The 1906 earthquake in San Francisco spawned fires that nearly destroyed the city, and defied all scientific theories of the day. You know the broad story; now here's the science to fill in the details.
Blended image of 1906 post-quake damage, and present-day San Francisco. Photography credit: Shawn Clover
The 1906 earthquake isn't one of the Top Ten largest earthquakes in the world. It isn't even the largest earthquake in Californian history. Yet it is one of the most significant earthquakes in history, shaping both cultural and scientific understanding of the devastation an earthquake can wreck.
At 5:12 am on April 18th, 1906, 296 miles of the San Andres fault ruptured, producing a magnitude 7.8 earthquake. The fault broke from northwest of San Juan Bautista all the way to the triple junction at Cape Mendocino. The fault produced horizontal displacements of 8 to 20 feet at the surface, and probably more at depth.
The earthquake was felt from Los Angeles to southern Oregon, as far inland as Nevada, and picked up by a seismogram in Germany. The reconstructed shakemap for the event follows a cheerful rainbow scale of intensity: red for severe shaking, down through violet for a gentle rumble.
Shakemap of the 1906 earthquake. Image credit: USGS
Fires burned for three days after the earthquake, destroying 28,188 buildings. Attempts to build firebreaks by proactively demolishing buildings failed miserably, instead creating flammable rubble pathways to further spread the fire. By the time the fires died down, a full quarter of the city was destroyed. In a bit of trickery that is both horrifying and impressive, insurance companies only had to cover damage to buildings that were still standing, not those destroyed by the fires.
Almost every building in the business district burned, with a few steel-framed exceptions. The St. Francis Hotel and the Fairmount Hotel are visible in the distance. Photography credit: National Archive
In a city of 400,000 people, over half — 225,000 people — were made homeless by the direct earthquake damage, or by the resulting fires that raged through the city. Major refugee camps were set up in the Presidio and Golden Gate Park, with 19 other camps run by the Army in other locations around the city.
The combined property damage from earthquake and fire totalled $80 million dollars in 1906 dollars. That's earlier than the Bureau of Labor Statistics's inflation calculator can reach, but works out to somewhere in excess of $8 billion dollars at current rates.
The devastation wasn't limited just to buildings and damage. At least 498 deaths in San Francisco, another 64 deaths further north in Santa Rosa, and 102 deaths to the south in San Jose. Those are official numbers — the social-cultural context of the time make actual numbers even more difficult to establish than usual for a catastrophe.
The earthquake was deeply puzzling to sciences. It spawned the first integrated government science investigation, funded by congress, that tied the importance of surficial materials on the felt intensity of shaking. In other words, soils shake more than rock, and the waterfront infill shakes more intensely than rocky hills.
Displacement and cracking in artificial infilled ground. Photography credit: National Archive RG 46
The research also involved careful measurements of how much the ground had displaced from the earthquake, recording maximum displacement along the fault line. These observations led to Harry Fielding Reid developed his theory for elastic-rebound theory to describe earthquakes. The idea is that as stress builds up, the fault is deforms to accommodate it, until eventually it builds up too much and the whole fault breaks and dislocates. This principle still guides our understanding of the relationship between faults, stress, and earthquakes.
Even with all these advances, it wasn't until plate tectonic theory was developed decades later that scientists were able to explain how so much of a fault could move together.