1906 SAN FRANCISCO EARTHQUAKE 5:12 am on April 18, 1906 Fires lasting for 3 days 490 blocks destroyed 25,000 buildings toppled Gas and water lines ruptured Loss of electric power
1906 San Francisco Earthquake William Alexander Coulter’s (1849-1936) panorama of the largest maritime rescue in United States history. The painting depicts the fleet of rescue vessels that ferried more than 30,000 people to safety from the burning city.
Earthquake Science Before & After1906 • Earthquake research in the U.S. had advanced slowly compared to efforts in Japan and Europe. • Only a small number of geology professors at U.S. universities and USGS geologists studied earthquakes. • Little was known about how and where they occurred, and the hazards they presented. • Theory of plate tectonics was still more than a half-century away. • 1906 Earthquake started the study of earthquakes and California geology in earnest. • State Earthquake Investigation Commission is formed – Andrew C. Lawson. • 1908 Lawson report was released.
San Andreas Fault • Amount and Rate of offset variable. • Consists of a complex system of parallel and interconnecting faults.
April 18, 1906 • 5:12 AM – Initial foreshock • 20-25 seconds later – The great earthquake hits. • Strong shaking lasted 45-60 seconds. • Rupture length ~290 miles.
Magnitude & Intensity • 1906 Earthquake magnitude was ~7.8. Traditional estimates went as high as 8.3, whereas modern estimates range from 7.7 to 7.9. • Shaking intensities of VIII (moderate damage) to IX (heavy damage) extended as much as 60 miles inland along a broad band paralleling the fault trace – depending on the “softness” of the subsurface materials.
Seismograms • Time advances from left to right. Small wiggles, beginning 1/2 inch from left end, signal arrival of first compressional (P) waves. Large wiggles half way along represent arrival of slower-traveling shear (S) waves.
The City of San Francisco “All of a sudden we had found ourselves staggering and reeling. It was as if the earth was slipping gently from under our feet. Then came the sickening swaying of the earth that threw us flat upon our faces. We struggled in the street. We could not get on our feet. Then it seemed as though my head were split with the roar that crashed into my ears. Big buildings were crumbling as one might crush a biscuit in one's hand. Ahead of me a great cornice crushed a man as if he were a maggot - a laborer in overalls on his way to the Union Iron Works with a dinner pail on his arm.” (P. Barrett).
The City of San Francisco The air was filled with falling stones. People around me were crushed to death on all sides. All around the huge buildings were shaking and waving. We rushed down Market Street. Men, women and children were crawling from the debris. Hundreds were rushing down the street and every minute people were felled by debris.(G.A. Raymond)
The City of San Francisco “When the fire caught the Windsor Hotel at Fifth and Market Streets there were three men on the roof, and it was impossible to get them down. Rather than see the crazed men fall in with the roof and be roasted alive the military officer directed his men to shoot them, which they did in the presence of 5,000 people.” (Max Fast). “The most terrible thing I saw was the futile struggle of a policeman and others to rescue a man who was pinned down in burning wreckage. The helpless man watched it in silence till the fire began burning his feet. Then he screamed and begged to be killed. The policeman took his name and address and shot him through the head.” (Adolphus Busch).
The City of San Francisco “The street car tracks were bent and twisted out of shape. Electric wires lay in every direction. Streets on all sides were filled with brick and mortar, buildings either completely collapsed or brick fronts had just dropped completely off. Wagons with horses hitched to them, drivers and all, lying on the streets, all dead., struck and killed by the falling bricks.” “The water mains had been broken by the earthquake, and so there was no supply for the fire engines and they were helpless. The only way out was to dynamite, and I saw some of the finest and most beautiful buildings in the city, new modern palaces, blown to atoms.” (Jerome B. Clark)
The City of San Francisco • Fire destroyed 2,831 acres of the city – more than 490 blocks. • 30 schools, 80 churches, and 250,000 homes were destroyed. • Modern estimates of at least 3000 people were killed. • Roughly 225,000 people were left homeless from a population of about 400,000. • Estimated property damage of $400,000,000 in 1906 dollars.
Aftermath The San Francisco Daily News was the only newspaper to publish on April 18.The Daily News’ plant, downtown, lost power and water in the earthquake, and moved to J.V. Rooney’s small printing shop at 1308 Mission Street, where this edition was turned out on a hand-cranked press capable of printing single sheets. New editions were printed until the shop was ordered evacuated because it was to be dynamited.
Propaganda and Corruption • Railroads controlled California politics at the time. • Southern Pacific Railroad Co. was aggressive in its attempt to rewrite the history of the earthquake. • Sunset magazine was devoted to extolling the wonders of California and was a promotion tool for the railroad. • Propoganda articles stressed the rebuilding of the city and highlighted the damage from fire and minimized the effects of earthquake. • The reason was to keep from destabilizing Eastern money markets and the economic interests of the railroad. • Mayor Schmitz, members of the Board of Supervisors, the police chief, and coroporate members of the railroads and other utilities were indicted for bribe giving and taking both before and after the earthquake.
Scientific Discovery • The Lawson Report • The Commission's final report, published in 1908, was an exhaustive compilation of detailed reports from more than twenty contributing scientists on the earthquake's damage, the movement on the San Andreas fault, the seismograph records of the earthquake from around the world, and the underlying geology in northern California.
Theory of Elastic Rebound • This theory, which forms the basis for our modern understanding of earthquakes, describes how the earth's crust gradually and elastically distorts with accumulating plate motion until it is suddenly returned to its undistorted state by rapid slip along a fault, releasing the years of accumulated strain and, in the process, generating seismic waves that produce shaking. • 1906 earthquake essentially turned off earthquakes of magnitude about 6 and larger for the next 73 years. • Central California has been experiencing a seismically quiet period caused by stress relaxation after 1906. The region may slowly be recovering from this "stress shadow" to a more normal state of seismicity as the tectonic plates continue to move, and the stresses on the major faults recover to the values that they had in 1905. Stress changes after 1906 for faults parallel to the San Andreas. In blue regions, parallel faults are less stressed; in red regions, more stressed. Nearly all major faults were relaxed after 1906.
The Next One? The rate of large earthquakes in the San Francisco Bay region abruptly dropped after the Great 1906 Earthquake. The San Andreas Fault slipped so much over such a great length in that quake that the strain was reduced on most faults throughout the region. Strain has been slowly building up again. However, the level of seismic activity has not yet reached that of the late 1800's.
Probabilities The threat of earthquakes extends across the entire San Francisco Bay region, and a major quake is likely before 2030. Knowing this will help people make informed decisions as they continue to prepare for future quakes.
Then & Now If a similar earthquake occurred in Northern California today, after many decades of rapid urban growth, thousands of people would likely be killed, and economic losses might be in the hundreds of billions of dollars. Studies of earthquake shaking, active faults, and the response of structures to shaking have already led to improved building codes and a better understanding of how to reduce the threat posed by earthquakes. Damaging earthquakes are inevitable in the Bay region, but taking actions based on the odds of future quakes will help save lives and protect property.