Experimental demonstration of a semi-brittle origin for crustal strain transients

Abstract

Tectonic motions give rise to destructive earthquakes and transient slip events. These movements are often described by friction laws for stick-slip motion on brittle fault surfaces and gouge-filled zones 1,2 . Yet, many transient slip events, such as slow earthquakes and aseismic creep, occur in rocks that exhibit mixed brittle-ductile rheology, where these friction laws are not clearly applicable 3,4 . Here we describe the flow and evolution of fractures as observed in a semi-brittle rock analogue exposed to shear stress in laboratory experiments. We find that, depending on the strength of the rock-analogue material, and thus the magnitude of yield stress, the material exhibits either creep-like or stick-slip behaviour. At low yield stress, deformation occurs as constant creep along a main fracture, whereas at high yield stress, the material exhibits stick-slip behaviour. However, the deformation does not involve frictional behaviour; it is instead accommodated by the initiation and growth of a system of tensional and shear fractures. The opening and interplay of such fracture systems could generate tectonic tremor and slow slip. Our laboratory experiments thus support a frictionless alternative mechanism for the development of tectonic strain transients.Semi-brittle deformation in geologic systems is generally defined micromechanically 5-7 , wherein fracturing and/or frictional sliding characterizes the deformation of one mineral phase while another flows through viscous deformation mechanisms. In the resulting meso-scale deformation, rather than accommodate strain via a single, brittle shear fracture or continuous, ductile creep, there is a combination of both brittle and ductile deformation. Field observations of semi-brittle rocks show that for a range of composition, temperature and pressure, the formation of fluid-filled brittle fractures and veins accompanies localized ductile flow 3,4,8 . At the scale of the lithosphere, the transition from brittle to ductile deformation is highly temperature-sensitive, and thus such semibrittle deformation is particularly important at a depth interval in the lithosphere near the base of the seismogenic zone referred to as the brittle-to-ductile transition (BDT; refs 6,7). A wide variety of slip behaviours have been geophysically resolved to originate in the BDT, and observations and physical experiments of semibrittle materials show that the coexistence of brittle and viscous behaviour gives emergence to some of the characteristics of strain transients and slow slip events 9-11 . However, most workers explain differences in slip transients with laws that describe how fluid pressure, temperature changes and friction interact 12 . Here, we explore an alternative mechanism where strain transients ranging from stick-slip to creep can be explained by the co-occurrence of brittle and ductile deformation.Semi-brittle deformation has been studied in a variety of rock mechanics experiments 5,13 . Although constraining flow laws under laboratory conditions, these experiments are...

Analogue earthquakes and seismic cycles: experimental modelling across timescales

“…Another polymeric viscoelastic material that has found application in analogue modelling of earthquakes by means of deformable slider-spring models is Carbopol ® (Reber et al, 2015). As with gelatine, Carbopol ® rheology depends on its concentration but also additionally on pH.…”

“…While elasticity is still controlled by a separate spring, the frictional element can be replaced by different plastic rheologies, e.g. a Bingham fluid (Reber et al, 2015).…”

“…feldspar vs. quartz). In the experiments by Reber et al (2015), viscoelastic material is used to induce both creep and fracture. The observation of slip transients at various speeds in such experiments may be equivalent to tremors and slow slip phenomena (Peng and Gomberg, 2010).…”

“…man et al 2016;Kaproth and Marone, 2013). The deformable slider-spring set-up of Reber et al (2015) is used to study transients in the brittle-ductile regime. The latter is defined as a two-mineral-phase regime where one phase deforms in a brittle manner, while the other is ductile (e.g.…”

Investigation of Wing Crack Formation With a Combined Phase-Field and Experimental Approach

“…We mixed the gelatin powder (Great Lakes Gelatin, USA, Type A, 255 Bloom, 40 Mesh) at 3 wt % with deionized water at 80 ∘ C. Before using the gelatin layers we cooled them to 10 ∘ C. The experiments were performed on a modified version of the experimental apparatus used in Reber et al [2015]. It has a split bottom where one side is stationary and the other moves at a constant speed of 0.1 mm s (Figure 3).…”