The movement of Earth’s crust is a constant process driven by the slow but relentless forces of plate tectonics. Where two tectonic plates meet, the fractures, known as faults, accommodate this motion. While the most dramatic and widely known form of fault movement is the sudden, violent release of accumulated energy in an earthquake, not all geological shifts result in catastrophic shaking. Some faults move in a far more subtle manner, slipping past one another in a slow, steady motion. This gradual displacement, which occurs without the damaging seismic waves associated with an earthquake, is known as fault creep.
Defining Fault Creep
Fault creep is defined as the measurable surface displacement along a fault that occurs without the generation of notable earthquakes. This process is categorized as “aseismic slip,” meaning it happens without the release of significant seismic energy. The movement is typically slow and continuous, though it can sometimes occur in short, episodic bursts lasting minutes to days. Fault creep rates range from a few millimeters up to a few centimeters per year.
For example, the creeping segment of the San Andreas Fault moves at a rate of approximately 28 to 33 millimeters annually. This continuous motion prevents the excessive buildup of stress that would otherwise be released in a large, destructive event. The continuous nature of fault creep serves to gradually accommodate the long-term tectonic motion.
Creep Versus Stick-Slip Earthquakes
The fundamental difference between fault creep and a typical earthquake lies in the mechanism of energy release. Most earthquakes are characterized by “stick-slip” behavior. This occurs when friction locks the two sides of a fault together, causing tectonic forces to gradually accumulate strain energy in the surrounding rock, similar to slowly stretching a rubber band. When the accumulated stress overcomes the frictional resistance, the fault abruptly slips, releasing the stored energy as seismic waves that cause ground shaking.
Fault creep, however, is a form of continuous or stable sliding. The fault surfaces slide past one another at a rate roughly equivalent to the rate of tectonic loading. This motion means that little elastic strain energy is stored in the rock surrounding the fault. Since the stress is relieved constantly, the potential for a large, stick-slip rupture is significantly reduced along creeping segments.
Geological Conditions and Locations of Creep
The presence of specific geological materials within the fault zone is why some faults creep while others remain locked. Creeping faults often contain weak, low-friction substances, such as clay-rich gouge or the metamorphic rock serpentinite. These materials reduce the shear strength of the fault, allowing the blocks to slide past each other more easily without building up substantial stress. This composition facilitates plastic deformation—smooth, stable sliding—rather than the brittle failure associated with an earthquake.
Creep tends to be confined to the shallow parts of the crust, typically extending only a few kilometers deep before transitioning into a locked zone. The most widely known examples occur along California’s San Andreas Fault system. A central segment of the San Andreas Fault, between Parkfield and San Juan Bautista, is famous for its steady creep. The Hayward Fault in the San Francisco Bay Area is another prominent example, where surface creep rates can reach up to 8 millimeters per year.
Monitoring and Infrastructure Effects
Geologists employ several methods to measure this displacement.
Monitoring Methods
- Creepmeters are installed directly across the fault trace to continuously record minute changes in distance between two fixed points.
- Space-based techniques, such as the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR), provide broader, regional measurements of crustal movement.
- Traditional ground surveys using alignment arrays and theodolites track the progressive offset of fixed monuments over time.
While fault creep does not generate destructive shaking, its continuous movement poses a long-term threat to human-built infrastructure. Structures that cross the fault zone, such as roads, sidewalks, pipelines, fences, and building foundations, are gradually bent, broken, and offset. For instance, the California Memorial Stadium, built across the Hayward Fault, requires costly, ongoing repairs due to measurable displacement. This persistent, gradual damage necessitates expensive and repeated maintenance to realign and repair structures that are slowly being pulled apart or compressed by the creeping fault.