Although not as widely known as Yellowstone and Yosemite, Point Reyes National Seashore is particularly loved by residents of the San Francisco Bay Area. Just an hour’s drive from downtown San Francisco, Point Reyes feels a world away and provides a welcome refuge from urban life. Most also know that the San Andreas fault (SAF) extends through the park and that this part of the fault moved during the Great San Francisco Earthquake in 1906.
The epicenter (place on Earth’s surface directly above an earthquake) was offshore, just west of the entrance to San Francisco Bay, and of course that part of the fault could not be directly observed. But a famous geologist—G.K. Gilbert—walked nearly the entire length of the on-land part of the fault after the earthquake, taking photographs and drawing diagrams to illustrate his observations. The fault ruptured for 477 km (296 miles), from Shelter Cover, located about a 4.5-hour drive north of San Francisco to San Juan Batista, located about a 2-hour drive south of San Francisco. The greatest amount of offset along the fault was at Olema, near the Point Reyes National Seashore Visitor’s Center, a worthwhile stop.
Modern seismographic instrumentation was not available in 1906, but geophysicists have since used various methods to estimate a magnitude of 7.9 for the earthquake, which was a breakthrough event for science. Prior to 1906, the cause of earthquakes was not understood, but as a result of this large and well documented event, geoscientists figured out that earthquakes occurred because of fault movement—forces associated with plate motions cause pressure to build up on fault surfaces until the amount of force exceeds the strength of the fault (i.e., overcomes friction), and the fault moves, sending out seismic waves that reverberate throughout the earth and shake the ground surface.
While a professor at San Francisco State University, I and my students completed many studies of the Quaternary sediments (yellow areas marked Q in the geology map) to better understand how the fault zone has evolved during the past 200,000 years (recent history for the planet!). While working in the fault valley, I kept looking at the peninsula to the west and wondering why it was elevated—the transform SAF was causing the peninsula to slide northward, but it was not responsible for uplifting the ridges.
Our investigations involved features on land and under the ocean. On land we discovered that a series of flat surfaces indent the western slope of Inverness Ridge. These features, called marine terraces, can be used to figure out how fast the uplift is happening.
But what faults have been responsible for the uplift? To find these faults, we needed to examine data from the seafloor west of the peninsula. In the 1970s and 80s, oil companies conducted seismic surveys that involve sending sound pulses from a ship that are strong enough to penetrate the seafloor and bounce off of rock layers below. Companies also drilled holes into the seafloor to collect samples of the rock types. Fortunately, they didn’t find much oil and gave up the pursuit, but the data are now in the public domain and we were able to access them through the U.S.G.S. office in Menlo Park, California (thanks Holly Ryan!).
Park History. Point Reyes is one of the younger parks in the national park system. A few small county parks had been established, but in the mid 1950s, there were plans for a large development at Limantour Beach (see geology map above). In 1958, the National Park Service proposed establishing a national seashore at Point Reyes, and in 1959, Congressman Clem Miller began his first term in U.S. House of Representatives and made establishing the Point Reyes National Seashore his greatest priority. He was successful when the park was established in 1962 by President John F. Kennedy. Unfortunately, Clem died less than a month after this, but I encountered his gravestone one day while hiking on the west slope of Inverness Ridge—his marker appropriately looks out to sea from the beautiful peninsula that he was instrumental in saving.
Earlier on, coastal Miwoks had occupied the region for at least 10,000 years, before European explorers landed here. The Englishman Sir Frances Drake, for whom Drakes Bay and Beach are named, arrived in the last 16th century; at the beginning of the 17th century, the Spaniard Vizcaíno arrived and named the location “Punta de los Reyes”. Unfortunately, within a few centuries, most of the native peoples were gone.