The San Andreas Fault extends over 400 miles (640 km), from Point Arena in northern California almost to the Mexican border in the south. The evidence of the fault (and the danger implied) is all around us, but few ever give it a second thought, despite many having vivid memories of Loma Prieta in 1989.
The first location overlooks one of the most distinctive fault features on the Earth. The linear valley is characteristic of a strike-slip fault. The San Andreas Fault marks the boundary of the Pacific and North American plates. If you look northwest, towards the lakes, the Pacific plate is on the left. Relative to the rest of North America, the area to the west of the fault is moving north at several centimeters per year.
The water that you see in the background is Crystal Springs Reservoir. There is another artificial reservoir in the fault zone just further to the north (and closer to this view), known as San Andreas. This valley of the two lakes is formed because of the almost constant motion of the rock against rock. The fault breaks down the rock as the two plates move past each other, creating soil that is more susceptible to erosion. Over time, much of this rock is washed away, leaving the narrow valley. The action of crushing the rock also creates an impermeable layer, preventing water from draining into the soil. Because of this, both the San Andreas and Crystal Springs lakes existed as much smaller sag ponds before the various dams were constructed.
This site is only accessible from Interstate 280, and is open from dawn to dusk.
Now proceed to N 37° 31.797 W 122° 21.773. This location is not on the freeway, but on Skyline Blvd. Exit I-280 onto highway 92 westbound, then turn right at the signal onto Skyline. Proceed about two miles (3km) until you have crossed the dam. Parking is available.
Now walk south until you can view the dam. Construction on Crystal Springs and San Andreas reservoirs was begun in the 1870s, with the clearing and removal (by hand!) of the foliage in the valley. This dam was completed in 1889, and an unseasonably rainy winter filled the reservoir well in advance of expectations. The dam was originally 120 feet (37m) high, and was raised to 145 feet (44m) in 1891. The dam escaped almost completely unscathed in 1906, despite the fact that the magnitude 7.8 earthquake was less than 18 miles (30km) to the northwest, and more than nine feet (3m) of ground displacement was recorded at a nearby estate. The height of the dam was raised again in 1911 to 149 feet (46m). In 1989 the magnitude 7.1 Loma Prieta earthquake struck about 45 miles (72km) SE, but once again the structure suffered virtually no damage.
Next, look to the west, across the water. Note the types and density of the foliage on the Pacific Plate. Now turn your attention to the east. Underneath the huge freeway overpass you see the natural drainage for the valley. Please notice where this drainage goes.
But one detail that has not been ignored is this area now comprises a "seismic gap", which is an area where no sizable earthquakes have struck in a number of years, therefore stress is likely building along the fault. The USGS says that there is a 62% chance of a magnitude 6.5 or greater earthquake in the Bay Area before 2033. And one of the most likely locations for that quake is along this very section of the fault, due to a bend in the fault zone under Black Mountain, near Los Altos Hills.
Photo courtesy of the United States Geological Survey.
Many thanks go to TerryDad2 for much invaluable assistance on this, my first EarthCache. He really is the master of the EarthCaches.
To log this, first email me with a comparison of the vegetation between the two sides of the valley, along with an explanation of (this is important) why they differ. Also include a description of the areas that would be at risk of flooding downstream if the dam were to fail in an earthquake. After you have sent the email, feel free to log the cache. I will contact you if there are problems. And please feel free to add any pictures of your visit.
