What's the difference between an INACTIVE fault and an ACTIVE fault? An Earthquake. Most of the last Bigger ones illustrate this principal. Longer the seismic gap the bigger the shock on some faults, that aren't completely dormant and left on the side of the crustal conveyer. Some are active often some not.
We haven't a long record to draw from in most cases and IF the period of re-occurrence on most faults was 300 years or more then we wouldn't see results much different than we've been getting. They'd say most faults were inactive until a quake told them different. Sound familiar?...mb.