EarthquakesThe Big One: Back to the Future on the San Andreas Fault
Maybe you’ve heard that the “Big One is overdue” on the San Andreas Fault. No one can predict earthquakes, so what does the science really say? Where does the information come from? And what does it mean? Earth 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.
Maybe you’ve heard that the “Big One is overdue” on the San Andreas Fault. No one can predict earthquakes, so what does the science really say? Where does the information come from? And what does it mean?
Earth 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 data show that at many places along the San Andreas Fault, we have gone past the average time between large earthquakes. Since we have exceeded the average, many people use the term “overdue,” but it’s more complicated than that. First, let’s zoom out and look at the big picture.
San Andreas Fault Zone – The Big Picture
USGS says that scientists have a good big picture understanding of the San Andreas Fault Zone (SAFZ). 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. 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 “zone” part of the name means it’s a system with a main fault and many sub-parallel faults that all together take up the motion between the two plates. 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.
