Mission of San Juan Bautista

August 04, 2005


Provided and copyright by: Tom McGuire
Summary authors & editors: Tom McGuire

Geology, history and Hollywood come together on this photo. Many of us fondly remember Alfred Hitchcock's 1958 thriller, Vertigo. James Stewart plays a troubled detective with a fear of heights. He's hired to tail a suicidal wife. Perhaps the most gripping scene occurs when the wife (Kim Novak) lures Stewart into a high bell tower for a memorable cinematic climax.

The truth of the tower of the Mission of San Juan Bautista is more than meets the movie lens. The famous bell tower sits directly on the 5-meter uplifted ridge of San Andreas Fault. As this photograph was taken, the North American Plate is on the right and the Pacific Plate is on the left. The stairs descend the fault scarp.

If the San Andreas is a right lateral strike-slip fault, why do we see vertical displacement? Nature is more complex than many textbooks (including my own --see link below) show. Natural faults are not perfectly straight. As a result, movement creates places where the two sides do not fit very well. This can result in a landform known as a pressure ridge.

The mission was constructed here because of the relatively flat terrace overlooking fertile farm fields. The elevated site sits on granite. Seismic shaking is usually more violent on loose sediments than on nearby bedrock. So the irony is that the Mission was built on a terrace created by the fault, but structures on the elevated site, resting on bedrock, are less prone to damage than structures on deep sediments across the fault. Perhaps proximity of Heaven has its advantages.

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