How to Explore the San Andreas Fault Near the San Francisco Bay

When you consider that the San Andreas Fault has shaped not only California history but California itself, it is surprising how few of the people who live nearby ever learn to spot it on a map, or stop to look for it in their travels. If you visit the San Francisco Bay Area, you have many opportunities to observe the San Andreas Fault, literally at your feet. Here is how to spot it and where to look for it.

Learn a little about the San Andreas Fault. This information will help you understand what you are seeing. The San Andreas is a transform fault, meaning that it is the boundary between tectonic plates. It is the boundary between the Pacific Plate (to the west) and the North American plate (to the east). It runs for 810 miles, mostly north-south along the coast of California.

Land to the west of the fault is moving roughly north to northwest; land to the east is moving roughly south-southeast.

The fault is a strike-slip fault, meaning that the movement on either side is mostly horizontal and roughly parallel to the fault, as shown in figure C in the diagram.

Take a hike. Near Palo Alto, California, visit Sanborn County Park and explore the earthquake trail there. [1]
The San Andreas Rift Valley, one of the places you can hike through, is along the fault. Looking at a topographical map or satellite photo of this area shows the line of the fault clearly. The valley is formed by a mixture of geological characteristics (the weak spot where the earthquake fault is) and the erosion that has happened as water flows through the landscape.

Visit the website of the US Geological Survey (USGS) in Menlo Park, California. [2]
This USGS office is a research studies earthquakes, along with other earth sciences topics.

Check their website for seminars and lectures open to the public, for news on recent earthquakes (large and small) and for earthquake preparedness information.

Also check out their earthquake map [3] to see recent earthquakes.

This image is an earthquake intensity map for a small (magnitude 3.4) quake that happened near Gilroy, California. It also shows faults in and around the area in red. The San Andreas is the one that passes offshore just south of San Francisco.

Drive north along highway 280, and stop to see the view. Watch for the blue Vista Point signs and take the one just north of Edgewood Road. You will drive up into the hills and get a great view of (south to north) the Upper Crystal Springs Reservoir, the Lower Crystal Springs Reservoir, and the San Andreas Lake. If you look at these bodies of water on a map or from the viewpoint, you’ll see that they all line up. That’s because they all lay along the fault.
There are hiking trails down along the Crystal Springs Reservoir, as well.

Visit Point Reyes National Seashore. You may have already explored this beautiful landscape, a short distance north of the Golden Gate. But did you know that the entire Point Reyes peninsula shifted north some 20 feet during the 1906 earthquake? [4] To see the fault, look for water all in a straight line, separating the peninsula from the mainland, where Highway 1 runs along Tomales Bay.
Hike the short Earthquake Trail. [5] Start at the Bear Valley Picnic Area (near the Bear Valley Visitor Center) and follow the signs for a short 1km (0.6 mile) loop.

How likely is it that California will experience another earthquake? According to the USGS, “The probability of a magnitude 6.7 or greater earthquake in the Greater Bay Area is 63%, about 2 out of 3, in the next 30 years.”[6]
Even if you’re only in California for a few weeks or months, check out the USGS earthquake map while you’re there. You may be surprised at how many earthquakes happen every week. Of course, most are too small to feel.

Olema, near Point Reyes, is the place where the ground displaced the most during the 1906 earthquake, as evidenced by fences, roads and other markers which straddled the fault. However, Olema was not the epicenter, or origin, of that earthquake. The epicenter, as best modern science can determine, was about a mile off the coast of Daly City.[7]
The epicenter of the magnitude 6.9 Loma Prieta Earthquake, in 1989, was in the Forest of Nisene Marks State Park, in the Santa Cruz Mountains. The earthquake was along the San Andreas Fault system. According to a USGS publication: [8]The earthquake occurred when the crustal rocks comprising the Pacific and North American Plates abruptly slipped as much as 2 meters (7 ft) along their common boundary-the San Andreas fault system. The rupture initiated at a depth of 18 km (11 mi) and extended 35 km (22 mi) along the fault, but it did not break the surface of the Earth.

Sources and Citations
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