SCEC Award Number 25063 View PDF
Proposal Category Individual Research Project (Single Investigator / Institution)
Proposal Title Experimental Investigation of Coseismic Fault Re-strengthening
Investigator(s)
Name Organization
Xiaofeng Chen Oklahoma State University
SCEC Milestones B1-1 SCEC Groups FARM
Report Due Date 03/15/2026 Date Report Submitted 03/14/2026
Project Abstract
 Tectonic stress accumulation and release are fundamental to earthquake cycles. While the majority of fault shear stress is generated during interseismic periods, coseismic fault stress recovery has received little attention. Furthermore, although dynamic weakening is critical for earthquake initiation and has been extensively investigated, dynamic restrengthening remains poorly understood. Building on our previous observations (SCEC#24113), this study specifically aims to assess coseismic shear stress recovery. I employ double-direct shear coupled with high-resolution mechanical data acquisition. A total of 29 stick-slip experiments were conducted on Westerly granite and 22 on Sioux quartzite under normal stresses ranging from 3 to 24 MPa, with peak slip velocities up to 0.63 m/s and 0.58 m/s for granite and quartzite, respectively. Both lithologies exhibit strong velocity dependence of friction at peak slip velocity, with granite faults showing slightly higher strength. Granite demonstrates a consistent coseismic velocity-strengthening trend across the entire normal stress range, with strengthening recovering approximately 50–80% of the preceding coseismic weakening during early slip. In contrast, quartzite exhibits a broader strengthening range of 5–70%. Granite faults display a symmetric velocity evolution with comparable acceleration and deceleration phases, whereas quartzite faults exhibit a Yoffe-type velocity profile characterized by rapid acceleration followed by prolonged deceleration. Microstructural analyses reveal intense gouge pulverization and evidence of frictional melting during stick-slip events. These results demonstrate that fault composition exerts a strong control on coseismic frictional behavior and that coseismic strengthening is a widespread feature in laboratory faults.
Intellectual Merit Our results demonstrate that coseismic strengthening is a widespread feature in laboratory faults. Despite identical loading conditions (double-direct shear) and only minor mineralogical differences between the siliceous lithologies tested, fault composition exerts a strong control on coseismic frictional behavior. Granite faults exhibit higher overall friction, more pronounced and consistent coseismic restrengthening, and a more symmetric slip velocity evolution. In contrast, quartzite faults show lower friction, more variable restrengthening ratios, and a Yoffe-type velocity profile that more closely resembles those inferred for natural earthquakes.
Broader Impacts This project provided hands-on training for two graduate students in MATLAB-based experimental data analysis. Beyond technical contributions, they participated in the interpretation of experimental findings, fostering critical thinking and research independence. The skills developed through this work directly enhance their ongoing research on induced seismicity and carbonate rock fracturing.
Project Participants Dr. Fred Chester at Texas A&M University facilitated access to the double-direct shear apparatus. Two graduate students at Oklahoma State University contributed to the analysis of the experimental data.
Exemplary Figure Figure 2. (a) Coseismic shear stress recovery versus maximum dynamic weakening during stick–slip events. Westerly granite shows a narrow recovery range of ~50–80%, whereas Sioux quartzite exhibits a broader range of ~5–70%. (b) Friction coefficient at peak slip velocity plotted against peak velocity for quartzite (blue circles) and granite (green diamonds). Both lithologies display strong velocity-dependent friction, with greater scatter at velocities <0.1 m/s and pronounced velocity weakening at >0.1 m/s. Granite consistently exhibits higher friction coefficients than quartzite. Best-fit relationships and corresponding equations are shown for each lithology.
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