San Andreas Fault
The San Andreas Fault ( engl. San Andreas Fault ) is a quite ingenious ( dextral ) transform fault along which the Pacific plate at the North American plate by drifts . It stretches for a good 1,300 kilometers lengthways through the US state of California .
Like any other plate boundary, the San Andreas Fault is a scene of earthquake activity. On April 18, 1906 , the city of San Francisco was struck by a severe earthquake measuring 7.8 on the moment-magnitude scale .
The San Andreas Fault is one of the few plate boundaries that run directly on a continent. Most of the plate boundaries run along the ocean floor.
course
The San Andreas Fault can be geologically divided into three sections: a southern, a central and a northern section.
The southern section begins on the Salton Sea ( 33 ° 21 ′ 48 ″ N, 115 ° 40 ′ 45 ″ W ) near the Mexican border, from where it goes southeast into the spreading zone of the Gulf of California . From the Salton Sea to the west-northwest, the fault runs, passing Los Angeles northeast, almost parallel to the coast to the northern end of the Transverse Ranges (triangle of the counties Santa Barbara , Ventura and Kern , 34 ° 55 '25 "N, 119 ° 22' 36" W ).
There a bend to the northwest marks the transition to the central section of the fault. This section also runs almost parallel to the coast. In contrast to the northern and southern sections, it is also characterized by the lack of significant side branches. Its northern end is in the area of the northern end of the Gabilan Range ( San Benito County , 36 ° 47 '36 "N, 121 ° 21' 54" W ).
From there the fault runs towards the coast, which it, passing San Jose and San Francisco just southwest, reaches in the northwest of the San Francisco Peninsula. Moving further to the northwest, it first forms the Olema Valley and Tomales Bay and covers a longer distance on the coastal mainland to the north of Point Arena ( Mendocino County ). Up to Punta Gorda ( Humboldt County , 40 ° 15 ′ 41 ″ N, 124 ° 21 ′ 42 ″ W ), which is considered the northern end of the northern section, its course can be easily followed using the seabed morphology. From there it will likely turn west towards the Mendocino Triple Point, where it meets the Mendocino Transform Fault and the Cascadia Subduction Zone .
Move
The annual displacement of the earth's crusts can be determined based on the decreasing plate distance between Los Angeles and San Francisco. Accordingly, it is about 6 cm per year. The North American plate pushes south and the Pacific plate pushes against it.
However, the movement is not always constant; some areas of the fault move almost constantly, while other areas get stuck and only occasionally shift jerkily by several meters against each other - up to six meters in the San Francisco earthquake of 1906. In the Fort Tejon earthquake of 1857 the Shift in places even have been up to nine meters.
Geologists believe the San Andreas Fault system must have been in motion for at least 31 million years. During this time, the Reyes Peninsula has moved 450 km from the south to its current location. The two pinnacles were formed in the same place about 23 million years ago. In the meantime, however, they have been moved 313 km along the San Andreas Fault. The northern Pinnacles are 313 km from their origin, today's Neenach Formation. And still does a lot in the system of the San Andreas Fault: A few hundred thousand years is Point Reyes to be an island on their way to Alaska , the Bodega Bay is "overrun". It was not until the late 1960s that scientists realized that the San Andreas Fault is the boundary between two continental plates and that earthquakes are therefore inevitable. Since then, people in the region have no longer asked whether the next earthquake will happen , but when . The United States Geological Survey has been carrying out intensive investigations since 1984, particularly in the small village of Parkfield, which is located directly on the San Andreas Fault . They should help to improve the ability to predict earthquakes. Since 2004, with the help of SAFOD (San Andreas Fault Observatory at Depth 35 ° 58 ′ 27 ″ N, 120 ° 33 ′ 8 ″ W ), attempts have been made for the first time to drill into an active plate boundary directly with a deep borehole in order to draw direct conclusions in the event of an earthquake to be able to draw on the processes in the fault.
Link to the Cascadia subduction zone
The San Andreas fault activities are related to the Cascadia subduction zone . Their seismic activities often precede the earthquakes of the San Andreas fault, on average 25 to 45 years. In a study with turbidites , 15 earthquakes in the San Andreas Fault found 13 quakes in the Cascadia zone that occurred simultaneously.
Web links
- United States Geological Survey (USGS), Southern California Earthquake Center: New Study - Researchers Predict Major Earthquake in California. In: Spiegel Online from April 15, 2008 (30-year probability for strength 6.7 by the year 2038 at 99.7 percent. For strength 7.5 or more is 46 percent.)
- NASA Earth Observatory, Image of the Day (June 23, 2009)
- Information from USGS: The San Andreas Fault (English)
literature
- Robert E. Wallace (Ed.): The San Andreas Fault System, California. US Geological Survey Professional Paper 1515, Washington (DC) 1990, doi: 10.3133 / pp1515
Individual evidence
- ^ Seismological Society of America : Earthquakes Along The Cascadia And San Andreas Faults May Be Linked, Affecting Risk To San Francisco Bay Region. , ScienceDaily , April 8, 2008. Retrieved July 29, 2015.
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