had an amplitude of approximately and an approximate period of 4045 minutes.Tsunami Record from the Great 1906 San Francisco Earthquake; , United States Geological Survey, 2008
Analysis of triangulation data before and after the earthquake strongly suggests that the rupture along the San Andreas Fault was about in length, in agreement with observed intensity data. The available seismological data support a significantly shorter rupture length, but these observations can be reconciled by allowing propagation at speeds above the S-wave velocity (supershear). Supershear propagation has now been recognized for many earthquakes associated with strike-slip faulting.
In 2019, using an old photograph and a recorded eyewitness account, researchers were able to refine the location of the hypocenter of the earthquake as offshore from San Francisco or near San Juan Bautista, confirming previous estimates.
Intensity The shaking intensity as described on the Modified Mercalli intensity scale reached XI (Extreme) in San Francisco and areas to the north like Santa Rosa where destruction was devastating.
The most important characteristic of the shaking intensity noted in Andrew 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 of former bay where soil liquefaction had occurred. Modern seismic-zonation practice accounts for the differences in hazard posed by varying geologic conditions.
Aftershocks The main shock was followed by many aftershocks and some remotely triggered events. As with the 1857 Fort Tejon earthquake, there were fewer aftershocks than would have been expected for a shock of that size. Very few of them were located along the trace of the 1906 rupture, tending to concentrate near the ends of the rupture or on other structures away from the San Andreas Fault, such as the Hayward Fault. The only aftershock in the first few days of near 5 or greater occurred near Santa Cruz at 14:28 PST on April 18, with a magnitude of about 4.9 M. The largest aftershock happened at 01:10 PST on April 23, west of Eureka with an estimated magnitude of about 6.7 , with another of the same size more than three years later at 22:45 PST on October 28 near Cape Mendocino.
Remotely triggered events included an earthquake swarm in the Imperial Valley area, which culminated in an earthquake of about 6.1 at 16:30 PST on April 18, 1906. Another event of this type occurred at 12:31 PST on April 19, 1906, with an estimated magnitude of about 5.0 , and an epicenter beneath Santa Monica Bay.
Damage
Early death counts ranged from 375William Bronson, The Earth Shook, the Sky Burned (San Francisco: Chronicle Books, 1996) to over 500. However, hundreds of fatalities in Chinatown went ignored and unrecorded. The total number of deaths is still uncertain, but various reports presented a range of 700–3,000+. In 2005, the city's Board of Supervisors voted unanimously in support of a resolution written by novelist James Dalessandro ("1906") and city historian Gladys Hansen ("Denial of Disaster") to recognize the figure of 3,000+ as the official total.Casualties and Damage after the 1906 earthquake USGS Earthquake Hazards Program – Northern California. Retrieved September 4, 2006 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. Newspapers described Golden Gate Park, the Presidio, the Panhandle and the beaches between Ingleside and North Beach as covered with makeshift tents. More than two years later, many of these refugee camps were still in operation.Displays at the U.S. Army Corps of Engineers Museum in Sausalito, California
Selected Mercalli intensities MMI Locations San Francisco, Santa Rosa Sebastopol, San Bruno San Jose, Point Arena Eureka, Salinas Truckee, Parkfield Willows, Fresno Chico, Paso Robles Dunsmuir, Bakersfield Santa Monica, Indio U.S. Earthquake Intensity Database, NGDC
The earthquake and fire left long-standing and significant pressures on the development of California. At the time of the disaster, San Francisco had been the ninth-largest city in the United States and the largest on the West Coast. Over a period of 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. Over 80% of the city was destroyed by the earthquake and fire. 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. Many of the city's leading poets and writers retreated to Carmel-by-the-Sea where, as "The Barness", they established the arts colony reputation that continues today.
The 1908 Lawson Report, a study of the 1906 quake led and edited by Professor Andrew Lawson of the University of California, showed that the same San Andreas Fault which had caused the disaster in San Francisco ran close to Los Angeles as well. The earthquake was the first natural disaster of its magnitude to be documented by photography and motion picture footage and occurred at a time when the science of seismology was blossoming.
Survivor accounts Writing from San Francisco on April 23, five days after the earthquake, resident John W. Nourse described the scene to his family in Indiana: woken at quarter past five by shaking that made the house "seem like it would break all to pieces," he looked out to see a large brick building completely destroyed in seconds, with fires visible simultaneously to the right, front, and left. Grabbing the baby's buggy, he and his wife Lena fled to Market Street, which he described as a terrible sight. Whole blocks reduced to masses of ruins, City Hall a complete wreck. He left his wife and baby at the mission and returned to try to save their belongings, but found the whole part of the city burning. He and his family spent the night in the hills, watching twelve and twenty story buildings burning and falling, "a sight no one can describe." He noted that some streets had sunk several feet while others had risen, the city was under martial law, and provisions were beginning to arrive from surrounding towns. "In traveling over the city it looks like there is not a building but what is more or less damaged," he wrote.
A traveler passing through San Francisco ten days after the earthquake, Mrs. C. C. Baughman of LeMars, Iowa, described a stove placed in front of every house, residents were not permitted to cook indoors, and Market Street sunken several feet in places, with one location four blocks from the Ferry five feet lower than the sidewalk and the cable car rails warped and twisted. She observed soldiers with blue coats throughout the begrimed crowds, groups of listless dazed men in the suburbs, and frame buildings near Valencia Street completely shattered within a radius of several blocks. "The sights of the ruins are awful," she wrote, "and the scenes that transpired there during the terrible catastrophe are beyond the description of either tongue or pen." She and her husband, who had planned to settle permanently in California, left with "no desire to locate there permanently."