| 15 Oct 2025
High-Speed Infrared Thermography for Measuring Flash Temperatures in Sheared Fault Gouge Analogues
Abstract. Flash temperatures induced by flash heating can lead to thermal softening or decomposition of fault-zone materials at microscopic grain contacts and, consequently, cause a rapid reduction in fault strength during seismic slip. To quantify the efficiency of short-term frictional heating at the contact scales and its impact on the mechanical fault strength, we conducted rotary-shear friction experiments on Ottawa quartz sand “gouges” with variable grain sizes of 250–710 µm at a range of normal stresses of 1–7.5 MPa and slip velocities of 1–50 mm/s under room-dry and wet conditions. We employed a high-speed infrared camera to monitor temperature fluctuations along the outer circumference of the ring-shaped gouge layer during sliding, utilizing a frame rate of up to 1200 Hz with a spatial resolution of 15 µm to capture flash temperature occurring at asperity contacts. We show that flash temperature can be captured within the gouge layer in both room-dry and wet conditions with a peak value up to ~220 °C and ~100 °C, respectively. In addition, the flash temperature increases with increasing slip velocity and grain size, while decreasing at higher normal stress, which is likely associated with enhanced grain size reduction. In our study, we showed that flash temperatures in shearing fault gouges can be constrained using a fast thermal camera. Although difficulties remain in the experimental set-up related to the need to confine the gouge layer and to the evolution of contact size due grain size reductions, the trends in maximum temperatures we observed agree with those predicted from theory.
Review of “High-Speed Infrared Thermography for Measuring Flash Temperatures in Sheared Fault Gouge Analogues” by Hung and Niemeijer,
This is a great paper reporting measurements of flash temperature in simulated quartz gouge using infrared thermography. I am very interested in this topic myself, and have also attempted to measure flash temperature as the authors have done. However, the infrared camera in our lab (InfraTec8800) has a very narrow factory-calibrated temperature range, which was optimized for studying the tempo-spatial evolution of the temperature field during earthquake nucleation in half-meter sample friction experiments. Due to technical restrictions, it is not easy to send the camera back to Germany for recalibration or adjustment. As a result, I eventually had to abandon the attempt. I am thus excited to read through the manuscript. The manuscript is logically structured, with accurate and precise descriptions and well-designed figures. While reading through the Results section, I had a few questions in mind; however, most of them were addressed or discussed later in the Discussion section.
My overall recommendation is thus minor revision, as I have only one major comment and a few minor comments detailed below.
My major comment concerns the authors’ interpretation of the much lower flash temperatures measured in the experiments compared to those predicted by the model. The authors should note that, when measuring the flash temperature from the flank of the gouge layer or from the flank of two grains in contact, the central portion of the contact is not actually exposed to the infrared camera. Due to the large stress concentration at the contact, there is likely to be an extremely steep temperature gradient in its vicinity, so even a small distance from the central portion, the peripheral areas of the contact may have much lower temperatures than at the center. This important limitation should be taken into consideration when interpreting differences in flash temperatures between the experimental measurements and the model predictions.
Two minor comments:
(1) Why was the highest slip velocity used in the experiments limited to 50 mm/s? Was this constraint due to the capabilities of the rotary-shear machine, or was it related to the allowable temperature measurement range of the infrared camera? At normal stresses of a few MPa, gouge layers may exhibit pronounced slip weakening at slip velocities exceeding several hundred mm/s. If an experiment reveals significant weakening concurrent with high flash temperatures (e.g., close to 1000 degC; I believe the weakening temperature Tw could be much lower than the melting temperature of undeformed standard quartz grains), you can have a more in-depth discussion of the flash heating mechanism.
(2) Line 331: Just to confirm: larger dots indicate higher temperature rise in a & c, and later occurrence in b & d, right? Also, is the unit of time on the Y-axis of panels b and d correct?
Lu Yao