The seismic cycle and the difference between foreshocks and aftershocks in a mechanical fault model

We examine the evolution of and the exchange between two forms of elastic energies stored in the quasistatic fault model of Ziv and Rubin [2003]. The first, E_tect, is due to the integrated slip deficit accumulated between the plate boundaries and the fault surface, and the second, E_fault, is the result of differential slip along the fault surface. The results of our analysis reveal cyclic exchange between the two energies. On a E_fault versus E_tect plot, the seismic cycle has a triangular shape with the large earthquakes occurring at the top corner of the triangle (where E_fault is maximum), and the foreshocks and the aftershocks occupying the right side and left side, respectively. While both foreshocks and aftershocks dissipate tectonic energies, the cumulative effect of the foreshocks is to increase the potential elastic energy along the fault plane and the cumulative effect of the aftershocks is to reduce it.