The effects of temporal changes of fault zone material properties on earthquake cycles
It was usually assumed that stress heterogeneities along the fault control earthquake behaviors, but material heterogeneities, such as fault damage zones associated with localized deformation, also influence when and how earthquakes occur. In this talk I will present results from 2-D fully dynamic earthquake cycle simulations to discuss how the styles of earthquake nucleation and rupture propagation change as fault zone material properties vary temporally. First, I will talk about the influence of fault zone structural maturity quantified by near-fault seismic wave velocities in simulations. The simulations show that immature fault zones promote small and moderate subsurface earthquakes with irregular recurrence intervals, whereas mature fault zones host pulse-like earthquake rupture that can propagate to the surface, extend throughout the seismogenic zone, and occur at regular intervals. The interseismic healing in immature fault zones plays a key role in allowing the development of aseismic slip episodes including slow-slip events and creep, which can propagate into the seismogenic zone and thus limit the sizes of subsequent earthquakes by releasing fault stress. In the second part, I will discuss how the precursory changes of seismic wave velocities of fault damage zones may affect earthquake nucleation process. Both laboratory experiments and seismic observations show that the abrupt earthquake failure can be preceded by accelerated fault deformation, which is accompanied by seismic velocity reductions of near-fault rocks. By systematically testing the effects of such precursory velocity changes, the simulations demonstrate that precursory velocity reductions can significantly reduce earthquake nucleation size, accelerate the nucleation of earthquakes, and shorten earthquake recurrence intervals.