Earthquake ruptures and volcanic eruptions are the most dramatic manifestations of the dynamic failure of a critically stressed crust. However, these are actually very rare events in both space and time, and most of the crust spends most of its time in a highly stressed but subcritical state. Under upper crustal conditions, most rocks accommodate applied stresses in a brittle manner through cracking, fracturing and faulting. Such cracks can grow at all scales from the grain scale to the crustal scale, and under different stress regimes. Under tensile stress, single, long cracks tend to grow at the expense of shorter ones. By contrast, under all-round compression deformation of rocks in the brittle field proceeds by the progressive growth and coalescence of many microcracks. Under nominally dry environmental conditions and rapid loading, crack growth is primarily governed by the applied stresses. Indeed, laboratory experiments show that rock strength is essentially time independent under such conditions (Paterson & Wong 2005). At the microscopic scale, time independence means that the crack growth criterion is well modeled by the concept of constant fracture toughness, i.e., a critical stress intensity factor at microcrack tips.

However, at lower strain rates and in the presence of aqueous fluids, i.e., under more realistic upper crustal conditions, experiments show that brittle rock deformation becomes time-dependent. This is well demonstrated by the fact that rocks can fail by static fatigue, following a preceding period of creep deformation, under conditions of constant applied stress (e.g., Scholz 1968; Kranz 1980; Baud & Meredith 1997; Heap et al. 2009; Brantut et al. 2013). Time-dependent crack growth arises from chemically activated subcritical crack growth processes, such as stress corrosion reactions (see Atkinson 1984; Atkinson & Meredith 1987). This process allows deformation of the crust to proceed over a wide range of strain rates, from the very low rates associated with tectonic loading up to the very high rates associated with earthquake rupture or impact events. Overall, cracking in the crust can therefore occur over a spatial scale spanning some 12 orders of magnitude, and a temporal scale spanning some 18 orders of magnitude. Establishing quantitative links between microscopic, grain-scale subcritical cracking and macroscopic, sample-scale to crustal scale brittle creep behavior is a key challenge for our understanding of the time-dependent mechanics of the Earth's crust.

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