San Francisco Bay Region Geology and Geologic Hazards

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Sliced Up

Many faults make up the plate boundary, and they slide blocks of rocks like a deck of cards on edge, pulling them apart or bringing them together over great distances.

In addition to sliding the rocks of the Salinian complex north from southern California, the San Andreas Fault system has broken up and moved the rocks of the region. Although the San Andreas Fault itself is often thought of as the boundary between the Pacific and North American plates, many faults in the region work together to take up the motion of the plates sliding against each other. Today these faults, which are found from the Pacific Ocean to Mount Diablo, are moving together about an inch and a half a year (about one-billionth of a mile per hour). The movement of all these faults over geologic time has sliced up many rock bodies and moved the pieces far apart. For example, Eocene sedimentary rocks located south of Monterey are the same deposit as those found at Point Reyes, sliced up and moved apart by the San Gregorio Fault. Many more offset rock bodies have been recognized by geologists, allowing them to estimate how far and how fast the faults in the region have moved, as well as to understand how this faulting has changed the shape of California.

illustration - description below
These maps show how far the rocks of the San Francisco Bay region have slid along the faults of the San Andreas Fault system, beginning about 34 million years ago (lower left) and continuing until today (upper right). The arrows show the distance that the faulted blocks of rocks have moved relative to the central block that contains San Francisco, Oakland, and San Jose. The main active faults of the San Andreas Fault system are shown on the present-day map as black lines. Faults that were once important but are no longer active are shown in magenta.

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