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OK... Now I am Nervous.... Sitting on the San Andreas Fault in California..........
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The San Andrea's Fault:
The San Andreas Fault
Despite its benign name (Saint Andrew), the San Andreas fault is a violent and destructive killer. Over its long history, spasms of its enormous energy have been released in countless earthquakes along its great length. These have ranged in magnitude from slight tremors to cataclysmic upheavals and rupturing of Earth's surface.
It is a well known and familiar geologic feature in western America. The remarkably straight fault trace is prominent on any California map. It looks like a knife slash from hell in a northwest-southeast direction that attempts to separate the Coast Ranges from central and southern California.
The fault ranks among the longest such structures in the world. Overland, it is a visible break that can be traced for 625 miles from Point Arena, north of San Francisco, to the Salton Sea, near the border with Mexico.
The inferred extensions of the fault are even more dramatic. Northward, the break is thought to extend under the sea another 400 miles, possibly merging into the Murray Fracture Zone on the deep sea floor. At the southern end, the fault is believed to follow the axis of the Gulf of California for almost 1,000 miles.
Because of the fault's great length, as much as 2,000 miles, movement is intermittent as to time, and random as to place. At any place along the fault, movement may be a one-time event, or movement may be recurrent over thousands of years. In any one episode, displacement may be a fraction of an inch or it may be many feet.
The surface trace of the fault first appears at Point Arena, a peninsula north of San Francisco. It takes a straight path southeastward through a series of elongated valleys to San Francisco Bay, it crosses the bay with San Francisco on the western block and the bay cities on the eastern or continental block.
In the San Francisco Bay area, the San Andreas has two major companion branches. Generally defining the east side of the Bay is the Hayward fault, and a few miles inland is the Calaveras fault. The Hayward fault presently carries a high probability forecast for an earthquake.
In an undeviating course, the trace passes San Jose and enters the Coast Ranges. It runs parallel to the Gabilan Range, passes the towns of Coalinga and Parkfield (both with vivid earthquake histories), thence it defines the elongated Temblor Range to Tejon Pass.
This segment of the fault in central California is very old. Between the Bay area and the Tehachapi Mountains, rock formations appear to have been displaced as much as 160 miles. This suggests that the northern section of the fault may have originated about 30 to 40 million years ago, during Oligocene time.
The fault planes of the faults in the system are usually near vertical. In some sections, however, the fault plane is locally tilted as much as 60 degrees to the northeast. A local example of this effect is the Banning fault near Cabazon in San Gorgonio Pass. Here, the fault tilted to the point where the moving western block slid under the eastern block, a thrust fault.
The San Andreas fault is a shallow fault, generally extending into the earth's crust four to ten miles. Earthquakes associated with shallow faults are more destructive at the surface than those originating from deep-seated faults.
The San Andreas is an uninterrupted zone of many braided fault traces. These fractures branch and interlace in a swath two to eight miles wide along the fault's great length. With these multiple fractures creating great slices or slabs, and over countless episodes of earth movement, the rocks within the fault zone have been severely deformed, smashed and ground up. Thus weakened and broken, they become subject to rapid weathering and erosion.
So, the fault course in central California has been eroded into a series of elongated valleys for more than one hundred miles through the Coast Ranges as a shallow trench only a few miles wide. This distinctive landform was made both from erosion of the shattered and weathered rocks and from the effect of recent faulting. The valleys contain many typical fault-related features such as offset streams, ponds, and scarps. This string of continuous valleys is recognizable on any large scale road map.
In southern California, the fault is much younger. Major right-lateral movement began about 12 million years ago, or during late Miocene time.
This considerably predates the opening of the present Gulf of California which began about 5.5 million years ago. The progressively younger age of segments of the fault from north to south is consistent with ideas concerning plate movement along the west coast of North America.
The southern California segment of the San Andreas fault is described from the vicinity of Gorman, about 60 miles northwest of Los Angeles. There, it bends abruptly to the east for six miles, then resumes its original southeast heading.
This "Big Bend" area is possibly the most significant tectonic area in California today. Here, the San Andreas intersects the left-lateral Garlock fault, the only major east-west trending fault in southern California. Earthquakes are common in this area. The Fort Tejon earthquake in 1857 (magnitude 8-plus) is thought to have been at least as violent as the San Francisco earthquake 49 years later. The San Fernando earthquake of 1971 (magnitude 6.6) was also associated with this zone of intersection.
From Tejon Pass, the main trace of the San Andreas fault passes through the high desert north of Los Angeles, defining the north face of the San Gabriel Mountains. It then separates the San Gabriel Mountains from the San Bernardino Mountains, creating Cajon Pass that takes Interstate 15 out of southern California.
From the vicinity of Cajon Pass, the southern California segment of the San Andreas fault becomes very complex with no distinctive single trace as in the north. Instead, it is divided into several right-lateral elements, all somewhat parallel to each other. Principal among these are the San Jacinto and Elsinore faults. Several branches go through the San Bernardino Mountains and along the north margin of San Gorgonio Pass into the Coachella Valley.
One important associated fault is the San Gabriel fault, defining the southern edge of the San Gabriel Mountains in the Los Angeles basin. This fault was probably the most active strand for thousands of years, but there is no evidence of recent activity. Another important branch in the Los Angeles area is the Newport-Inglewood fault, the source of the destructive Long Beach earthquake of 1923.
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The Next big one will be San Francisco....
Take a look at the map [link to geology.com]
I am located outside of Stockton.
Edit to add >>> [link to www.sci.sdsu.edu]
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