Instrumental monitoring began in the early 20th century, but the historic catalog extends back to the 19th century through newspaper accounts and early seismographs. Across both states, the average annual count of magnitude 4.0‑4.9 events has risen modestly since the 1970s, reflecting denser networks rather than a true increase in activity.

Large‑magnitude earthquakes (≥ 6.5) remain clustered along the southern San Andreas corridor, the Eastern Sierra, and the Nevada–California border. The 1906 San Francisco quake, the 1994 Northridge event, and the 2019 Ridgecrest series illustrate the spatial variability of major ruptures.