San Andreas Fault
The San Andreas fault, with a length of more than 800 miles (1,200 kilometers), is the longest fault in California, and one of the longest in North America. Because of the aridity of the Carrizo Plain, the trace of the San Andreas Fault has not been significantly eroded. Photographs of the stark hills and clear trace of the fault in the Carrizo Plain have been used in numerous earth science text books.
On January 9, 1857 at 8:20 am, an earthquake with an estimated magnitude of 8.0 occurred just north of Carrizo Plain. This quake caused nearly 30 feet (9 m) of lateral offset within Carrizo Plain, and ruptured the surface along the trace of the fault for about 220 miles (350 km). It was one of the greatest earthquakes ever recorded in the United States. Buildings in Los Angeles were severely shaken, and the quake was felt from Marysville south to San Diego and east to Las Vegas, Nevada. The current of the Kern River was turned upstream, and water ran four feet deep over its banks. The waters of Tulare Lake were thrown upon its shores, stranding fish miles from the original lake bed. The waters of the Mokelumne River were thrown upon its banks, reportedly leaving the bed dry in places. The Los Angeles River was reportedly flung out of its bed, too.
Since that event, this portion of the San Andreas fault has been locked in place, seismically quiet for decades; however, it is obvious a similar magnitude quake today would cause billions of dollars of damage to Los Angeles, the southern San Joaquin Valley, and Antelope Valley.
There has been extensive geologic research conducted in the Carrizo Plain over the last 100 years, and during the last 20 years the pace of research has increased. The first geologic investigations conducted by A. C. Lawson nearly 100 years ago were driven to provide understanding of the San Andreas fault which had caused the devastating 1906 San Francisco Earthquake. Soon after that, US Geological Survey geologists mapped the Carrizo Plain and vicinity to provide a geologic framework for oil exploration in the region.
Literally hundreds of geologic investigations have been conducted within the Carrizo Plain since Lawson's 1908 study of the San Andreas Fault. These investigations can be grouped as follows:
1) Study of the San Andreas and other faults (paleoseismic and geophysical studies): These studies have revealed details about the nature and intensity of earthquakes that have occurred here, potential for future quakes as well as the deep structure of the fault.
2) Study of fossils (paleontology): Correlation of fossils with rock units (stratigraphy), description of species which occurred here, interpretation of paleoclimate and past geographic setting of the area.
3) Basic geologic frame work (stratigraphy and structure): Geologic mapping with descriptions of rock units their composition and source, and interrelationship with other rock units.
4) Study of soils. The highly saline and alkaline soils surrounding Soda Lake provide a natural laboratory which has agricultural implications.
5) Paleoclimate. By coring Soda Lake, scientists have found that the sediments of Soda Lake preserve a high-resolution record of climatic change in southern California.
If you were standing at Wallace Creek, on the San Andreas Fault, in 1857, just after the last earthquake, you would have witnessed a large tear stretching across the land, as far as your eye could see. A casual observer today might conclude that erosion has covered up signs of the fault that broke with such great force—but the fact is, anyone can see it—and so can you!
How can you tell there is a fault here? Walk along the trail, read the trail guide and look at the unusual landforms and geological features. Figure out, just as a geologist would do, what happened in the Earth’s past that formed the land you see now.
When the “modern” channel of Wallace Creek first formed about 3800 years ago (see “Science in Action” on the back of the trail guide), it cut a path directly across the fault. There were no bends. Repeated motion along the fault, due to large earthquakes every few hundred years, caused the upstream half of Wallace Creek to break away from its downstream half. Geologists call this feature an offset channel (From the Wallace Creek Trail Guide).
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Soda Lake, a normally dry playa, covers an area of about 3,000 acres, and is one of the dominant geographic features of the Carrizo Plain. A crust of sodium sulfates and carbonates are the result of evaporating mineral-laden surface water.