On April 18, 1906, an enormous earthquake ripped through San Francisco and shook most of northern California. 700 people died in the earthquake and the ensuing fire, and damage was estimated at $350,000,000. Why? The answer lies deep under the city.
If you were to drill a hole from San Francisco down to the center of the Earth, you would hit a series of layers. In the middle is the Earth's core, which is made mostly of iron and nickel. The inner core is solid, and the outer core is liquid. Above the outer core is the mantle, which makes up most of the volume of the earth. The rocks that make up the mantle are extremely dense and hot, and under a lot of pressure. The heat makes the rocks flow very slowly, like extremely dense hot wax. Hot material in the mantle rises, cools, and sinks again, just as boiling soup rises and sinks in a pot. However, the mantle rocks take millions of years to complete the circuit. These circular paths that the mantle rock makes are convection cells.
The solid outer part of the Earth, the part we live on, is the crust. The crust is attached to the uppermost part of the mantle. Together they form a rigid, brittle layer called the lithosphere. (Lithos is the Greek word for rock.) The lithosphere is broken into unevenly shaped pieces of different sizes, called plates. The plates float on the hot, dense mantle like large crackers on hot, very thick pea soup. They break apart, collide, and grind past each other. Sometimes one plate will be driven under another into the mantle in a process called subduction. When two convection cells in the mantle are moving apart near the surface, the plates above them also move apart. If the hot material in two adjacent convection cells comes together at the surface, the plates also come together. In other places, the plates grind past each other; such boundaries are known as transform faults. The soft layer of the upper mantle on which the lithosphere floats is called the aesthenosphere, from the Greek word for "weak".
Click here for a map of the world's tectonic plates and a more detailed explanation of tectonics, courtesy of the U.S. Geological Survey.
Most of the North American continent is part of the North American plate. But a large sliver of western California is part of the Pacific plate, which underlies much of the Pacific ocean. The Pacific plate is moving northward relative to the North American plate. The plates are not colliding or separating; they are passing each other. This transform boundary manifests itself as the San Andreas fault system, a skein of breaks in the Earth's crust that lies along most of the length of California.
The plates do not glide smoothly past one another. There is great friction between them. They often get stuck in spots until they build up enough stress to snap the rocks and jolt a little farther along. When this happens, shock waves travel from the rupture through the surrounding rocks and an earthquake occurs.
Sometimes a section of the fault can get stuck for a long time. Instead of releasing its stress in a series of small earthquakes, it builds up energy until it violently lets go. This is what happened to the San Andreas fault near San Francisco. In 1906, the ground jolted 6 meters (18 feet) along the fault. Houses and buildings collapsed. Streetcar tracks were bent like pipe cleaners. Huge rifts opened in the earth. Broken gas lines and overturned lamps started a fire that burned for days and destroyed whole neighborhoods. The fire brigades, hampered by a lack of water, could only stop the fire by dynamiting buildings ahead of it. The city was a nightmare of fire, smoke, dynamite explosions, and destruction. The Museum of the City of San Francisco has many photographs of the city after the earthquake and fire.
Only two buildings remained standing on Nob Hill: the Flood mansion and the recently completed Fairmont Hotel, which was also built with Comstock money. Fires gutted both, but they did not collapse. The Flood mansion was repaired, and two wings were added to the first floor.
San Francisco lies on a variety of substrates. Some neighborhoods are on solid rock, while others are on sand; much of the city, however, is on fill--land that was made by dumping dirt into water and wetlands. Such material, when shaken, behaves like a liquid instead of a solid and amplifies the shaking, the way a bowl of jello shakes when you bump the table it's sitting on. This liquefaction is accompanied by a sudden loss of strength. In the 1989 earthquake in San Francisco, many buildings in the Marina district in the north of the city were badly damaged because the fill under them liquefied. In some places, the mud churned up debris from the 1906 earthquake that had been dumped as fill.