Final answer:
A city without a recent earthquake over an active fault is likely over a seismic gap, a region on a fault that is locked and where stress is accumulating. The longer the interval between earthquakes, the greater the accumulated stress and potential energy for a future earthquake.
Step-by-step explanation:
A city that is located over an active fault, but has not experienced an earthquake for a long period of time, is most likely located over a seismic gap. A seismic gap refers to a section of a fault that has had relatively few earthquakes, often because it is locked, allowing stress to build up over time. This suggests that the city might be at a higher risk for future seismic activity due to the accumulated stress.
Since the motion along fault zones is not smooth and continuous, stresses build up in the crust, which are eventually released in sudden, violent slippages that cause earthquakes. The longer the interval between such seismic events, the greater the stress accumulation and the more energy that can be released in a future earthquake. It is crucial to use sensitive instruments and conduct regular geologic assessments to monitor the region for potential seismic risk, as shown by the use of instruments in the Los Angeles basin to detect distortion and contraction caused by building pressures beneath the surface.