The spectrum of tectonic failure modes and earthquake precursors: Insights from rock deformation experiments.

Collettini C., Scuderi M.M., Tinti E.

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IV - Geofisica e fisica dell'ambiente
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Seismic and geodetic observations show that fault slip occurs via a spectrum of behaviors that range from seismic (fast dynamic) to slow (quasi-dynamic) and aseismic (creep). These phenomena have been observed in active tectonic environments and also in areas affected by fluid-injection activities during modern energy production. However, the processes that control fast or slow fault-slip speed are still poorly understood. Here we report on two types of laboratory experiments aimed at the characterization of slow slip phenomena. In the first suite of experiments we show that the full spectrum of slip behavior, from slow to fast stick slip, is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the fault surrounding material. In addition, during the earthquake preparatory phase, accelerated fault creep causes reduction of seismic velocity. In the second suite of experiments we document that during fluid pressure stimulation of a shale-rich fault, upon failure, slip velocity remains slow (i.e., $v \sim 200 \mu$m/s), not approaching dynamic slip rates. We relate this fault slip behavior to the interplay between the fault weakening induced by fluid pressurization and the strong rate-strengthening behavior of shales. Our laboratory experiments provide the opportunity for a step forward in our understanding of earthquake physics based on stick-slip friction. The observed reduction in $P$-wave velocity during the earthquake preparatory phase suggests that if similar mechanisms operate in nature, high-resolution monitoring of the evolution of the elastic properties of faults may be a promising avenue for reliable detection of earthquake precursors. The failure mode of shale-rich faults via accelerated creep might induce stress transfer to adjacent fault patches more prone to generate a seismic instability.

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