The San Andreas Fault System
Donald R. Prothero
Abstract
California is legendary for its earthquakes, and a lot has been learned about seismology by studying earthquakes and faults in California. In particular, they have a set of distinctive surface features, and by trenching into them and dating the layers, paleoseismologists can study their prehistoric movements. The San Andreas fault is the largest and most important fault in the state, since it is a transform fault 148 that marks the boundary between the North American and Pacific plates. A tour of the San Andreas along most of its length allows the geologist or seismologist to witness much evidence of how and when it has moved. Figure 11.19 Piercing points that restore the original position of the blocks west of the San Andreas. Point 1 is the offset of Jurassic rocks in the Gualala block, which have moved 350 mi since 150 Ma (or 2 mi/m.y.). Point 2 is the contact with the Cretaceous basement (which has moved 320 mi since 65 Ma, or 5 mi/m.y.) on both sides of the fault. Point 3 is the match of the Eocene Butano Sandstone on the west with the Point of Rocks Sandstone on the east side of the fault (225 mi since 37 Ma, or 6 mi/m.y.). Point 4 is the match of Oligocene–lower Miocene rocks across the fault (175 mi since 20 Ma, or 8.75 mi/m.y.). Point 5 is the late Miocene match (65 mi since 8 Ma, or 9 mi/m.y.). Point 6 is the offset of Pleistocene gravels (14 mi since 2 Ma, or 7 mi/m.y.). Point 7 is the offset of the young Big Pine and Garlock faults (5 mi in 100,000 years). In the inset diagram in the upper right is a cartoon showing the magnitude of the offset of the western block (if treated as one coherent unit) since the late Miocene and Late Jurassic. In reality, the western block is not a single coherent terrane but a composite of many smaller blocks. For example, much of the Mesozoic offset (points 1 and 2) may have taken place on the San Gregorio–Hosgri fault, since the modern San Andreas did not develop until about 30 Ma (see Chapter 15 ). https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003301837/e36b5725-87f5-48a6-82f9-0113d8e8ec0b/content/fig11_19.jpg "/> Source: From California Division of Mines and Geology Bulletin, 170, 1954.