The San Andreas Fault (SAF) is a major geological feature in western North America, defining a significant part of the boundary between two massive tectonic plates. Geologically, the San Andreas Fault is classified as a strike-slip fault, characterized by blocks of the Earth’s crust moving primarily in a horizontal direction past one another. The fault operates as a transform plate boundary, which explains the large-scale mechanism driving this lateral movement.
Classifying the San Andreas Fault
The classification of a fault depends on the direction of movement along the fracture plane, which is the surface separating the moving rock masses. Geologists recognize three primary types of faults based on this movement: normal, reverse, and strike-slip. Normal faults occur where the crust is being pulled apart, causing one block to slip downward relative to the other in a process known as extension. Reverse faults, or thrust faults, are the result of compression, where the crust is squeezed, forcing one block upward over the other.
The San Andreas Fault falls into the strike-slip category, where motion is almost entirely horizontal and parallel to the fault trace. Instead of one side moving up or down, the two blocks slide past each other, driven by shearing forces. This movement means that locations on opposite sides of the fault are slowly being carried away from each other along the horizontal plane.
The fault plane itself is nearly vertical, which is typical for a strike-slip fault, allowing the horizontal shearing motion to dominate. The San Andreas is an extensively studied example of this fault type, extending for approximately 750 miles through California.
The Transform Plate Boundary Setting
The classification of the San Andreas Fault as a strike-slip type is a description of its mechanical motion, but the larger context is defined by its role as a transform plate boundary. A transform boundary is specifically where two plates slide horizontally past one another. The San Andreas Fault accommodates the relative motion between the Pacific Plate and the North American Plate.
The Pacific Plate, which lies to the west of the fault, is moving in a general northwesterly direction relative to the North American Plate on the east. This lateral, side-by-side motion is the fundamental reason the fault exhibits strike-slip movement.
The fault system is not a single, continuous break but a zone of complex, crushed, and broken rock that can be up to a mile wide in places. This intricate network of fractures and smaller faults within the broader zone collectively handles the displacement between the two major plates.
Understanding Right-Lateral Movement
The San Andreas Fault is further defined as a right-lateral strike-slip fault, which specifies the precise direction of the horizontal movement. Right-lateral, or dextral, motion means that if an observer stands on one side of the fault and looks across to the other side, the block on the far side appears to move to the right. For the San Andreas, the Pacific Plate moves northwest relative to the North American Plate, resulting in this right-lateral displacement along the fault trace.
This constant, relative motion does not occur as a smooth, continuous slide everywhere along the fault’s length. Instead, the movement occurs at an average rate ranging from about 30 to 50 millimeters (1.2 to 2 inches) per year. This seemingly slow pace, accumulating over millions of years, has resulted in the total offset of geological features by hundreds of miles.
Geological evidence, such as the offset of ancient stream beds, provides a measure of this long-term slip rate. This slow, persistent movement is the underlying cause of the strain that leads to seismic events.
Geological Features and Seismic Risk
The right-lateral, horizontal movement of the San Andreas Fault creates distinct geological features visible on the surface. Features such as offset stream channels, where a river or stream flowing across the fault has been bent or shifted, provide clear visual evidence of past movement. Other landforms like sag ponds (small depressions formed where the ground has subsided) and linear fault scarps (subtle ridges or cliffs) also mark the path of the fault.
The seismic risk associated with the fault arises from the fact that friction prevents the two sides from sliding smoothly past each other in most segments. Instead, stress slowly builds up in the crust until the accumulated strain exceeds the strength of the rock, leading to a sudden, rapid slip known as an earthquake. The earthquakes produced by the San Andreas Fault are typically shallow and intense, as the rupture occurs close to the surface.
In some segments, particularly the central section, the fault exhibits a phenomenon known as aseismic creep. In these areas, the fault moves continuously and slowly, which releases stress without generating large, damaging earthquakes. However, the northern and southern sections are considered “locked,” meaning they build up tremendous strain over long periods before releasing it in major events, such as the great earthquakes of 1857 and 1906.