In the previous two chapters we discussed the statics and dynamics of faulting by treating the fault as an isolated system. We now place the fault into the tectonic engine and consider its behavior when coupled to the loading system. Observational results and models are combined to determine the nature of this loading system, and we explore the nature of the seismic cycle.

Historical

G. K. Gilbert understood that slip on faults must accrue through a repetition of earthquakes, and his writings show that he devoted considerable thought to just how such repetition must occur. In Gilbert (1909), he wrote both of rhythm in the recurrence of earthquakes, and of alternation, by which he meant that earthquakes alternate in position along major seismic zones – anticipating the concept of the seismic gap, as it is phrased today.

The credit for a full conceptualization of the loading cycle, however, is usually given to H. F. Reid. In summarizing the mechanism of the California earthquake of 1906, Reid (1910) presented his elastic rebound theory in which the earthquake was the result of a sudden relaxation of elastic strains through rupture along the San Andreas fault. The causative strains, according to his theory, were accumulated over a long period of time by the steady motion of the regions on either side of the fault, which under normal conditions remains locked by friction.