Parkfield Bridge and San Andreas Fault
August 09, 2008
This bridge across Cholame Creek at Parkfield, California marks the boundary between the North American and Pacific Tectonic Plates. The sign on the far side of the bridge identities entry back onto the North American Plate. Most plate boundaries are hidden from our view beneath the world’s oceans. Where they do occur on land they can be difficult to locate precisely. But the San Andreas Fault is remarkably easy to follow as it runs the length of California from the Salton Sea northwest approximately 800 miles (1,280 km), nearly to Oregon. There the fault curves out to sea.
A series of six earthquakes occurred in Parkfield at roughly 22-year intervals between 1857 and 1966. Another strong seismic event was predicted for the early 1990s. This led to the installation of a wide variety of instruments by the United States Geological Survey and a variety of other partners at Parkfield. But the next strong seismic event did not come until the 2004 magnitude 6.0 event. Note the extra wide supports under the bridge that have helped the structure survive. The far (Pacific) side has shifted more than 5 feet (1.5 m) relative to the North American Plate since the bridge was constructed in 1936.