Gazing Across a Plate Boundary EarthCache

Rather than one single fault, the San Andreas is actually a complex system of parallel and interconnecting faults. Some of these faults creep along almost continually, as at Hollister, California, while other sections are locked during periods between large earthquakes, as in Portola Valley. In general, the Pacific Plate is moving northward at about 2 inches (5.1 cm) per year relative to the North American Plate. While much of this motion is along the San Andreas Fault, much also occurs along known (such as the Hayward Fault along the southeast side of the San Francisco Bay) and unknown faults. Some of the movement is side to side (lateral), but other movement is more of a squeezing motion (compressional). These compressional forces have resulted in the landscape that you are standing on.

The San Francisco Bay portion of the San Andreas Fault may be one of the most spectacular settings in the world. Complex mountains, amazing and varied vegetation, and ecosystems ranging from wetlands, coastlines, grasslands, redwoods, and many more, combined with a tremendous human impact of 8 million people make for a fascinating place to study.

Natural hazards are commonplace, and include landslides, wildfires, and earthquakes. What hazards do you think occur most frequently? How are landslidees, wildfires, and earthquakes related in terms of time and space? From 1800 to the present, the San Francisco Bay Region has been shaken by 21 earthquakes of magnitude 6.0 or greater. This area is characterized by very little seismicity since the 7.9 magnitude San Francisco earthquake ruptured this section of the fault.

To the east is the San Andreas Fault and the Pilarcitos Fault, an inactive ancestral strand of the San Andreas. This area is the “Permanente Terrane,” a belt of ancient oceanic volcanic and sedimentary crustal rocks of Cretaceous Age. This includes pillow basalt, sandstone, mudrocks, chert, and limestone in various degrees of metamorphic process and from ancient and modern fault movement. Past the communities of Palo Alto and Menlo Park in the distance lies the southern part of the San Francisco Bay.

To the west lies the San Gregorio Fault, and in the hazy distance, the Pacific Ocean. Can you see the ocean today? These Santa Cruz Mountains where this Earthcache is located are composed of granitic and mafic igneous rocks that have intruded older Paleozoic and Mesozoic metamorphic rocks, called the Salinian Complex. These crystalline basement rocks are overlain by thick Cretaceous and Tertiary sedimentary rocks and some basaltic volcanic rocks.

Fortunately, despite continued population growth and urban sprawl, not all of this region is paved. Windy Hill Open Space is one of a series of preserved areas (www.openspace.org) in the MidPeninsula Regional Open Space District. A map of the open space can be found here.

The Windy Hill Open Space occupies several of the “balds”—a term for the grassy hilltops, where windy, dry conditions and nutrient-depleted soils favor the development of grass cover in some upland areas and downslope lands. Why do you think "balds" is the term used for these hilltops?

Other vegetation visible from Windy Hill is “mixed evergreen." Mixed evergreen forests include oaks, redwoods, Douglas fir, pines, chaparral, laurel, madrone, and others. Can you find each of these species as you are looking out over the landscape below?

To the east, Portola Valley is visible, the community sitting on top of the San Andreas Fault. Around 1900, this area became a place of small farms and large estates, where immigrants from Ireland, Portugal, Croatia, Italy, China, the Philippines, Chile, and Germany joined the Californians who were here earlier to raise strawberries, herd cattle, and cut firewood. Large landowners came from San Francisco to escape the summer fog. Extensive residential development began after World War II, and by the early 1960s, residents concerned about increasing pressures for housing and business expansion voted in 1964 to incorporate in order to have local control over development. Their goals were preserving the beauty of the land, fostering low-density housing, keeping government costs low by growing a cadre of volunteers, and limiting services to those necessary for local residents.

Many consider the community a success story—a good balance between modern development and pastoral quiet. Portola Valley is home to 4,500 residents in 1,700 households, but over 1,900 acres of permanent open space exist within the community. Indeed, there is no “downtown” but rather, a mixture of homes and vegetation with a distinctive rural feel. Numerous bicyclists and hikers were present on the day I set up the Earthcache. Do you see any bicyclists or hikers today?

The community’s lush surroundings do not seem to have lulled its citizens to sleep regarding natural hazards. I suppose it is difficult to ignore the enormous redwoods that were snapped in two from the 1906 earthquake. Ever since Portola Valley incorporated in 1964, numerous geologists have been pushing the town to develop zoning and building regulations that recognize geologic hazards. In fact, in 2003, the community earned the first Earthquake Risk Reduction Award for “leadership, innovation, and outstanding progress in reducing earthquake risks,” presented by the Northern California Chapter of the Earthquake Engineering Research Institute. George Mader, Portola Valley's town planner for over 40 years, has provided continuity in planning, and in converting information on local geologic risks into innovative regulations that guide planning and building. Would you like to live in this community? Why or why not?

This Earthcache is near Stop 9 in the Section 7—Field Trip to the Skyline Ridge Area in the Central Santa Cruz Mountains, in the excellent USGS book "Where’s the San Andreas Fault?"