A deep dive into the infamous San Andreas Fault and the seismic risks it poses to California.

The San Andreas Fault is a continental transform fault that extends roughly 1,200 kilometers (750 mi) through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk. The southernmost segment, which begins near the Salton Sea, has a recurrence interval of about 180 years, but it has been unusually quiet for more than 300 years.

The fault was first identified in 1895 by Professor Andrew Lawson of UC Berkeley, who discovered the northern zone. It is often described as having been named after the San Andreas Lake, a small body of water that was formed in a valley between the two plates. However, some of his reports from the 1890s and early 1900s suggest that he may have named it after the surrounding San Andreas Valley.