The San Andreas Fault
The San Andreas Fault is a roughly 1,200-kilometre strike-slip fault running most of the length of California, marking where the Pacific plate slides north-west past the North American plate at about five centimetres a year. It is not one clean line but a system of segments, and it produced both the 1906 San Francisco earthquake and the 1857 Fort Tejon earthquake, each around magnitude 7.9.
Three segments that behave differently
The fault is usually described in three parts, and they do not behave alike. One creeps continuously; the other two are locked and release their strain all at once.
| Segment | Behaviour | Last major rupture |
|---|---|---|
| Northern, Cape Mendocino to Juniper Serra | Locked | 1906, magnitude 7.9 |
| Central, Parkfield to Hollister | Creeping, releases strain steadily | No great rupture recorded |
| Southern, Parkfield to the Salton Sea | Locked | 1857, magnitude 7.9 |
The creeping central section is the interesting one. Because it slips continuously, it never accumulates enough strain for a great earthquake, and it appears to act as a barrier that stops a rupture propagating from one locked section into the other.
The southern segment has not produced a major earthquake since 1857. At five centimetres a year, more than eight metres of slip has accumulated on it since, which is the basis of the long-standing expectation of a large southern California earthquake.
Will the San Andreas Fault cause California to fall into the ocean?
- No. It is a strike-slip fault, so the two sides move horizontally past each other rather than one dropping away. Los Angeles is drifting north-west towards San Francisco at about five centimetres a year, and in roughly 15 million years the two will be neighbours. Nothing sinks.
When is the next big one?
- Nobody knows, and nobody can. USGS forecasts probabilities rather than dates: its most recent assessment puts the chance of a magnitude 6.7 or larger in the greater San Francisco Bay Area at 72 percent within 30 years. That is a rate, not a prediction of any particular earthquake.
Can you see the San Andreas Fault?
- In places, clearly. At the Carrizo Plain the fault has offset stream channels by tens of metres, and the trace is visible from the air as a line running across the landscape. Elsewhere it is buried under sediment or built over.