Fault surface morphology as an indicator for earthquake nucleation potential

Abstract

Laboratory measurements can determine the potential for geologic materials to generate unstable (seismic) slip, but a direct relation between sliding behavior in the laboratory and physical characteristics observable in the field is lacking, especially for the phyllosilicate-rich gouges that are widely observed in natural faults. We integrated laboratory friction experiments with surface topography microscopy and demonstrated a quantitative correlation between frictional slip behavior and fault surface morphology of centimeter-scale samples. Our results show that striated, smooth fault surfaces were formed in experiments that exhibited stable sliding, whereas potentially unstable sliding was associated with rougher, isotropic fault surfaces. We interpret that frictional stability and fault surface morphology are linked via the evolution of asperity contacts on localized slip surfaces. If fault surface roughness obeys a fractal relationship over a large range of length scales, then we infer that the morphological characteristics observed in the laboratory could indicate the earthquake nucleation potential on natural fault surfaces.

How Fault‐Normal and Shear‐Parallel Stiffness Influence Frictional Sliding Behavior

“…Most experiments show slip instabilities during the last millimeter of sliding at 10 μm/s (Figure 5). Strain dependence of sliding instabilities is often attributed to microstructural changes with increasing displacement (Scuderi, Marone, et al., 2016; Shreedharan et al., 2019), however previous data using the same device and material show limited changes in microstructure and frictional parameters with increasing slip (Eijsink et al., 2022). Out of all possible combinations of fault‐parallel stiffness k S and normal stiffness k N , there are only two experiments that show stable sliding with no slip events according to our definition; for k S = 14 MPa/mm, and k N = 4.1 and k N = 223 MPa/mm.…”

“…From our experiments, we use the first sequence of velocity increases (5-8 mm sample displacement) to analyze the velocity-dependent behavior of the material, which in most experiments is complicated in the second sequence (8-11 mm sample displacement) by the occurrence of stick-slip behavior and large stress drops directly after a velocity step (Figure 3A). Frictional behavior can change with displacement, although from a previous study using the same material and type of apparatus, we have shown that the variation in a-b and D c is small after the initial run-in phase (Eijsink et al, 2022). We use the RSFit3000 Matlab script (Skarbek & Savage, 2019) to find the optimal fit for the rate-and-state friction parameters.…”

“…The quartz powder was mixed with deionized water and placed into the sample holder cell, which consists of two steel plates with a 25.4 mm diameter cylindrical void in the middle, where the sample material is compressed between two porous sintered‐steel frits. The sample and holder cell are loaded into a Giesa RS5 single‐direct shear device (see Ikari, 2019; Eijsink et al., 2022 for more details), and normal load is applied to the sample using a servo‐controlled steel vertical ram (Figure 2). After the normal stress is applied, the sample is left to compact for 12–16 hr, after which the compaction rate is negligible.…”

“…The measured friction coefficients, modeled rate‐and‐state friction parameters and characteristics of all analyzed slip events shown in this manuscript can be found as Eijsink and Ikari (2023).…”

Characteristics of slow slip events explained by rate-strengthening faults subject to periodic pore fluid pressure changes

“…We consider uniform friction properties on the fault, where a = 0.005 and b = 0.004 (a/b = 1.25); these values are within the range (10 −4 to 10 −2 ) obtained in friction experiments on incoming sediments to the Hikurangi margin (e.g., Boulton et al, 2019;Eijsink & Ikari, 2022;. These experiments show that frictional stability trends span rate-strengthening, rate-neutral, and rate-weakening behaviors.…”