The 1906 earthquake rupture trace of the San Andreas fault north of San Francisco, with stops at points of geotechnical interest
Published:January 01, 2006
Tina M. Niemi, N. Timothy Hall, Alexander Dahne, 2006. "The 1906 earthquake rupture trace of the San Andreas fault north of San Francisco, with stops at points of geotechnical interest", 1906 San Francisco Earthquake centennial Field Guides: Field trips associated with the 100th Anniversary Conference, 18–23 April 2006, San Francisco, California, Carol S. Prentice, Judith G. Scotchmoor, Eldridge M. Moores, Jon P. Kiland
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The main destination of this field trip is the San Andreas fault in Marin County, where the ground rupture of the 1906 earthquake is well preserved within the boundaries and easements of Point Reyes National Seashore. In addition to three stops along the fault, the field guide also describes stops to view the Golden Gate Bridge and White's Hill slide on Sir Francis Drake Boulevard near the town of Fairfax, and it discusses the geology along the way. Figure 1 shows the location of the stops for this field trip. Excellent online fieldtrip guides to the geology of Point Reyes peninsula, the Marin Headlands, and the San Andreas fault are available on the Internet (Stoffer, 2005; Elder, 2005).
The great San Francisco earthquake of 18 April 1906 was generated by rupture of at least 435 km of the northern San Andreas fault (Lawson, 1908). The earthquake produced maximum horizontal offsets of 16–20 ft (5–6 m) along the San Andreas fault north of San Francisco and smaller offsets south of the city. In Marin County, there has been very little urbanization along the fault. Prior to the establishment of the National Seashore in 1962, most of the region was used for dairy farming and cattle ranching. Because the region remains largely as it was in the late nineteenth century, conditions are ideal for investigating how the morphology of the rupture has changed in the 100 years since the earthquake. Furthermore, this section of the San Andreas fault continues to yield important data about dates of prehistoric earthquakes and the slip rate of the fault.
Two fundamentally different types of bedrock underlie Marin County (Fig. 2). Right-lateral shear along the San Andreas transform plate boundary during the late Cenozoic has juxtaposed Franciscan subduction zone rocks on the east against the Salinian terrane of Point Reyes peninsula to the west. The Franciscan Assemblage (Complex) is a highly deformed, lithologically heterogeneous sequence of metamorphosed volcanic and sedimentary rocks accreted to western North American during subduction of the Farallon plate in the Mesozoic. The Salinian terrane is a displaced fragment of continental crust that consists of Cretaceous plutonic and older metamorphic rock overlain by lower Eocene to Pliocene marine sedimentary rocks (Clark and Brabb, 1997). In between the Franciscan and Salinian terranes lies a valley created by the San Andreas fault zone that is characterized by Quaternary deposition and low ridges and depressions elongated parallel or subparallel to the fault.
Along the route of this field trip on our way to the San Andreas fault, road cuts expose the world-famous, Franciscan Accretionary Complex rocks including oceanic pillow basalts (greenstone) overlain by radiolarian chert, graywacke sandstone, and “mélange” (from the French word for “mixture”), with inclusions of greenstone, chert, serpentinite, and graywacke. Isolated outcrops or knobs of erosion-resistant rocks within a surrounding matrix of highly sheared shale of the mélange typify the topography of grass-covered slopes of eastern Marin County. During the trip we will also travel through a forest of redwood trees near Samuel Taylor State Park en route to the Douglas-fir–covered Point Reyes peninsula.
Figures & Tables
1906 San Francisco Earthquake centennial Field Guides: Field trips associated with the 100th Anniversary Conference, 18–23 April 2006, San Francisco, California
The twenty field trip guides in this volume represent the work of earthquake professionals from the earth science, engineering, and emergency management communities. The guides were developed to cross the boundaries between these professions, and thus reflect this diversity: trips herein focus on the built environment, the effects of the 1906 earthquake, the San Andreas fault, and other active faults in northern California. Originally developed in conjunction with the 100th Anniversary Earthquake Conference held in San Francisco, California, in April 2006, this book is meant to stand the test of time and prove useful to a wide audience for general interest reading, group trips, or self-guided tours.
- engineering properties
- fault zones
- field studies
- field trips
- Franciscan Complex
- Marin County California
- Point Reyes
- road log
- San Andreas Fault
- San Francisco California
- San Francisco County California
- San Francisco earthquake 1906
- United States
- Point Reyes National Seashore
- San Francisco earthquake 19096