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DISCOVERY FILE |
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San Francisco is "earthquake country. " Most everyone knows about the devastating earthquake that hit San Francisco in 1906. But many people don't know other moderate to great earthquakes (over magnitude 6.0) have also hit San Francisco--22 in the last 160 years. The average is one significant earthquake every seven years. And it's not just San Francisco that's affected. Big earthquakes usually damage areas much larger than San Francisco. The 1906 San Francisco earthquake also damaged Oakland, San Jose, and Santa Rosa. More recently, the 1989 Loma Prieta earthquake caused extensive damage in the Santa Cruz Mountains, as well as in Oakland, and San Francisco tens of miles away. From 1911 to 1979--when most of the San Francisco Bay Area was developed--there were fewer earthquakes. But in recent years, earthquake activity there has increased. Major earthquakes are now occurring at the same rate that they were before the 1906 earthquake. Earthquakes happen in the Bay Area when forces cause faults under the ground to suddenly break loose and slip. If the break reaches to the surface, we can see visible movement along a fault. But strong earthquakes can also happen deep in the earth without any visible cracking of the surface. Slippage along a fault generates vibrations or waves. We feel these waves as the ground shaking. Because faults are weaknesses in the rock, earthquakes tend to occur over and over in the same areas. Almost all of the major faults in the Bay Area are strike-slip faults where the break extends more-or-less vertically into the ground. In a strike-slip fault the ground on one side of the fault moves past the ground on the other side. Thrust faults are much more common in the Los Angeles area. With a thrust fault, the ground on one side of the fault rises over the ground on the other side of the fault. Thrust faults in southern California are caused by the bend to the west that the San Andreas fault makes in that area. |