San Francisco's City Hall the day before the 1906 San Francisco earthquake and resulting fire. � Because of the earthquake, over half of the city's population of 400,000 were left homeless to live in tents. � It has been speculated that most of the damage in the 1906 quake was caused not directly by the earthquake itself, but by the resulting inflagration. � The 1989 Loma Prieta earthquake resulted in a collapse of the middle portion of the Bay Bridge.. Motorists were trapped and killed between the beams, accounting for many of the injuries and deaths in that area. � This garage collapsed because of the earthquake and poorly built apartments above it.

General Earthquake Facts 1. In 1769, the earliest reported earthquake was felt by Spanish explorers. It took place about 30 miles southeast of Los Angeles. 2. The average range of motion along the San Andreas fault zone is about 2 inches per year, about the same rate of growth of human fingernails. 3. When primitive seismographs were first built in 1751, no one knew the reason earthquakes happened. That wasn't discovered until 1855, when faults were determined to be the source of earthquakes. 4. Many people think that the words "tsunami" and "tidal wave" are interchangeable. However, they are two unrelated phenomena. A tidal wave is a large wave produced by high winds, while tsunamis are initially caused by an underwater earthquake, which displaces ocean water relatively quickly. 5. In an earthquake, the hypocenter is the origin of the rupture of the fault. The epicenter of the earthquake is located directly above the hypocenter, on the surface. 6. It is a popular opinion that more damage was done during the fire after the 1906 San Francisco earthquake than by the earthquake alone. 7. A seiche ("SAYSH") is the movement of water in a body of water caused by the ground shaking in an earthquake. It often happens in swimming pools during a California earthquake. A seiche can continue from minutes to hours after original force is gone. It can also be caused by wind or tides. 8. Every year, southern California has an average of about 10,000 earthquakes. The majority of these quakes are minor and go unnoticed by the general public. Only a few hundred of these quakes are above an M3.0, and about 15-20 are greater than M4.0. If a large earthquake takes place, the aftershocks will continue for many months. 9. The magnitude of an earthquake is the measure of how big an earthquake is. It does not vary with location. However, the intensity of an earthquake is how much shaking is generated by an earthquake. The intensity does vary with location. 10. There are many people who attribute earthquakes with muggy weather or unseasonably cold weather, but science determines that there is no correlation between earthquakes and weather. There is an equal likelihood of earthquakes in warm, cold, rainy, foggy, and windy weather. There is no physical way for the weather to affect the forces beneath the earth's surface. Barometric pressure on the earth's surface doesn't affect the changes in the earth's crust because it cannot go beneath the soil. 11. The typical depth of an earthquake is about 50 miles below the earth's surface. 12. The San Andreas fault is not one single fault, but a series of faults called a fault zone. Movement can occur along one section along the fault zone at any time. This fault zone is over 800 miles long, and up to 10 miles deep. 13. In 350 B.C., Aristotle discovered that soft ground (landfill) shakes more than hard rock (bedrock) in an earthquake. 14. European settlers first experienced an earthquake in America in 1663. � California Earthquake Facts California's complex and interesting landscape can be attributed to a network of faulting running underneath the state. Faults are seen when there are abrupt changes in the structure of rocks. Typically, faults are observed in seacliffs along the beach or places where layers of rock are exposed. Faults, including fault zones, are classified by how the rocks on each side of the fault move past each other. There are two different kinds of faults: strike-slip faults and dip-slip faults. Strike-Slip Faults In a strike-slip fault, the rocks move past each other horizontally. The San Andreas is a good example of a strike-slip fault, having displaced rocks up to hundreds of miles. The San Andreas is not a single fault, but a fault zone, where movement can occur on one section without affecting another section. The effects of this fault zone can be observed for more than 600 miles. Dip-Slip Faults Dip-slip faults are faults which move parallel to one another, almost vertically. The rocks that generally form ridges are called horst, while the rocks that make up valleys are called graben. When the dip of a normal fault's surface is steep, it is called a normal fault. Two examples of a normal fault are the Sierra Nevada fault zone and the Owens Valley fault zone. This created the Owens Valley, an example of a graben. A version of a dip-slip fault is called a detachment fault, in which the dip of a fault surface is nearly flat. These are common in the deserts of California.

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