A section of a fault that has not ruptured in a long time is called a seismic gap. The term describes a stretch of an active earthquake zone where relatively little seismic activity has occurred compared with neighbouring sections.
Seismologists study seismic gaps because a locked segment may be accumulating strain as tectonic plates continue to move. If that strain is released suddenly, the segment can produce a damaging earthquake. The idea became especially important in earthquake forecasting and hazard assessment, particularly along long subduction zones and major continental faults.
A seismic gap is not a guaranteed prediction. A quiet section may reflect incomplete historical records, slow deformation, aseismic slip or a fault segment that is not currently ready to rupture. “Asperity” is different: it means a particularly strong, locked patch on a fault that resists sliding. “Fault creep” describes gradual movement without a large earthquake, while “seismic shadow” generally refers to an area of reduced seismic activity after or around an earthquake. These terms can overlap in discussion but are not synonyms.