San Andreas Fault: Earthquake Risk, History & Live Activity
The San Andreas Fault is a right-lateral strike-slip fault that forms much of the boundary between the Pacific and North American plates. It runs roughly 1,200 kilometres through California, from the Salton Sea in the south to Cape Mendocino in the north, and slips an average of about 20 to 35 millimetres a year. The two plates grind horizontally past each other rather than one diving beneath the other.
How likely is a large earthquake?
The U.S. Geological Survey's Third Uniform California Earthquake Rupture Forecast (UCERF3, 2015) puts the chance of a magnitude 6.7 or larger earthquake somewhere in California at more than 99 percent over 30 years, and the chance of a magnitude 8 or larger at about 7 percent. The southern San Andreas carries one of the highest single-fault probabilities in the state.
These figures are long-run averages across decades, not forecasts of a specific date; no method today can predict the day, month, or year of a large earthquake. What they describe is a fault that has been storing strain faster than it has released it in recent history. The southern reach last broke in the great earthquake of 1857 and has been quiet since, which is why it draws so much attention.
Live seismic activity near this fault
Counts include M2.5+ events — the smallest size USGS reports consistently worldwide.
| Magnitude | Time (UTC) | Depth | Location |
|---|---|---|---|
| M2.5 | Oct 26, 13:10 UTC | 5 km | 4 km SE of Aromas, CA |
| M2.7 | Oct 25, 01:35 UTC | 1 km | 32 km WSW of Tecopa, CA |
| M2.5 | Oct 24, 20:39 UTC | 2 km | 7 km W of Cobb, CA |
| M2.5 | Oct 24, 18:42 UTC | -0 km | 17 km SW of Toms Place, CA |
| M2.9 | Oct 24, 10:54 UTC | 2 km | 19 km SW of Toms Place, CA |
| M3.0 | Oct 22, 21:58 UTC | 6 km | 15 km S of Tres Pinos, CA |
| M3.1 | Oct 22, 04:18 UTC | -1 km | 23 km WSW of Gustine, CA |
| M2.5 | Oct 20, 16:23 UTC | 0 km | 2 km NW of The Geysers, CA |
History
The San Andreas system has produced some of the largest earthquakes in the recorded history of the western United States.
- 1857 Fort Tejon (M7.9) — ruptured about 350 kilometres of the central and southern fault.
- 1906 San Francisco (M7.8) — broke roughly 430 kilometres of the northern fault; the earthquake and the fires that followed destroyed much of the city.
- 1989 Loma Prieta (M6.9) — struck the Santa Cruz Mountains section of the San Andreas system and caused 63 deaths across the Bay Area.
- 2004 Parkfield (M6.0) — a moderate, heavily instrumented event on the central section, where the fault creeps.
Frequently asked questions
Is the San Andreas Fault overdue for a big earthquake?
Overdue is a loose word. The southern San Andreas last produced a great earthquake in 1857, and trenching shows large events there have recurred on the order of every 100 to 150 years on average, so the elapsed time is comparable to past intervals. But faults do not keep a schedule, and an average is not a deadline. UCERF3 expresses the hazard as a probability over decades, not a due date.
What would a big San Andreas earthquake do?
A USGS scenario for a magnitude 7.8 on the southern San Andreas estimated one to two minutes of strong shaking across Southern California, heavy damage to older buildings, and disruption to water, power, and transport lines that cross the fault. Modern engineered buildings are designed to remain standing; the greatest risk is to older unreinforced masonry and to lifelines that cross the fault trace.
Nearby regions
Live counts from the USGS earthquake feeds within 400 km of the fault. Magnitudes and depths are automated readings and may be revised. Not for emergency use.