San Jacinto Fault Zone: Earthquake Risk, History & Live Activity
The San Jacinto Fault Zone is a right-lateral strike-slip system running roughly 210 kilometres through the Inland Empire and desert of Southern California, from near San Bernardino southeast toward the Imperial Valley. Part of the wider San Andreas plate-boundary system, it takes up Pacific-North American motion at a slip rate of about 12 millimetres a year and is generally considered the most seismically active fault zone in Southern California.
How likely is a large earthquake?
The San Jacinto zone produces moderate earthquakes more often than any other Southern California fault, with magnitude 5 events roughly every decade and magnitude 6 events every few decades. UCERF3 (USGS, 2015) includes it among the higher-probability contributors to the state's greater than 99 percent chance of a magnitude 6.7 or larger earthquake over 30 years.
Parts of the zone, notably the Anza section, have not produced a ground-rupturing earthquake since at least 1918 and possibly longer, accumulating strain that makes them a focus of study. Because the fault threads through growing Inland Empire communities, even a moderate event can affect a large population.
Live seismic activity near this fault
Counts include M2.5+ events — the smallest size USGS reports consistently worldwide.
| Magnitude | Time (UTC) | Depth | Location |
|---|---|---|---|
| M2.7 | Sep 12, 07:34 UTC | 19 km | 5 km E of Lennox, CA |
| M2.6 | Sep 11, 10:19 UTC | 8 km | 6 km W of Edwards Air Force Base, CA |
| M2.7 | Sep 8, 21:15 UTC | 2 km | 39 km NE of Big Bear City, CA |
| M3.1 | Sep 6, 07:13 UTC | 16 km | 4 km SSE of Compton, CA |
History
The San Jacinto zone has the busiest historical record of any Southern California fault, with several damaging earthquakes over the past century.
- 1899 San Jacinto (M6.6) — destroyed adobe buildings in San Jacinto and Hemet; six people were killed at Soboba.
- 1918 San Jacinto (M6.8) — the largest instrumentally recorded event on the fault, again damaging San Jacinto and Hemet.
- 1968 Borrego Mountain (M6.5) — ruptured the Coyote Creek section in the desert to the southeast.
- 1987 Superstition Hills (M6.6) — struck the southern end of the system near the Salton Sea.
Frequently asked questions
Is the San Jacinto Fault overdue?
Some sections are candidates. The Anza section has not produced a ground-rupturing earthquake in over a century, longer than the intervals seen elsewhere on the zone, so it is watched as a strain-accumulating gap. The fault as a whole, though, is unusually active and releases energy in frequent moderate earthquakes rather than staying silent between rare large ones.
Why is the San Jacinto Fault so active?
It carries a large share, roughly 12 millimetres a year, of the motion between the Pacific and North American plates, comparable to neighbouring strands of the San Andreas system. That steady loading, combined with a structurally complex set of fault segments, produces frequent moderate earthquakes across the zone.
Nearby regions
Live counts from the USGS earthquake feeds within 200 km of the fault. Magnitudes and depths are automated readings and may be revised. Not for emergency use.