M3 Seismic Cycle

Topic

The seismic cycle describes how elastic energy accumulates and is released within an active fault zone. During the interseismic phase—which can last anywhere from decades to millennia—tectonic plates continue to move while the fault remains locked by friction; this friction causes elastic deformation to build up in the surrounding rock, a deformation that can be measured via satellite positioning as a velocity gradient around the fault itself.

When the shear stress acting on the fault exceeds its frictional resistance, coseismic rupture occurs, releasing all the energy accumulated during the interseismic phase in mere seconds or minutes in the form of seismic waves.

An elastic rebound model, well-established for over a century, serves as the conceptual basis for this entire seismic cycle: the lithosphere undergoes elastic loading during the interseismic phase and then rebounds elastically during the earthquake, abruptly releasing the accumulated deformation. The earthquake's magnitude is determined by its seismic moment—the product of rock rigidity, rupture area, and the average slip produced by the rupture.

Following the earthquake, the postseismic phase combines additional slow slip along the fault itself with the viscoelastic relaxation of the underlying mantle; both processes remain detectable via satellite positioning for years or even decades after the main earthquake. Earthquake recurrence follows, on the one hand, the same magnitude distribution described for earthquakes in general, and on the other, renewal models that describe when a characteristic earthquake is likely to recur on a specific fault segment: some of these models assume that recurrence is completely random and independent of the time elapsed since the last earthquake, whereas others assume the existence of a preferred recurrence interval, albeit with some variability around that interval.

The expected behavior of a complete seismic cycle includes an acceleration of slip just prior to the main rupture—during the earthquake nucleation phase—and a long sequence of aftershocks that decays over time according to the same empirical relationship described for earthquakes in general, thereby completing the cycle of elastic energy accumulation and release with which this description began.

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