The San Andreas Fault (SAF) is the primary tectonic boundary in California, where the Pacific Plate grinds northwest past the North American Plate. This movement is not a smooth slide but a stick-slip motion along the approximately 750-mile-long continental transform fault. The question of when the fault last “went off” is complex because it does not rupture as a single unit but in distinct, independent sections. Understanding the answer requires examining the fault’s segmented structure and the specific historical events that have reshaped the state’s landscape.
Defining the San Andreas Fault and Its Segments
The sheer length of the San Andreas Fault means its behavior varies significantly from north to south, which is why seismologists conceptually divide it into three primary sections. The Northern segment extends from the Mendocino Triple Junction down to the town of Hollister. This segment is known for its tendency to lock up and accumulate stress. This locked behavior allows tectonic strain to build over long periods, setting the stage for very large earthquakes.
The Central segment, running roughly from Hollister down to Parkfield, behaves differently than the sections to its north and south. This middle portion releases stress continuously through a process known as aseismic creep, which prevents the build-up of strain for major ruptures. The Southern segment begins near Parkfield and continues southeast toward the Salton Sea. This southern section is locked, making it capable of generating the largest and most destructive events near the state’s most populated areas.
The Last Major Rupture: The 1857 Fort Tejon Earthquake
The most recent major release of stress on the southern half of the San Andreas Fault occurred during the 1857 Fort Tejon earthquake. This event is estimated to have reached a moment magnitude of 7.9. The rupture propagated along a substantial portion of the fault, stretching for approximately 225 miles (350 kilometers).
The ground displacement extended from the Parkfield area in the north, through the vast Carrizo Plain, and continued southeastward, likely stopping near the Cajon Pass. Along the Carrizo Plain, the fault movement was dramatic, with maximum horizontal offsets recorded up to 30 feet (9 meters). Though the event is named after Fort Tejon, where it caused significant damage, the epicenter is thought to have been farther north, near Parkfield.
While the 1857 quake was the last major event on the southern part of the fault, the Northern segment experienced its most recent large rupture in 1906. This event, the San Francisco earthquake, also had an estimated magnitude of 7.9 and ruptured a fault section approximately 270 to 296 miles long. The 1906 event relieved stress along the northern fault section but did not extend far enough south to affect the section that ruptured in 1857.
Evidence of Past Events: Paleoseismology and Recurrence
To determine the fault’s long-term history beyond written records, scientists employ the technique of paleoseismology, which involves excavating trenches across the fault trace. These trenches reveal layers of sediment and soil that have been offset, folded, or faulted by past earthquakes. By collecting organic material from these layers and applying carbon dating techniques, researchers can establish the approximate age of prehistoric ruptures.
Studies at sites like Wrightwood, located on the Southern segment northeast of Los Angeles, have provided a detailed record of the fault’s ancient activity. This data shows a pattern of major ruptures over the last 1,500 years. The average time between these large-scale events, known as the recurrence interval, has been calculated at around 100 to 111 years in this section.
This paleoseismic record indicates that the interval between major earthquakes is highly variable, with individual periods ranging from a few decades to over a century and a half. The time that has passed since the 1857 Fort Tejon earthquake is now approaching or exceeding this average recurrence interval for the Southern segment. The long-term record demonstrates that this region of the fault has a history of frequent events, suggesting that the current period of quiet is not a permanent state.
Current Activity: Locked Sections vs. Creeping Sections
The San Andreas Fault today can be characterized by two contrasting modes of movement: locked and creeping. The Northern and Southern segments, which ruptured in 1906 and 1857 respectively, are currently considered locked. In these locked sections, the Pacific and North American plates are firmly stuck together, and tectonic forces continue to push, causing strain to accumulate in the crust.
In contrast, the Central segment, which runs approximately 90 miles between Parkfield and Hollister, is a creeping section. Here, the fault slips slowly and continuously at a rate of roughly 0.8 to 1.4 inches (20 to 35 millimeters) per year. This constant, gradual movement releases stress, preventing the build-up of energy necessary to fuel a large earthquake.
Modern scientific monitoring, including GPS arrays and strain gauges, tracks the movement of the plates across the fault. These instruments confirm that the Northern and Southern locked sections are currently accumulating strain at a rate consistent with the long-term plate motion. The accumulated strain in these sections represents the energy that will eventually be released in a future large-magnitude earthquake when the friction holding the plates together is overcome.