There are only two large known historic earthquakes on the San Andreas Fault in southern CA, the most recent in 1857, and before that one in 1812. Thick red lines show the extent of historic ruptures. Bold numbers show the average time between big earthquakes, determined at paleoseismic sites (triangles). Let's start in southern California and work our way north. Many of the sites paleoseismologists have been studying are along key sections of the SAFZ where there is a large population or major infrastructure that would be affected by a large earthquake in the future. The plate is moving slowly all the time, but the edges move in fits and starts. The stuck section slips, and the edge of each block catches up to the rest of the plate. An earthquake occurs when the stress from the force of the moving plate overcomes the friction causing the plate boundary edges, the fault, to stick. However, the edges of the blocks, the faults themselves, are stuck and only move where there is a large earthquake (some faults creep a little bit, but most are locked). The faults are boundaries between blocks, and each block is constantly moving, which we can see by analyzing GPS (Global Positioning System) data. The relative motion between these two tectonic plates is 50 mm/yr (about 2 inches/yr), but that rate is distributed across all the faults that are part of the SAFZ. Visit Media to see details.Ĭartoon sketch of the Pacific Plate-North American Plate boundary showing the San Andreas Fault. In northern California, the zone includes the Hayward, Calaveras, as well as the Northern San Andreas and other faults, and in southern California, the zone is even wider, encompassing the Southern San Andreas, the San Jacinto, and other faults in the Los Angeles area. The “zone” part of the name means it’s a system with the main fault and many sub-parallel faults that all together take up the motion between the two plates. The SAFZ is the main part of the boundary between the Pacific tectonic plate on the west side and the North American plate on the east side. The SAFZ started moving about 28-30 million years ago and has horizontally slipped (transform motion) a total of about 300-350 km (186-220 mi)since it began moving. Scientists have a good big picture understanding of the San Andreas Fault Zone (SAFZ). First, let's zoom out and look at the big picture. Since we have exceeded the average, many people use the term “overdue,” but it’s more complicated than that. The data show that at many places along the San Andreas Fault, we have gone past the average time between large earthquakes. Public domain.)Įarth scientists have been gathering data at key paleoseismic sites along sections of the San Andreas Fault to figure out the past timeline of earthquakes at each spot. The red line is traced on a fault that offsets the layers. The horizontal colored lines highlight different layers of sediment. Exposure of the San Andreas Fault in a trench.