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The San Andreas Fault
September 23, 2013, 05:00 AM By Darold Fredricks

On Oct. 30, 2007, the earth shook in the Bay Area. We were having an earthquake. This tremor reached a 5.6 on the now-famous Richter scale. People in the Bay Area were jolted back to reality, and many first-timers were terrified. It created a lot of anxiety, but little damage was done at this time.

In 1906, a much larger earthquake occurred (8.3 Richter), with a great deal of damage and loss of life. This quake moved the earth in some places 20 to 30 feet. Again in 1957, we had a good jolt, but this was not excessive and did a minimum of damage. In 1989, the earth moved again. The 15-second Loma Prieta quake (6.9) caused extensive damage through-out the Bay Area, including the collapse of the Cypress Freeway and a portion of the Bay Bridge. The cost was over $6 billion. And it cost 63 people their lives.

On Nov. 30, 1774, Captain Fernando Rivera’s exploratory group stopped in a little valley they named Canada de San Andres Valley (later spelled Andreas) to honor their patron saint, St. Andrew. Eventually this valley was discovered to have been formed by a tremendous fault that ran the length of California — the San Andreas Fault.

Planet Earth has been found to be neither flat nor static. The topography of the surface has been ever-changing and it will never cease its restless adjustments to time and pressure. There is a seemingly endless source of energy generated in the interior of the earth in the form of heat which slowly works its way to the surface by convection. In its movement, it creates a number of geologic phenomena that result in a multitude of surface configurations and features. One of man’s greatest insights into the understanding of nature has crystallized only recently when it was understood that Earth’s crust is not immovable, but has a number of large and small plates that move and have been moving since the Earth formed. Some of this heat is responsible for huge slabs of crust to constantly be shifting into new positions. They meet resistance that has to be overcome and this has created additional pressure and movements. The numbers of plates that move on the surface of our globe have produced startling geologic events and social reactions on the western edge of our continent. The study of these actions, called Plate Tectonics, is quite new and still evolving.

As the Pacific Plate moved eastward against the stable Northern American Plate, the Pacific Plate ducked under the North American Plate in the California region and the ensuing slide scraped off some of the millions of years of accumulated debris nearer to the surface. This debris accumulated above the water level heaped up unevenly, forming a barrier to the ocean’s force and becoming what is called the Coast Range. It is a jumbled-up mess of sandstone and shale that formed on the ocean’s floor and is called greywacke by geologists. The present maximum height of it around the Bay is about 4,000 feet at Mount Hamilton, Mount Diablo and Mount Tamalpais. A few miles eastward from the Pacific Ocean its influence diminishes and a flat, dry valley many miles wide occurs. This valley is formed from eroded rocks that washed down from the Sierra Nevada Mountains to the east. As soon as this scraped-off debris was lifted above sea level, the water from the atmosphere in the form of rain and fog started eroding the range.

The Coast Range forms a great barrier to the ocean moisture that rides with the clouds as they flow to the east. At the crest of the Coast Range water flowing toward the stream that ran from the San Jose area has cut gullies and ravines deep into the hills, creating a rugged topography with soil that scrub oak trees and low bushes and chaparral have thrived on. Related to the great ebb and flow of the ocean currents is a seasonal change due the movement of the Earth around the sun and moisture falls mainly in the cool season, winter. Continual rainfall in the millennium of winters since their formation have forced the streams and the rivulets to overflowing with torrential rain water on its way to the Bay. The scouring ability of this water forced solid rocks to become mud. In this state it could be moved to the Bay, forming miles of mud flats. Study of these muddy records in the rocks and mud flats has revealed these hidden secrets of past geologic events.

The rapidly-moving water gouged the canyons deeper each year and carried the dissolved mud toward the lower parts of the river in the center of the Bay and settled there. The smaller particles traveled far out to the ocean, but the heavier particles settled and formed extensive mud flats around the Bay. The slower the water, the more it dropped its load of sediment. The hills kept giving its sediment for millions years, forming what was to eventually become land that would create favorable places for mankind to live ... until Mother Nature decides to rearrange her furniture with the help of another earthquake along the San Andreas Fault.

Rediscovering the Peninsula by Darold Fredricks appears in the Monday edition of the Daily Journal.



Tags: water, earth, valley, plate, formed, surface,

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