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

Review of “High-Speed Infrared Thermography for Measuring Flash Temperatures in Sheared Fault Gouge Analogues” by C. Hung and A. Niemeijer

This is a terrific manuscript focusing on flash heating in gouge using rotary shear experiments and IR temperature data across a wide range of sliding velocities and normal stresses. The manuscript is well-structured and concise, the methodology and modelling are well-conceived, and the figures are well-crafted. My recommendation is for minor revisions with only one major comment and a handful of minor comments detailed below. I look forward to seeing this manuscript published soon.

Major comment:

I think the discussion could benefit by briefly addressing energy budgets in these experiments, and more specifically the partitioning of energy going toward friction vs. fracture and comminution. I agree with many points in the discussion that at higher normal stresses (and sliding velocities), discrepancies between the predicted and measured temperatures could be related to either grain size reduction changing the asperity size and/or IR limitations. I also wonder if more energy is going to fracturing of grains and comminution in your higher normal stress experiments, and if the flash heating model predictions are overestimating the peak temperatures because they typically assume all the work is done by friction. This seems compatible with your measurements of enhanced grain size reductions at higher normal stresses and sliding velocities.

Minor comments:

Line 67 – Regarding the phrase “no visible dynamic weakening was observed”, I’m a little unsure what this line references. I assume it refers to the measured surface temperatures not approaching flash heating breakdown temperatures of ~800 C? If this is the case, can you reword slightly for clarity? This disconnect between the maximum measured temperatures and the flash heating breakdown temperature could also be a function of the 75 mm resolution of the IR camera, which is roughly an order of magnitude larger than typical expectations for asperity sizes.

Figure 1 a & b: is the window 90° from the pore pressure inlet?

Table 1: I’m not sure exactly what number of flashes means. I presume this is the number of distinct contacts measured via IR in the window during experiments. Would that be correct? From lines 261-264 later, this seems to be the case. It could be helpful to define flashes here when introducing Table 1.

Figure 5 a: There seems to be some evolution or hysteresis regarding the minimum measured temperatures in the 1 MPa experiment. Was there a temporary decrease in the sliding velocity or a hold around 400 mm of displacement?

Figure 7 b: There are 3 spots with times close to 50s that are very hard to notice unless you look very closely, and 3 more I was unable to locate (I could only find 37 out of 40). A thin black line around these spots could perhaps help the reader see these spots more clearly. A scale bar for occurrence time (a,c) or temperature rise (b,d) could also be helpful here.

Figure 9 Line 414: Should this say “…sufficient spatial nor temporal resolution” instead of and/or? Assuming I followed correctly, the region above the red box has sufficient temporal resolution but not spatial resolution, and to the right of the red box you have sufficient spatial resolution but not sufficient temporal resolution, while the red region has neither the spatial nor temporal resolutions necessary to image contacts?

-Monica Barbery