California – San Andreas Fault
In California, the boundary between the Pacific Plate and the North American Plate is designed as a horizontal shift (San Andreas Fault). The Pacific plate moves northwards, the North American plate moves southeast. Fault areas run perpendicular to the earth’s surface or to the ocean floor in the direction of the earth’s interior. The two plates rub against each other, they exert pressure on each other and interlock. A uniform horizontal movement cannot develop in this way in the area of the rigid earth’s crust. The granite intrusions on both sides of the fault surface are particularly resistant parts of the earth’s crust. They hold back part of the movements and store elastic parts of tension until the rock’s resistance to breakage is exceeded. Then the tensions suddenly dissolve in earthquakes. This explains the accumulation of earthquakes along the active fault lines; including numerous very strong earthquakes with a strength of 7 or more on the Richter scale. Such events occur around the world almost 20 times a year and lead to large-scale destruction of buildings, bridges or dams in populated areas. This damage goes hand in hand with changes in the landscape and ever new accompanying breaks.
The strongest earthquake in the region to date was the San Francisco quake of 1906. It had a magnitude of 8.3, making it one of the strongest earthquakes of the 20th century. With fewer than 1,000 deaths out of a population of around 400,000 at the time, it still had a comparatively mild course. 30,000 houses collapsed; Above all, however, high-rise buildings in reinforced concrete remained standing. Over a length of 600 kilometers there were horizontal shifts – in some places up to seven meters – as well as new crevices and faults. For geologists, this catastrophe was the starting point for modern earthquake research. (Up until 1935, earthquakes were classified according to their strength using the Mercalli scale; the information on the map according to the Richter scale was estimated retrospectively.)
The seismic hazard potential in California varies. It is largest along the main fault, the San Andreas Fault. Its course between Los Angeles and San Francisco is identical to the coastal mountains, north of San Francisco it first follows the coast and then changes to the ocean floor. Other areas of high risk are located at secondary faults in the Serra Nevada and south of California’s long valley. The seismic hazard potential is lowest on the eastern edge of California’s long valley. For more information about the continent of North America, please check areacodesexplorer.com.