Los Trancos Open Space Preserve: San Andreas Fault Trail

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The San Andreas Fault Trail in Los Trancos Open Space Preserve is a short but pleasant and informative loop trail in the foothills above Palo Alto.  I had hiked in Los Trancos on other occasions, but this was my first time to take the self-guided earthquake tour on the San Andreas Fault Trail.  As the trail name implies, the informational brochure and signed stations focus on earthquake geology.  The San Andreas Fault runs through the park.

GPS track

GPS track

The trail dips slightly down the hillside toward San Francisco Bay, but there is an elevation loss/gain of only 270 feet.

Elevation profile

Elevation profile

I followed the brochure to pass by the stations in numerical order.  At the first station I noted that a marine layer still hovered above San Francisco Bay, so that Mt Diablo seemed to be floating on a cloud.

image of Mt Diablo above the marine layer

Mt Diablo above the marine layer

To the southeast, Mt Umunhum and Loma Prieta rise above the Stevens Creek Canyon, which essentially follows along the fault line that is the boundary between the Pacific and American plates.  This station is actually on the Pacific plate, while Loma Prieta is on the American plate.  There are rocks near the station that, some 2 million years ago, came down Loma Prieta and ended up on the Pacific plate, which moves northwest (relative to the American plate) at an average rate of about ¾ of an inch per year.

picture of view southeast along San Andreas Fault, with Mt Umunhum and Loma Prieta above Stevens Creek Canyon

View southeast along San Andreas Fault, with Mt Umunhum and Loma Prieta above Stevens Creek Canyon

From nearby station 2, Crystal Springs Reservoir is visible to the northwest in the crease between ridgelines in the Santa Cruz Mountains.  Crystal Springs Reservoir is about 15 miles away, while Loma Prieta is about 23 miles away in the opposite direction.  The marine layer that often pours through the Golden Gate into San Francisco Bay is still present over the reservoir.

photo of view northwest along San Andreas Fault to Crystal Springs Reservoir

View northwest along San Andreas Fault to Crystal Springs Reservoir

At another station there is a depiction of how a fence might look after the 1906 earthquake, if it had been built straddling the fault line before the earthquake.  The offset in this depiction is 2 or 3 feet, reflecting the amount of fault movement in this area.  In contrast, the movement north of the Golden Gate was more like 15-16 feet.

image of depiction of fence offset due to 1906 earthquake

Depiction of fence offset due to 1906 earthquake

The main loop of the trail crosses the fault line in two places.  At least within Los Trancos, a series of markers with yellow stripes near their tops has been placed along the fault line.  Being able to see the exact location of the fault was a particularly interesting feature of this trail.

picture of markers indicating the San Andreas Fault

Markers indicating the San Andreas Fault

Other stations describe local land features that characterize seismically active areas, such as benches, pressure ridges, and sag ponds.

Much of the lower part of the trail passes through forested areas.  Here the trail passes among bay trees with their characteristic growth pattern of multiple trunks fanning out from a central base.

photo of bay trees beside the trail

Bay trees beside the trail

I hiked this trail later the same day I hiked the Stevens Creek Nature Trail across Page Mill Road from Los Trancos in Monte Bello Open Space Preserve.  The San Andreas Fault Trail was a nice way to finish the afternoon.

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