Direct Observation of Faulting by Means of Rotary Shear Tests Under X‐Ray Micro‐Computed Tomography

Abstract

Friction and fault evolution are critical aspects in earthquake studies as they directly influence the nucleation, propagation, and arrest of earthquake ruptures. We present the results of a recently developed experimental approach that investigates these important aspects using a combination of rotary shear testing and X‐ray micro‐computed tomography technology. Two sets of experiments at normal stresses (σn) of 2.5 and 1.8 MPa were conducted on synthetic laboratory faults. We identified real contact areas (Ac) on the fault surfaces and estimated sizes of contact patches by means of micro‐computed tomography image analysis. The number of contact patches and their sizes showed positive correlations with σn, and contact patch size distributions followed power law relations. The total number of contact patches decreased with increasing slip distance, and large contact patches endured longer slip distance than small ones. Secondary off‐fault fractures created by interlocking and breakdown of large contact patches were closely related to the sudden drops of frictional resistance, suggesting the dominant role of surface roughness on shear behavior especially at low stress.

Assessing Energy Budget of Laboratory Fault Slip Using Rotary Shear Experiments and Micro‐Computed Tomography

“…The noncontacting surface area on the shear surface, A nc can be calculated from the nominal contact area ( A n ) and A c using where the multiplier 2 accounts for the fact that we are considering top and bottom sides of the noncontacting surface. The real contact area ( A c ) is calculated following the method in Zhao et al ().…”

“…10.1029/2019GL084787 (Zhao et al, 2018). In this study, we improve our comprehension of the data collected in Zhao et al (2018) by quantitatively assessing the energy budget of the laboratory fault slips.…”

“…where the multiplier 2 accounts for the fact that we are considering top and bottom sides of the noncontacting surface. The real contact area (A c ) is calculated following the method in Zhao et al (2018).…”

“…Over the last several decades, X‐ray micro‐computed tomography (micro‐CT) has been increasingly used to investigate microscopic structures of geomaterials (e.g., Ketcham & Carlson, ; Renard et al, ; Tatone & Grasselli, ), and the state‐of‐art in this research area evolved from examining post‐mortem specimen (i.e., remove tested specimen from the testing facility and scan) to in situ and operando observations (e.g., Renard et al, ; Tisato et al, ; Zhao, ; Zhao et al, ; Zhao et al, ). The recently developed ERDμ‐T rotary shear testing facility that is paired with X‐ray micro‐CT allows observations of the gradual variation within the tested sample after each incremental slip step (Zhao et al, ).…”

“…The recently developed ERDμ‐T rotary shear testing facility that is paired with X‐ray micro‐CT allows observations of the gradual variation within the tested sample after each incremental slip step (Zhao et al, ). This apparatus has been used to conduct a series of laboratory fault slip tests and gain intriguing insights of the gradual fault evolution and development of off‐fault fractures (Zhao et al, ). In this study, we improve our comprehension of the data collected in Zhao et al () by quantitatively assessing the energy budget of the laboratory fault slips.…”