Aseismic creep is slow ground movement that releases tectonic strain without producing a large earthquake.
Along some faults, the two sides gradually slide past one another rather than remaining locked until a sudden rupture. This motion can occur over months or years and may be measured with surveying instruments, creepmeters, or satellite-based positioning.
The San Andreas Fault provides a well-known example, especially in parts of central California. Creep does not mean that a fault is harmless: a fault can contain creeping sections and locked sections at the same time, and locked sections may still accumulate strain capable of producing damaging earthquakes.
Aseismic creep is sometimes confused with an earthquake swarm. A swarm consists of many earthquakes clustered in time and place, whereas creep is primarily gradual fault motion that may occur with little seismic shaking.