Off‐Fault Damage Characterization During and After Experimental Quasi‐Static and Dynamic Rupture in Crustal Rock From Laboratory P Wave Tomography and Microstructures

Abstract

Elastic strain energy released during shear failure in rock is partially spent as fracture energy Γ to propagate the rupture further. Γ is dissipated within the rupture tip process zone, and includes energy dissipated as off‐fault damage, Γoff. Quantifying off‐fault damage formed during rupture is crucial to understand its effect on rupture dynamics and slip‐weakening processes behind the rupture tip, and its contribution to seismic radiation. Here, we quantify Γoff and associated change in off‐fault mechanical properties during and after quasi‐static and dynamic rupture. We do so by performing dynamic and quasi‐static shear failure experiments on intact Lanhélin granite under triaxial conditions. We quantify the change in elastic moduli around the fault from time‐resolved 3‐D P wave velocity tomography obtained during and after failure. We measure the off‐fault microfracture damage after failure. From the tomography, we observe a localized maximum 25% drop in P wave velocity around the shear failure interface for both quasi‐static and dynamic failure. Microfracture density data reveal a damage zone width of around 10 mm after quasi‐static failure, and 20 mm after dynamic failure. Microfracture densities obtained from P wave velocity tomography models using an effective medium approach are in good agreement with the measured off‐fault microfracture damage. Γoff obtained from off‐fault microfracture measurements is around 3 kJ m2 for quasi‐static rupture, and 5.5 kJ m2 for dynamic rupture. We argue that rupture velocity determines damage zone width for slip up to a few mm, and that shear fracture energy Γ increases with increasing rupture velocity.

Dilatancy stabilises shear failure in rock

“…The four experiments yield best fits for δ c that vary between 1.1 mm to 1.4 mm (Table 1), similar to values measured for the slip-weakening distance of granite (Aben et al, 2020) (Figure 1). We use this to quantify the transition from stable to dynamic failure in our experiments, which occurs when the loading stiffness of the machine k (see Method section for computation of k) is lower than the critical stiffness of the fault k cr .…”

“…We use the spring-slider model and our experimental data in an inverse problem to estimate fault zone storage S f w and cohesion-weakening distance δ c for each experiment that exhibited stable shear failure (see Methods section). δ c in the model is akin to the slip-weakening distance measured in triaxial (controlled) rupture experiments on intact granite (Figure 1, sample WG6 in Table 1) (e.g., Wong, 1982;Lockner et al, 1991;Aben et al, 2020). However, δ c in the model depends on the definition of cohesion-weakening function and may vary from the measured slip-weakening distances.…”

“…Doing so for the experimental data provides values for W b at stable shear failure of 83 kJm −2 (sample WG12), 84 kJm −2 (sample WG5), and 100 kJm −2 (sample WG4) at 40 MPa effective pressure, and 118 kJm −2 at 80 MPa effective pressure (sample WG8). The minimum breakdown work necessary to form a fault zone and reach the residual frictional strength of a rock (also known as shear fracture energy) may be measured from a quasi-static or "controlled" shear failure experiment (e.g., Lockner et al, 1991;Aben et al, 2020). We performed such experiments on a notched, thermally cracked Westerly granite samples at 40 MPa effective pressure, and obtain W b = 60 kJm −2 .…”

A Travel‐Time Path Calibration Strategy for Back‐Projection of Large Earthquakes and Its Application and Validation Through the Segmented Super‐Shear Rupture Imaging of the 2002 Mw 7.9 Denali Earthquake

“…Segments 2 and 3 show many more aftershocks and are featured with asymmetric distribution, with more aftershocks located to the southwest of the DF (the extensional side of the rupture), suggesting that more off‐fault structures were activated there. Numerical simulations (e.g., Okubo et al., 2019) and laboratory experiments (e.g., Aben et al., 2020; Griffith et al., 2009) also show that the dynamic stress mainly generate off‐fault damages (mainly opening, Mode‐I cracks) in the extensional side of a mode II (i.e., strike‐slip) rupture. Compared with the aftershocks around segment 3, in both relative and absolute senses, more aftershocks around segment 2 are located to the northeast of the fault (the compressional stress quadrant of the rupture).…”

Energy Partitioning, Dynamic Fragmentation, and Off‐Fault Damage in the Earthquake Source Volume

“…Even fewer geological studies have estimated (Fulton et al., 2013 updated by Brodsky et al., 2020; Matsumoto et al., 2001; Pittarello et al., 2008; Reches & Dewers, 2005), the results also spanning a range of . High‐strain‐rate experimental estimates of and/or are equally sparse (e.g., Aben et al., 2016, 2020; Barber & Griffith, 2017; Doan & Billi, 2011; Liu & Zhao, 2021).…”