The San Andreas Fault zone is one of Point Reyes National Seashore's defining features, but most days you can't even see it. Why? The fault zone is deep underground, where two tectonic plates meet and rub against each other. The plates are rigid (or almost rigid) slabs of rock that comprise the crust and upper mantle of the Earth. The plates are continually moving but where they touch each other, they get stuck. As the rest of the plates moves, the stuck parts deform like compressing a spring so they build up stress in the rocks along the fault. When the rock breaks or slips, the plates move suddenly, causing an earthquake.
The two plates in our region most affected are the Pacific Plate (most of Point Reyes) and the slowly moving North American plate (everything east of Highway 1). Along this stretch of the San Andreas Fault, the last big release happened during the great San Francisco Earthquake of 1906. The greatest displacement in this area was measured at 24.5 feet, but on average the plate is estimated to creep northwestward about two inches a year. For years, many people thought Point Reyes was the epicenter of the earthquake because of its huge displacement, but with better technology and understanding of plate tectonics, scientists determined the epicenter was acutally offshore of Pacifica, CA.
You can see the fault and learn more about it by visiting the Earthquake Trail, a 0.6-mile self-guided wheelchair accessible loop along the faultline. Accompanying exhibits help to explain plate tectonics. Allow about 30 minutes.
Or join a ranger for a guided walk around the Earthquake Trail at 10:30 am on most Saturdays. Check in at the Bear Valley Visitor Center for details.
Check out some interesting facts about the San Andreas Fault:
- The fault is roughly 28 million years old and in another 28 million years may be in present day Alaska!
- The fault is about 700 miles long as the crow flies and about 800 miles long when its curves are measured.
- The fault zone can be up to ten miles deep, and reaches from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County.
Are you prepared for an earthquake? Use these tips and links to help you prepare for the next big one.
Did you feel it? Learn more about the earthquake rating scale and where to go to post your observations, here.
Check out interactive maps of the fault zone.