Fault rock heterogeneity can produce fault weakness and reduce fault stability

The mechanical behavior of faults is a critical area of study in earthquake science. A key factor influencing this behavior is the geological heterogeneity present within fault zones. While the presence of heterogeneity is widely acknowledged, its precise role in controlling fault strength and stability remains poorly understood. This research addresses this gap by investigating the impact of small-scale heterogeneity on fault mechanics. Understanding this impact is crucial because it has significant implications for predicting earthquake behavior. The occurrence of slow-slip events versus earthquake ruptures, and the characteristics of the resulting events, are directly influenced by the strength and stability of faults. This study employs laboratory experiments to isolate and quantify the effects of heterogeneity, thus offering a more refined understanding of the complex interactions within fault zones. The results provide valuable insights into the factors governing fault strength and the transition between stable and unstable slip.

Previous research has highlighted the importance of fault zone heterogeneity in controlling fault behavior. However, most studies have focused on large-scale heterogeneities, overlooking the potential impact of small-scale variations in material properties. While the presence of different materials within a fault zone is well-documented, the precise mechanisms by which these heterogeneities affect fault strength and stability are still debated. Some studies have suggested that large-scale heterogeneities are responsible for the observed spectrum of slip behavior on natural faults. This study builds upon previous experimental and numerical modeling work on rate-and-state friction, which has established the fundamental relationship between frictional properties and fault stability. The rate-and-state framework provides a basis for understanding the evolution of fault behavior under various loading conditions. However, the majority of this work has focused on homogeneous fault systems, neglecting the influence of material heterogeneity.

The researchers conducted laboratory friction experiments using a direct-shear arrangement within a triaxial deformation apparatus. Gouge layers, approximately 1.3 mm thick, were prepared with either heterogeneous patches or a homogeneous mixture of quartz and clay. The heterogeneous layers consisted of patches of strong, rate-weakening quartz gouge and weak, rate-strengthening clay gouge. The direct-shear arrangement included soft silicone spacers to accommodate displacement without load support, and grooves were cut into the sliding blocks to minimize boundary shear. A low-friction PTFE sleeve minimized jacket friction. The jacketed arrangement was placed within a pressure vessel, allowing for the application of normal stress through confining pressure and the introduction of pore-fluid pressure via porous disks. Experiments involved applying controlled shear displacement and measuring the resulting frictional forces. The stability-controlling rate-and-state friction parameter (a-b) was analyzed to assess the impact of heterogeneity on fault stability. The experiments systematically varied the clay fraction in both homogeneous and heterogeneous gouge layers, allowing for a quantitative comparison of their strength and stability characteristics.

The experiments revealed that laterally heterogeneous faults exhibited significantly reduced strength and frictional stability compared to homogeneous faults with the same overall composition. The (a-b) parameter, which quantifies fault stability, consistently showed lower values for heterogeneous faults, indicating reduced stability. This reduction in stability was observed across various clay fractions and displacement levels. The researchers identified three primary weakening mechanisms resulting from heterogeneity: 1) smearing of the weak clay gouge, which reduces its effective strength; 2) differential compaction of the quartz and clay gouges, redistributing normal stress and favoring slip along the weaker clay; and 3) shear localization within the strong quartz patches, concentrating stress and promoting instability. The degree of instability was sensitive to the proportion of rate-weakening material. When the rate-weakening material comprised less than 70% of the layer, the heterogeneous faults remained stable, although closer to rate-neutral behavior than their homogeneous counterparts. Only when the strong rate-weakening patches comprised ≥80% of the layer did stick-slip instabilities occur. The findings highlight the significant impact of small-scale heterogeneities, which are abundant in natural fault zones, on fault strength and stability.

The results of this study demonstrate that small-scale heterogeneity within fault zones exerts a significant control on fault strength and stability. This has substantial implications for understanding the mechanisms governing earthquake ruptures versus aseismic slip. The observed weakening effects—smearing, differential compaction, and shear localization—provide a mechanistic explanation for the reduced strength and stability of heterogeneous faults. The findings highlight the need to incorporate the effects of small-scale heterogeneity into models of earthquake source processes. The commonly-used assumption of homogeneity may significantly underestimate the complexity and variability of fault behavior in natural systems. The results show similarities between the behavior observed in these small-scale heterogeneous experiments and the behavior believed to be controlled by large-scale heterogeneities in natural faults. Further research is needed to better understand the interplay between the scale of heterogeneity and its impact on fault behavior.

This study conclusively demonstrates that introducing simple heterogeneity into a fault zone substantially reduces fault strength and decreases stability compared to homogeneous gouges. The abundance and complexity of heterogeneity in natural faults strongly suggest that interactions between heterogeneously distributed materials significantly influence whether faults experience aseismic creep or earthquake slip. The challenges posed by smaller-scale heterogeneities in earthquake modeling necessitates further laboratory experiments and modeling to better understand and incorporate these effects into larger-scale constitutive laws for accurately predicting fault processes.

The study focused on a simplified representation of fault zone heterogeneity, using two distinct materials (quartz and clay) with contrasting frictional properties. Natural fault zones are far more complex, exhibiting a wider range of materials and microstructures. The experiments were conducted under controlled laboratory conditions, which may not perfectly replicate the complex stress and temperature conditions found in natural fault zones. The limited size of the experimental setup could also restrict the scaling to larger natural faults. Despite these limitations, the findings offer valuable insights into the fundamental mechanisms governing the impact of heterogeneity on fault behavior.