Event

Earthquake science is in the midst of a revolution. Our understanding of tectonic faulting has been shaken to the core by the discoveries of seismic tremor, low frequency earthquakes, slow slip events, and other modes of fault slip. These phenomena represent modes of failure that were thought to be non-existent and theoretically impossible only a few years ago.  Despite the growing number of observations of slow earthquakes and the fact that they can trigger catastrophic large earthquakes their origin remains unresolved.  Here I focus on two of our recent works to illuminate the physics of slow earthquakes (Leeman et al., 2016; Scuderi et al., 2016).

In the work by Scuderi et al. (2016) we monitor the evolution of elastic wave speed during the laboratory seismic cycle of failure and reloading. Such temporal changes in seismic velocity have the potential to illuminate physical processes associated with fault weakening and connections between the range of fault slip behaviors including slow earthquakes, tremor and low frequency earthquakes.  We show that precursory changes of wave speed occur in laboratory faults for the complete spectrum of failure modes observed for tectonic faults. We find systematic variations of elastic properties during the seismic cycle for both slow and fast earthquakes indicating similar physical mechanisms during rupture nucleation. Our data show that accelerated fault creep causes reduction of seismic velocity and elastic moduli during the preparatory phase preceding failure, which suggests that real time monitoring of active faults may be a means to detect earthquake precursors.

In the work by Leeman et al. (2016) we document the complete spectrum of stick-slip behaviors, with systematic measurements of repetitive, stick-slip from transient slow slip to fast stick-slip. We find that slow slip occurs near the threshold between stable and unstable failure, controlled by the interplay of fault zone frictional properties, including rate dependence of the critical rheologic friction parameter Kc, and elastic stiffness of the surrounding rock. Our results suggest that slow earthquakes and transient fault slip behaviors can arise from the same governing frictional dynamics as normal earthquakes. This mechanism could act in concert with other mechanisms that have been proposed for slow earthquakes, including fault zone dilatancy, and is consistent with the broad range of geologic environments where slow earthquakes are observed. An unresolved issue with this explanation of slow earthquakes, based on the stability transition, involves the critical patch size for nucleation of dynamic rupture.  I discuss this issue and how it could be resolved with fault zone heterogeneity or fracture energy dissipation.