Feldspar reduces fault frictional healing rate under hydrothermal conditions

Domain: earth_science
Published: 2025-12-23
Editorial outcome: minor_revision

Abstract
Fault restrengthening after earthquakes enables country rocks to restore elastic strain energy for subsequent seismic cycles. It originates from the contribution of several processes resulting in fault sealing and fault frictional healing, the latter arising from contact area growth and formation of new chemical bonds between grains. Laboratory studies demonstrate that fault frictional healing typically increases logarithmically with time during quasi-stationary contact (i.e., positive healing rate). Here we show that this logarithmic healing does not hold under certain (hydrothermal) conditions for feldspar, the most abundant mineral in the Earth’s crust, and for common feldspar-rich rocks. The healing rate switches from positive to negative with increasing quasi-stationary contact time at temperatures ≥ 200 °C in the presence of pressurized water. This unusual healing behavior suggests that thermally-activated friction weakening processes may exist at feldspar interfaces. This hypothesis is further validated through nano-scale friction experiments by sliding single-feldspar crystals on a feldspar substrate. Given the abundance of feldspars in the Earth’s crust, their peculiar healing behavior under hydrothermal conditions can lead to unexpected variations in fault strength, changing our understanding of fault mechanics and the seismic cycle, and challenging the direct extrapolation of laboratory-derived healing parameters to natural earthquake recurrence intervals.

Introduction
Fault healing is a well-documented phenomenon that describes the time-dependent recovery of fault strength between earthquakes1–6. Fault healing is due to the contribution of several physical and chemical processes that lead to (1) fault sealing (mineral dissolution and precipitation, vein deposition, etc., and in extreme cases, fault welding by solidification of seismic friction melts)7–10, (2) fault frictional healing (contact area growth plus formation of new chemical bonds between asperity contacts at the sub-microscale)5,11–16 and, (3) a combination of the two17. The underlying mechanism of fault frictional healing is generally attributed to an increase in the real contact area of the microscopic asperities (contact “quantity”)18,19 or an increase in contact strength of chemical bonds (contact “quality”)15,20. Although specific mechanisms differ, laboratory studies on centimeter-size rock specimens, mostly conducted at room temperature, and micro-scale single asperity friction tests have shown that 1) frictional healing (static friction minus dynamic steady-state friction, Fig. 1) usually increases linearly with the logarithm of the hold time th, and 2) the rate of healing (change of frictional healing with log(th)) is positive for a broad range of rock types, including Westerly granite, and a variety of minerals11,14–17,21–25. However, the results of recent experiments also conducted on Westerly granite, but under hydrothermal conditions, cast doubt on the occurrence of this log-linear frictional behavior: at T = 200 °C, frictional healing decreases, rather than increases, with log(th) (i.e., negative healing rate), although the healing is still positive within the investigated hold times. But, at T = 250 °C, the frictional healing becomes negative when hold time is longer than a threshold26. Consequently, faults in granitoids in the presence of hot and pressurized fluids, a common situation at hypocentral depths4,27, may have negative healing rates, which will alter the seismic cycle. But do other common crustal rocks have similar frictional healing behavior under hydrothermal conditions? And what is the mechanism for the negative healing rate?

In this work, we measure the frictional healing behavior of a wide range of common crustal rocks at temperatures typical of hypocentral depths. We show that the negative healing rate behavior is closely related to the presence of feldspar and water. The key role played by feldspar in the process of decreasing fault frictional healing rate is confirmed by nanometer-scale friction experiments with a single feldspar micro-grain translating along a feldspar substrate using an atomic force microscope. We conclude that thermally-activated and water-assisted chemical processes operating along feldspar interfaces limit the rate of contact strengthening, thereby reducing fault frictional healing.

Results
Frictional healing Δμ is normally investigated by laboratory slide-hold-slide (SHS) experiments11,19,22. Here we present results of 30 SHS experiments performed in the presence of hot and pressurized water using a rotary shear machine (LHV-Beijing) equipped with a dedicated hydrothermal pressure vessel28–30 (Supplementary Fig. 1 and Supplementary Table 1).

We first report frictional healing data for powders of gabbro, a common oceanic crust rock. Experiments are performed under an effective normal stress (σeff = σn - Pp) of 50 MPa, a loading velocity of 10 μm/s, temperature (T) ranging from 25 to 400 °C and pore fluid pressure (Pp) of 0 (nominally dry, relative humidity < 2% at T ≥ 100 °C) or 30 MPa. Each experiment includes two identical SHS sequences (SHS-seq1 and SHS-seq2) with a slip interval of 40 mm in between (see the representative experiment LHV2760 in Supplementary Fig. 2). The SHS procedure involves: (1) sliding at V = 10 μm/s for 1 mm of slip displacement, (2) holding at zero load-point velocity for hold times (th) ranging from 3 s to 10,000 s, and (3) resuming the sliding at the original velocity (V = 10 μm/s). Figure 1a shows the evolution of the friction coefficient (μ = τ/σeff, with τ the shear stress) with slip displacement (d ~ 52-59.5 mm) in the SHS-seq2 for experiments performed at temperatures of 100 °C to 400 °C. At these loading and temperature conditions the rheology of gabbro and its forming minerals is elasto-frictional31. Typically, frictional healing Δμ is measured as the difference between peak μ value (μp) upon re-sliding and steady state friction (μss) before the hold. However, in the cases of stick-slip, a well-defined μss cannot be obtained. We adopt the peak friction during the stick phase before the hold as a reference point. The apparent Δμ is then measured as the difference between the peak μ value of the stick-slip events after and before the holds (see Methods; referred to as Δμpp, Fig. 1a inset). The rate of healing (β) is conventionally determined as the slope of Δμ versus Δlog10(th) over the entire dataset, expressed as β = Δμ/Δlog10(th)19. Since some of the datasets in our study cannot be adequately described by a single log-linear relation, we determine individual healing rates (βloc) within intervals of monotonic change to explicitly capture the time-dependent evolution of β. Typically, two βloc values are sufficient to describe the change of Δμ with hold time. A positive βloc indicates that frictional healing increases with hold time, whereas a negative βloc indicates that frictional healing decreases with hold time.

Gabbro shows different healing behaviors in the presence or absence of pressurized water. Here we focus on the frictional healing behavior of SHS-seq2 (results of SHS-seq1 can be found in Supplementary Fig. 3). Under dry conditions, β is positive and Δμ generally increases monotonically with log10th except for a slight deviation from the log-linear trend at T = 300 °C and 400 °C at longer hold times (≥ 1000 s) (Fig. 1b). For the experiments in the presence of water (Pp = 30 MPa), at T = 25 °C and 100 °C, Δμ increases linearly with log10th, as observed under dry conditions. However, at T ≥ 200 °C, the linear relation between apparent Δμ and log10th does not hold. Specifically, at 200 °C apparent Δμ starts to decrease with increasing th when th > 300 s, indicating that frictional healing rate has switched from positive (βloc > 0) to negative (βloc < 0) values. For the test at T = 400 °C, the negative transition of frictional healing rate (βloc < 0) occurs at th = 100 s. Most strikingly, the peak μ value after the 3000 s hold is lower than the sliding μ before the hold, suggesting a negative value of apparent Δμ (Fig. 1c).

The transition in healing rates from positive to negative with increasing hold time apparently results from a reduction in (apparent) Δμ at longer hold times. In order to check whether or not the decrease in frictional healing at long hold times depends on the order in which the hold time is applied, we performed experiment LHV3047 at T = 200 °C by applying an increasing (3–3000 s) and decreasing (3000–3 s) order of hold time sequence. We find that both sequences produce a very similar pattern of healing behavior, as shown in Supplementary Fig. 4. This observation demonstrates that the decrease in Δμ at longer th is independent of hold sequence, and depends solely on the absolute duration of individual hold prior to the re-sliding.

X-ray diffraction (XRD) analysis of gabbro indicate that its mineralogical composition consists mainly of 47 wt% andesine and 28 wt% diopside (Supplementary Table 2, see Methods). To investigate whether the unusual frictional healing behavior of gabbro is sensitive to its mineral composition, we performed similar SHS experiments but on pure andesine and diopside gouges. These gouges are prepared by selecting the two minerals from gabbro gouges under the optical microscope. We find that pure diopside gouges have a linear relationship between Δμ and log10th under all investigated conditions (positive β, Fig. 2a). In contrast, pure andesine and a mixture of andesine and diopside with a ratio of 1.68:1 (or 47/28, same as in gabbro) show the same pattern of healing at 200 °C, including the occurrence of negative β after long th, as that of gabbro (Fig. 2b, c). These experiments indicate that the presence of andesine is responsible for the decreased Δμ and the negative β at long holds in the presence of pressurized water. To further demonstrate the role of temperature in the negative frictional healing rate, we conducted an additional SHS experiment LHV3240 on pure andesine gouge. The experiment includes three temperature sequences: two room temperature stages separated by a T = 200 °C stage. We observe the behavior of negative healing rate only in the T = 200 °C stage, not in the initial and final room temperature stages (Supplementary Fig. 5).

If andesine indeed accounts for the reduction in healing rate at elevated temperature conditions, we would expect this effect to occur in other types of feldspar and feldspar-rich rocks. To test this hypothesis, we present the fault frictional healing data for various feldspars, i.e., albite (Na-rich), orthoclase (K-rich) and andesine (Na-Ca rich) and feldspar-rich rocks (Etna basalt: 34% andesine + 11.3% sanidine, diorite: 49% albite, and Westerly granite: 35% albite + 19% microcline). As shown in Fig. 3, all types of feldspar and feldspar-rich rocks investigated exhibit the same time-dependent, negative frictional healing rate (i.e., frictional healing rate transiting from positive to negative) after long holds at temperature above 200 °C to 300 °C, in contrast to constant, positive healing rate at lower temperatures (Supplementary Fig. 6). Mechanical data from these common (non-altered) crustal rocks further support the critical role of feldspar in producing negative frictional healing rates at long holds.

Methods
Hydrothermal friction experiments and slide-hold-slide tests
The sample powders used in the experiments include “Jinan dark green” gabbro, Westerly granite, Etna basalt, diorite, and orthoclase and albite. The mineralogical makeup are presented in Supplementary Table 2, based on XRD data. The samples are crushed and sieved using a 180-mesh sieve to produce gouges with grain size < 88 μm for the friction experiments.

Friction experiments are performed with a Low to High Velocity Rotary shear apparatus (LHVR-Beijing) equipped with a hydrothermal pressure vessel at the Institute of Geology, China Earthquake Administration in Beijing, China (Supplementary Fig. 1)28–30. All experiments are conducted at a constant effective normal stress (σeff) of 50 MPa and temperature (T) ranging from 25 to 400 °C, under nominally “dry” or with a pore fluid pressure (Pp) of 30 MPa. In the dry experiments, the vessel is open to the atmosphere. The Pp = 30 MPa experiments are conducted under drained conditions (distilled water as pore fluid) and constant pore pressure is maintained by a syringe ISCO pump. A list of all the experiments can be found in Supplementary Table 1.

In each experiment, a 1.5 mm thick gouge layer is sandwiched between two ring-shaped and surface-grooved holders and flanked by inner and outer metal rings (for 22/28 mm and 14/20 mm internal/external diameter, ~0.8 g and 0.56 g gouges are used, respectively). The sample assembly is subsequently mounted inside the hydrothermal vessel and to the machine. Then, the sample assembly and the vessel are evacuated to 2000 Pa with a vacuum pump. Afterward, the normal stress σn is slowly increased to the desired value, while the pore fluid pressure (Pp) is increased in a stepped manner and kept at a constant target value before heating. Once the desired pore pressure and normal stress are achieved, the vessel is heated to target temperature. Heating normally takes 30 min for T = 100 °C and ~1.5 h for T = 400 °C.

After achieving the desired experimental conditions, the gouge layer is sheared (run-in) at V = 10 μm/s for 7 mm slip displacement (d), allowing friction to evolve to a steady state value. After this “run-in” stage, slide-hold-slide tests are imposed. The SHS test consists of three steps: (1) sliding the gouge layer at V = 10 μm/s for 1 mm of slip displacement, (2) holding the gouge for hold times (th) ranging from 3 s to 30,000 s by setting the load point velocity as null and, (3) resuming the sliding velocity at the original rate (V = 10 μm/s). Upon reloading, the friction coefficient typically reaches a peak value and then decays to a steady state value. Frictional healing Δμ is defined as the difference between μ peak value (μp) upon re-sliding and steady state friction (μss) before the hold. For stable sliding, μss is calculated as the average values of μ (with standard error) measured over the last 0.2 mm of displacement before the hold. In the cases of stick-slip, a μss counterpart is determined by averaging peak μ values (with standard error) during the stick phases within the last 0.2 mm of displacement (normally 6–8 cycles). It is worth noting that the measurement of Δμ in the case of stick slip is not affected by where the hold happens to start in the loading curve (Supplementary Fig. 11). Normal stress is kept constant during the holds and frictional strength relaxes non-linearly due to the creep of the machine and gouge samples. To ensure that no rotation is applied by the machine to the gouge layer during holds, the servo motor control system is set to stop once the imposed revolution rate is less than 1 rpm (corresponding to a slip velocity ~ 1.3 μm/s for the sample assembly used). By doing so, the servo motor remains stopped even if zero-voltage input signal is disturbed by electric noise, and no angular rotation is detected during the holds. In each experiment, two SHS sequences separated by a slip displacement of 40 mm are imposed. At the end of the experiment, the deformed samples are collected for micro-analytical (XRPD and FESEM) investigations. Note that the samples required a cooling period of approximately 1 h for T = 200 °C before being removed, this process might have potentially modified the microstructures.

During the experiments, axial load, shear torque, fluid pressure and temperature, velocity, revolution, and axial displacement are acquire data frequency of 100 Hz. Evolution of axial displacement, i.e., dilatation or shortening (Δw) of the gouge layer, is recorded using a LVDT (1 μm resolution and 10 mm stroke) placed on the axial column of the machine. No correction for system stiffness is applied for the calculation of the shear displacement. The measured shear torque is converted to shear stress (MPa) without correcting for the additional shear resistance from O-rings and confining rings.

Atomic force microscopy and slide-hold slide tests
Sample preparation. Feldspar/Pyroxene tips are fabricated from tipless atomic force microscopy (AFM) probes and mineral particles. The feldspar/pyroxene particles, separated from the samples used in rotary shear experiments, are attached to tipless AFM probes (HQ-NSC35 Mikromasch) using UV (ultraviolet) glue. The selected feldspar and pyroxene particles have a diameter of several tens of micrometers. The solid gabbro substrate is polished by Argon Ion Polishing, producing a smooth surface with a root-mean-square roughness of 3.0 nm, measured within an area of 4 × 4 μm2. The gabbro substrate is cleaned with air plasma beam before each experiment.

AFM experiments. Friction force microscopy is conducted using an AFM (Ntegra, NT-MDT Inc.) installed at the Tsinghua University, China. The experiments are conducted in an AFM chamber at approximately 80% humidity. The humidity is kept constant (uncertainty ± 2% RH) controlled by bubbling pure dry nitrogen vapor from a liquid nitrogen dewar through water. The experimental system is equilibrated for at least 1 h after the initial humidity change.

The friction experiments are performed after initially making contact between the feldspar/pyroxene tip and the substrate, then sliding for a run-in distance (256 cycles of 1 μm back-and-forth lateral displacements). The slide-hold-slide procedure is then conducted at a constant normal load of 150 nN. The tip is first slid 1 μm, then held stationary for a set time, followed by sliding another 1 μm at the original velocity. After this, the tip is slid backward to the original position. The friction was determined by calculating the difference between the forward and backward lateral forces. The hold time is predetermined ranging from 3 s to 700 s. Each hold time sequence is repeated 10 times. To exclude the potential effect caused by different interfacial chemical states, the tip is slid for a pre-run distance before each different hold time.

The healing of friction is described as normalized friction force increment, called the relative friction drop ΔF/Fss, which is the ratio of the increment of friction force during the hold stage (ΔF) and the steady-state friction (Fss) after the hold. To estimate the uncertainty of ΔF/Fss for each hold event, we first determined the peak force value (Fp) and subtracted from it the individual force values (Fi) during post-hold steady-state sliding, thereby obtaining a dataset of ΔFi = Fp - Fi. Based on the ΔF and Fss datasets, we then calculated the mean value (ΔF and Fss) and standard deviation (σΔF and σFss) of ΔF and Fss, respectively. Finally, the standard deviation of ΔF/Fss was estimated using the error propagation formula:

σR = (ΔF/Fss) * sqrt((σΔF/ΔF)^2 + (σFss/Fss)^2) (1)

X-ray powder diffraction (XRPD)
Measurements are performed using a Philips X’Pert Pro MPD diffractometer installed at the Department of Geosciences at the University of Padova, Italy. The instrument is equipped with a long-fine-focus cobalt anode tube working at 40 kV–40 mA and a 240 mm goniometer radius that operates in the θ/θ geometry. Samples are prepared using the front-loading procedure onto a Si-crystal sample holder that lacks any diffraction lines (zero-background). Measurements for phase identification are carried out between 3° and 85° 2θ angle, using a 0.017° step size, counting 100 s per virtual step on a spinning sample (1 revolution per second).

Field Emission Scanning Electron Microscope (FESEM)
Measurements are conducted using a Tescan 468 Solaris Field-Emission SEM of the Department of Geosciences at the University of Padova. Backscattered electron images have been acquired with an in-beam mid-angle backscattered detector using an accelerating voltage of 5 KeV, current of 300 pA and a working distance of 3 or 4 mm.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This paper presents a set of experimental results from laboratory friction tests using slide-hold-slide protocols to measure the time dependent frictional strengthening, or frictional healing, using gouges of feldspar, feldspar-bearing rocks, and some other minerals for comparison. The experiments are conducted in rotary shear at temperatures up to 400 °C, 50 MPa effective stress and 30 MPa pore pressure to explore mid-crustal conditions. The premise of the paper is important and in line with a recent increased focus on frictional healing within the community. I don’t see any problems procedurally or with the dataset itself. I agree that the data are quite intriguing, however, I do see some pretty serious problems with the presentation and interpretation of the results.\n\nMain Comments\nMy main comment/concern here regards the definition of terms and the concept of “weakening” as presented here. Throughout the text, “frictional healing” and “healing rate” are used interchangeably. This is incorrect, because they are two distinct quantities. The “frictional healing” is the increase in friction during the hold, or Δµ. The frictional healing rate is the healing as a function of hold time, or Δµ/Δlog10(t). What the authors call here ”weakening” is really a reduction in healing RATE, but there is no actual weakening because the Δµ values are still positive. In other words, you may get less healing for a longer hold time, but it is a lower but still positive value of healing. To call something weakening, you need the absolute value of friction to be LOWER after the hold.\n\nNow, there are actually a few values of negative Δµ reported here, which would be actual weakening. Two such values are shown in Figure 1C, for 400 °C. The raw data curve for these holds is shown in Figure 1A. The data is obscured by stick-slip (more on this later), but for the two holds for which negative Δµ is reported, the peaks in the stick-slips are clearly higher after both the 1000 and 3000 second holds. I do not think there is any way these two holds should be interpreted as “weakening”, and the issue likely lies with trying to pick a Δµ value on data that is overprinted by stick-slip. There are also 4 data points in Figure 3 that are reported as negative Δµ, these may have the same problem but I cannot check these without the raw data curves.\n\nA related problem is that a main point in this paper seems to be that specifically feldspar at hydrothermal conditions (> 200 °C, 30 MPa pore pressure) cause weakening. Again, there is only a reduction in the rate of frictional healing, so saying feldspar causes weakening is incorrect and not consistent with the data. For example, in Figure 1A I take it that the values in gray are the background values of friction for the four experiments. If so, the experiments under hydrothermal conditions are clearly stronger – the 400 °C experiment is the strongest. In the text on the AFM measurements, a point is made that the feldspar data is scattered and therefore show a “suppressed healing” or “little to negligible healing” (Line 202, 213), but a closer look at Figure 4B shows that most of the data points for the middle plot (the feldspar-feldspar contact) are actually larger values than in the other two plots – the figure shows that the feldspar heals the most!\n\nTo summarize the point here, the data here do not show actual weakening. They show, at most, a reduction in healing rate at certain conditions. The main points of the text (including title) are therefore inaccurate or incorrect and will have to be revised to reflect this.\n\nMy second concern regards the stick-slips that appear in the data. These are interesting in there own right, and maybe something may come out of analyzing these data. However, they obscure the healing and I do not think that you can obtain an accurate measure of healing in a material that is stick-slipping. The Δµ measurement requires an initial steady state friction value, which is clearly never the case for a stick-slipping material. The cyclic nature of stick-slips induces inaccuracies in the measurement, which is illustrated in the lower inset of Figure 1; the Δµ value is clearly affected by where in the loading curve the hold happens to start. Using the stick-slip peaks cannot be substituted for a steady-state friction rate, because you do not know what the steady state value would be without the stick-slips.\n\nAnother point related to the stick-slips, is that they in themselves indicate some degree of healing. Assuming repetitive stick-slips are laboratory repeating earthquakes, healing needs to occur in order for the stick-slips to repeat in the laboratory. Going back to the 400 °C experiment in Figure 1, there is no way that the 1000 3000 s holds can be classified as “weakening” or have negative healing, because the stick-slips indicate that healing has to be occurring.\n\nThe AFM measurements are a good idea, however I am not sure that they support the rotary experiments. The authors have clearly attributed the lower healing rates to feldspar under hydrothermal conditions. It is unclear to me how the AFM measurements under room temperature and humid air are controlled by the same mechanisms. The authors then speculate that the process is “thermally activated, water assisted and chemical”, which sounds like pressure solution to me. Is this even realistic in the AFM measurements conducted here?\n\nSpecific Comments:\nLine 44: healing *usually* increases log-linearly – there are healing studies showing examples where this is not the case\nLine 46: what is meant by “minerals”?\nLine 67: please explain the experiment protocols for the SHS sequences here\nLine 75-76: this is the “standard” definition of healing rate that is usually applied to materials with a constant healing rate, in other word materials where multiple healing peaks can be fit with a single log-linear regression. Since your data deviates from this, it may be advisable to use a “local” or “individual” healing rate, or a β calculated between two individual hold times.\nLine 91: Δµ does not appear to always increase monotonically in this figure, there looks like a clear rollover for the 300 and 400 °C experiments at the longer hold times.\nLine 132: it would be helpful here to provide a brief classification (K, Na, Al) of the feldspars chosen here.\nLine 240-247: as in the main comments, I do not see time-dependent weakening in this study, and the data do not indicate reduced stress levels.\nLine 247-249: This text on spectrum of slip modes comes out of nowhere and has no context, I suggest deleting.\nLines 262-264: this text also comes out of nowhere and rate-and-state friction is not introduced in the text, I suggest deleting this as well.", "reviewer_2": "The manuscript discusses an interesting study regarding the evolution of friction strength in different materials at elevated temperatures. The observation of frictional weakening under geothermal conditions is still new and these results help to validate the very limited previously published work. Additionally, these results come from a rotary shear apparatus showing that time-dependent frictional weakening doesn’t not depend on measurement apparatus. The project is well designed and interesting. A significant amount of work went into verify the behavior and isolating factors that could influence the observed weakening. The results are potentially very significant with relevance to earthquakes and geothermal systems. However, there are details that are either missing or need clarification, figures need to be reviewd for accuracy, and there are some grammatical improvements needed.\n\nThe AFM results are interesting but did raise some questions regarding uncertainties associated with this type of measurement. Admittedly I am not very familiar with AFM methods, but my understanding is that friction determined with AFM is based on comparison of forces measured in both the forward and reverse directions as the lateral force depends on if you are moving up or down a slope. I gather from the manuscript that the change in force reported is based on measurements in one direction. I would think that surface roughness and location on the surface could then have a significant impact on the measured healing. How did the roughness of the different mineral substrates compare? Also how did the tips evolve during the tests? Did the pyroxene or feldspar tip experience more wear? How would this affect your measurement.\n\nI would like to see some discussion of uncertainty in the measurements. There are never any error bars or uncertainty estimates in the figures or text, but we know there is uncertainty associated with measurements. I think this is especially important as the authors acknowledge system stiffness and seal friction are not accounted for.\n\nLine 25: The use of “particularly” here implies that negative friction healing rates were observed in some tests at temperatures ≥200 °C when pressurized water was not present. However, supplementary figure 3 shows the results of the dry test and negative healing rate are not observed at any temperature.\nLine 43 and 46: remove excess punctuation\nLine 46: “… including Westerly granite and [a variety of] minerals.”\nLine 51: “… may have negative healing rates which impact on the seismic cycle.” Wording issue here – doesn’t make sense\nLine 60: Experiments aren’t being performed in the manuscript, but the results are being presented.\nLine 103: We already know that (cid:1)μ depends on the duration of the hold period prior to resliding, I think you mean to specify that depends only on the prior hold and the dependent on previous history of healing.\nFigure 3: This is a nice figure presenting initial work looking at the weakening of multiple materials.\nLine 164: Were the samples used for FESEM removed from the experimental apparatus following an extended hold period or immediately after completion of a sliding period? Secondary minerals maybe easier to see if examined at the end of a hold period while they would likely be very disturbed in difficult to see in a sample taken at the conclusion of a sliding period.\nLine 200 & Figure 4b: Supplementary figure 10b shows the highest value of F/Fss (~ 4.8) for the 100 s hold but Figure 4b only shows values less than 4 for the 80% humidity case. Why is does the supplementary data not match the data presented in the main text?\nLine 206: wording here is awkward, the suppressed frictional healing is not observed at 5% humidity\nLine 208- 210: not clear what you mean by “with certain statistics” I found no discussion of statistical analysis in the text.\nLine 222: if contact area does not change for holds less than 300s what is causing the initial strengthening?\nLine 243-249: If the fault surface is becoming weaker with time the elastic strain energy stored would also be low because it would be creeping under constant stress loading. In this case we would not expect earthquakes to occur at all let alone with a shorter recurrence interval. What is written here contradicts line 14-15.\nLine 251: Not sure why you bring up the 40 wt% feldspar here as the composition required for negative healing. The presented data in figure 3 examined feldspar content down to around 40 wt% and all wt% exhibited weakening so nothing can be said based on the data available about the amount needed to observed weakening.\nLine 252: “… behavior in common crustal rocks…”\nLine 391-392: I think there are some words missing. Sentence doesn’t quite make sense.\nLine 395: was displacement dependence discussed?\nExtended data figure 7 – I recommend plotting using the same scale for easier comparison. Also, in caption the statement is made “these data show a great consistency, proving the reliability of the results obtained with the machine used.” I think you intended to indicate that the results are independent of the machine used.", "reviewer_3": "This manuscript presents new slide-hold-slide experiments to investigate “rate and state friction” sliding of feldspars and feldspar-bearing rocks under hydrothermal conditions that are very common in the Earth’s subsurface (including conditions of active seismogenic fault zones), and on feldspar individual contacts at different values of relative humidity. The results presented are new and exciting, and this study deserves close attention by those who study frictional properties and their significance for the earthquake cycle. It is well placed in a high-profile journal such as Nature Communications. These novel results are closely linked to the current hydrothermal conditions of the experiments, and it is not even clear to me that I should use the terms “rate and state friction” given how different the results are from the large data set of frictional properties of rocks at varying rates and slide-hold-slide, which are mostly restricted to ambient lab conditions.\n\n”Rate and state friction” of rocks at room temperature and relative dry atmospheric conditions typically shows a direct effect (during which frictional resistance to shear correlates with sliding rate) and an evolution effect, that is related to changes in strength that occur during hold stages of slide-hold-slide experiments. Typically, frictional sliding stability and, by inference, fault sliding stability are governed by whether the evolution effect changes the frictional resistance to sliding more or less than the direct effect. More generally, as pointed out by the authors of this manuscript, the evolution of frictional strength in slide-hold-slide experiments is marked by an increase in frictional resistance with hold time. This increase in frictional resistance is sometimes interpreted to result from increases in true area of contact of the frictional surfaces with hold time, as normal stress across the frictional surfaces and local stress concentrations lead to plastic (or other) deformation of the rough surface of the adjacent rocks. Alternatively, increases in frictional resistance with hold time have been interpreted to represent increasing strengths of contacts, as might happen due to formation of van der Waals (or other chemical bonds) at points of contact. All of these characteristic frictional responses are apparent for feldspar, diopside, and feldspar-bearing rocks under room temperature and/or dry conditions shown in Figures 1a, 1b, 1c, 2a. The single mineral sliding experiments show similar (typical) frictional sliding results for feldspars at low relative humidity and pyroxene of increasing strengths with hold time in Fig. 4b (top and bottom).\n\nThe new results of this study, which are so interesting, are the apparent frictional sliding reductions ascribed to reductions in frictional healing (where changes in friction coefficients show negative slopes with hold time (Figs 1c, 2b, 2c, 3, and 4 (middle)). The authors point out that the reductions in frictional healing correspond to the presence of feldspar at points of contact and water at high temperature and pressure. They also suggest that the loss of frictional healing is thermally activated, water-assisted and chemical in origin. I am in broad agreement with this interpretation, and wonder, in fact, if this isn’t good evidence for the role of pressure solution to frictional properties. In particular, it seems that dry surfaces in contact under compressive normal forces must show increasing contact areas and/or bond strengths of contacts, unless a process such as pressure solution dissolves the solid (feldspar) contacts, reducing real areas of contact (relative to the dry behavior) or replaces bonds across loaded solid contacts by nm-scale fluid films. That is, the results suggest that areas of contact are reduced with hold time (relative to healing under dry conditions) or that contacts have reduced strengths with hold time (relative to healing under dry conditions), contrary to most micro-contact models of frictional sliding. And yet, the hydrothermal conditions of these experiments are probably much more representative of conditions on real faults than the many room temperature, dry experiments on rocks.\n\nThe lack of evidence for product minerals due to reaction at feldspar surfaces by XRD and SEM is not surprising, as the amounts of reaction products and likely nm-scale clays and other new phases will be difficult to detect by these methods. It is worth trying, of course, but powder XRD typically requires %-level concentrations minerals and feldspar reaction to clays and components in solution may be very slow (thus leading to only small new growths). I wonder if Raman spectroscopy of feldspar surfaces might allow detection of nm-scale films or precipitates. I am not suggesting that this is necessary for publication but if the authors have access to a Raman lab, they may wish to confirm existence of clays on their clay gouge particles.\n\nWhatever the microscopic mechanisms, this behavior has major implications for long-term fault strength, fault stability, and the aseismic periods of the earthquake cycle. I recommend that this manuscript be accepted for publication in Nature Communications.\n\nThis manuscript is well written and my editorial suggestions are minor. I would like to suggest that the authors choose better headings for this manuscript. In particular, I note the heading “Main” rather than something like “Introduction” or “Prior Studies of Fault Healing”. However, most of my editorial suggestions are just minor rewording; I will give these line-by-line:\nLine 29 – I suggest starting this line with “crystals on a feldspar substrate. …”\nLine 30 – I suggest “… variations in fault strength, chaing our”\nLine 37 – “etc., and in extreme cases, fault welding …”\nLine 38 – “… between asperity contacts at the sub-“\nLines 39-40 – “… and, (3) a combination of the two11. The underlying mechanism of fault frictional healing is generally attributed to …”\nLine 51 – “… healing rates which will alter the seismic”\nLine 52 – delete “here”\nLine 53 – “rate” rather than “rates”\nLines 54-55 – “we measured the frictional haling behavior of a wide range of common crustal rocks at temperatures of hypocentral depth. We concluded …”\nLine 56 – “… feldspar in the process of decreasing fault”\nLines 57-58 – “… nanometer-scale friction experiments with a single feldspar micro-grain translating along a feldspar substrate …”\nLine 64 – “We first report frictional healing data …”\nLine 69 – “… the evolution of the friction coefficient …”\nLine 74 – “… between the peak   value of the stick-slip …”\nLine 78 – “… test and result of gabbro under hydrothermal”\nLine 81 – “… Hold times (th) are”\nLine 82 – what is meant by “bottom”? Do you mean the low stress value? Please reword.\nLine 95 – “… indicating that   has”\nLine 100 – “performed” rather than “perform”\nLine 102 – “… Extended Data (Figure 4). This observation”\nLine 106 – “performed” rather than “perform”\nLine 111 – delete “both”\nLine 115 – “healing rate, we conducted an additional SHS experiment LHR3240 on pure andesine gouge. The”\nLine 120 – “… rock-forming minerals of gabbro. …”\nLine 123 – “… at all temperatures investigated …”\nLine 135 “… time-dependent, negative …”\nLine 144 – “feldspars investigated (hollow symbols)… “\nLine 154 – “… water (Figures 1-3). Thus, the negative …”\nLine 155 – “… impact the seismic cycle. This frictional”\nLine 161 – “A similar negative   value has been reported recently for granite …,”\nLine 162 – delete “hypothetically”\nLine 171 – “formed clay at original silicate mineral surfaces may contribut to the negative healing … “\nLine 172 – “Time-dependent fault …”\nLines 182- 183 – “long holds for felspar-rich rocks we must conclude that the frictional strength   a decreases during long holds.”\nLine 186 – “difficult to determine the shear strength of individual nano- to micro-contacts from the overall”\nLine 189-190 – “(AFM) and an individual mineral grain, either feldspar or pyroxene, mounted to the AFM cantilever, measuring the force as the individual mineral in contact with the substrate of the same type of mineral is displaced at room temperature under”\nLine 198 – delete “although”\nLines 199-200 – “… (blue-shaded in color line) but many of the data have lower values, and in some cases  F/Fss (pink …”\nLines 201 -202 – “domain, Figure 4b) is negligible. Although the “usual” frictional increase in healing with log (th) is detected occasionally, this healing is markedly suppressed. Similar behavior is evident”\nLine 206 – “find that conventional frictional healing is more apparent than in flujid-present SHS tests (Figure 4b). Due”\nLines 208-215 – “… pressurized water. However, we are still able to observe the suppressed frictional healing at room temperature if sufficient SHS tests are performed to improve measurement statistics. Individual contact healing (ie.  F/Fss up to ~6, Fig. 4b) is more pronounced than the equivalent quantity,   / ss of ~0.1 for multiple-contact macro-scale rotary shear experiments (Fig. 1) and values reported previously for nano-scale SHS experiments16,38. Nevertheless, the negligible friction healing for the feldspar-feldspar contacts after long hold times at high relative humidity is consistent with our results for macro-scale gounge …”\nLine 234 – delete “in color”\nLine 235 – delete “in color”\nLine 236 – “… diagram …”\nLine 243 – “… healing may potentially influence the earthquake mechanism and”\nLine 250 – “Although the expeirments discussed …”\nLines 253-254 – “… remains challenging. By comparison with conditions of natural fault systems, (1) the experiments presented here are of short duratrion and reaction kinetics are too slow to observe …”\nLine 260 – “may lead to a decrease in shear strength of faults …”\nLines 363 – 364 – “Etna basalt, diorite, and orthoclase and albite. The mineralogical make up of the rocks are presented in Extended Data Table 1, based on XRD data. The samples are crushed …”\nLine 366 – “… performed with a Low to High …”\nLine 367 – “… with a hydrothermal pressure vessel …”\nLine 370 – “…under nominally “dry” or with a pore fluid pressure …”\nLine 381 – “… is heated to target temperature. Heating normally take …”\nLine 388 – “The original rate (V = 10  m./s). …”\nLine 391 – “… by the machine to the gouge layer …”\nLine 392 – “… revolution is less than 1 rpm …”\nLine 398 – “… using a LVDT (1  m”\nLine 399 – “… column of the machine. No”\nLine 405 – “… feldspar/pyroxene particles, composed of the same materials as in rotary shear”\nLine 407 – delete “approximately”\nLine 408 – “… solid gabbro substrate ins polished …”\nLine 410 – please specify what kind of plasma\nLine 412 – “installed at the Tsinghua …”\nLine 418 – “… lateral displacements). The slide-hold-slide procedure is then conducted …”\nLine 420 – “1 mm at the original velocity. The hold time is …”\nLines 424-425 – “… which is the ratio of the increment of frictional force during the hold stage, and the steady-state friction Fss after the hold, called  F.\nLine 431 – “holder that lacks any diffraction lines …”\nLine 435 – “… Solaris Field-Emission SEM of the Department”\nI look forward to seeing this paper in print,\nAndreas Kronenberg"}, "author_response": "Response to reviewers’ comments\nWe would like to sincerely thank all three reviewers for their constructive\ncomments on our manuscript titled “Feldspar weakens fault frictional healing”. Based\non their insightful suggestions, we have substantially modified and improved the\nmanuscript. Below, you will find our point-by-point response (in blue font) to the\nreviewers’ comments (in black font). Line numbers refer to those in the clean version\nof the revised manuscript.\nThis paper presents a set of experimental results from laboratory friction tests using\nslide-hold-slide protocols to measure the time dependent frictional strengthening, or\nfrictional healing, using gouges of feldspar, feldspar-bearing rocks, and some other\nminerals for comparison. The experiments are conducted in rotary shear at temperatures\nup to 400 °C, 50 MPa effective stress and 30 MPa pore pressure to explore mid-crustal\nconditions. The premise of the paper is important and in line with a recent increased\nfocus on frictional healing within the community. I don’t see any problems procedurally\nor with the dataset itself. I agree that the data are quite intriguing, however, I do see\nsome pretty serious problems with the presentation and interpretation of the results.\nWe would like to express our gratitude to the reviewer for acknowledging the\nimportance of our work. We also thank the reviewer for the constructive and insightful\ncomments that motivated us to interpret our results more carefully. We have tried our\nbest to address these comments and the detailed responses are given as follows.\nMain Comments\nMy main comment/concern here regards the definition of terms and the concept of\n“weakening” as presented here. Throughout the text, “frictional healing” and “healing\nrate” are used interchangeably. This is incorrect, because they are two distinct quantities.\nThe “frictional healing” is the increase in friction during the hold, or Δµ. The frictional\nhealing rate is the healing as a function of hold time, or Δµ/Δlog10(t). What the authors\ncall here ”weakening” is really a reduction in healing RATE, but there is no actual\nweakening because the Δµ values are still positive. In other words, you may get less\nhealing for a longer hold time, but it is a lower but still positive value of healing. To\ncall something weakening, you need the absolute value of friction to be LOWER after\nthe hold.\nNow, there are actually a few values of negative Δµ reported here, which would be\nactual weakening. Two such values are shown in Figure 1C, for 400 °C. The raw data\ncurve for these holds is shown in Figure 1A. The data is obscured by stick-slip (more\non this later), but for the two holds for which negative Δµ is reported, the peaks in the\nstick-slips are clearly higher after both the 1000 and 3000 second holds. I do not think\nthere is any way these two holds should be interpreted as “weakening”, and the issue\nlikely lies with trying to pick a Δµ value on data that is overprinted by stick-slip. There\nare also 4 data points in Figure 3 that are reported as negative Δµ, these may have the\n2\nsame problem but I cannot check these without the raw data curves.\nA related problem is that a main point in this paper seems to be that specifically feldspar\nat hydrothermal conditions (> 200 °C, 30 MPa pore pressure) cause weakening. Again,\nthere is only a reduction in the rate of frictional healing, so saying feldspar causes\nweakening is incorrect and not consistent with the data. For example, in Figure 1A I\ntake it that the values in gray are the background values of friction for the four\nexperiments. If so, the experiments under hydrothermal conditions are clearly stronger\n– the 400 °C experiment is the strongest. In the text on the AFM measurements, a point\nis made that the feldspar data is scattered and therefore show a “suppressed healing” or\n“little to negligible healing” (Line 202, 213), but a closer look at Figure 4B shows that\nmost of the data points for the middle plot (the feldspar-feldspar contact) are actually\nlarger values than in the other two plots – the figure shows that the feldspar heals the\nmost!\nTo summarize the point here, the data here do not show actual weakening. They show,\nat most, a reduction in healing rate at certain conditions. The main points of the text\n(including title) are therefore inaccurate or incorrect and will have to be revised to\nreflect this.\n1) We thank the reviewer for the clarification regarding the distinction between\n“frictional healing” and “healing rate”. We fully agree that they are two distinct terms,\nand we have reviewed our manuscript to ensure consistent use of the correct\nterminology throughout the main text.\nAs stated by the reviewer, the healing rate is defined as the frictional healing Δµ as a\nfunction of hold time t , expressed as Δµ/Δlog (t ). In most cases reported in the\nh 10 h\nliterature, frictional healing typically increases linearly with log (t ), resulting in a\n10 h\npositive and constant healing rate that remains unchanged with hold time. Regarding\nthe use of the term “weakening”, our intention was to describe the decrease in frictional\nhealing observed at long holds with respect to the expected values extrapolated from a\nlog-linear healing trend established by data from shorter holds (Rebuttal Fig. 1). In this\ncontext, “weakening” essentially refers to a reduced/suppressed frictional healing\ncompared with the normal behavior (but not necessarily resulting in a negative\nfrictional healing). However, we would also like to point out that this reduction can be\nsignificant enough to convert the healing rate from positive to negative values.\nEventually, under certain conditions, the frictional healing can become negative as 3000\ns hold for gabbro and 20000 s hold for orthoclase (see more discussion later in Rebuttal\nFig. 2). We acknowledge that \"negative healing\" and “weakening healing” are not\nprecise in most cases and we thank the reviewer for pointing this out. We have revised\n3\nthe main text and added more explanations accordingly to ensure accurate terminology\nthroughout. In the revised manuscript, some overlap between the use of \"decreased\nfrictional healing\" and \"negative healing rate\" may have occurred, and we believe they\nare consistent with our observations.\nRebuttal Fig. 1\n2) As the reviewer raised concerns about the negative Δμ values reported in Figures 1\nand 3, we have revisited the raw data and provided here two representative friction-slip\ncurves from the relevant experiments (Rebuttal Fig. 2a for the 3000 s hold of gabbro at\nT=400 °C; Rebuttal Fig. 2b for the 20000 s hold of orthoclase at T=200 °C), in which\nthe reviewer pointed out. In the zoom-in plots, we marked the peak friction values after\nthe hold and the “steady-state” friction before the hold. We agree with the reviewer that\nwith occurrence of stick-slip the accurate value of Δμ depends sensitively on the\ndetermination of the “steady-state” friction (see further discussion below about this\nterminology). For the 3000 s hold of gabbro, estimating the “steady-state” friction using\neither the stick-slip peak (solid red line) or the average friction (dashed red line) yields\na negative Δμ. For the 20000 s hold of orthoclase, the friction data is not obscured by\nstick-slip, and the post-hold peak friction is clearly lower than the pre-hold friction\n(Rebuttal Fig. 2b). These close-up plots confirm that negative Δμ values can indeed be\npossible under certain hydrothermal conditions. Again, we would like to emphasize that\nour main findings and discussions refer to the phenomenon of healing rate transitioning\nfrom positive to negative, but not necessarily negative frictional healing (Δμ, although\nit is possible under extreme conditions).\nIn summary, our use of terms, such as “decreased healing” and “suppressed healing”,\nwas intended to describe deviations from the commonly observed log-linear trend of\nfrictional healing in both hydrothermal rotary experiments and AFM experiments\n(Rebuttal Fig. 1). We primarily aimed to highlight the relative changes in frictional\nhealing compared to the log-linear extrapolation of short holds data. The reviewer also\nconcerned about the absolute magnitude of frictional healing across different\nexperiments, e.g., rotary shear experiments at different temperatures (25 to 400 °C). In\nthe AFM experiments section, we also did not aim to compare the absolute ΔF/F\nss\nvalues across the three different scenarios. We knowledge that the reviewer’s concern\n5\nis legitimate and warrants consideration. In the revised manuscript, we now use the\nphrase “decreased frictional healing” or “negative healing rate” to more precisely\ndescribe the observed breakdown of the linear relationship between frictional healing\nand the logarithm of hold time.\nWe also acknowledge the reviewer’s suggestion regarding the accuracy of the title. In\nthe manuscript we show that as the duration of exposure of the experimental fault to\nhydrothermal fluids (which are quite common conditions at hypocentral depths)\nincreases, the fault frictional healing decreases over time, and sometimes to negative\nvalues. We argue that is the presence of feldspar that, as opposed to pyroxene, that has\na key role in suppressing or drastically reducing the ability of faults to heal after an\nearthquake. We believe that this is still a weakening, at least compared to the role played\nby pyroxene in fault healing. Therefore, we believe that the original title reflects the\nmain findings of our study:\nFeldspar weakens fault frictional healing\nAlternatively, since in many experiments the frictional healing is still positive but the\nfrictional healing rate becomes neutral to negative, the result is that fault frictional\nhealing is less efficient with increasing time. So perhaps an alternative title could be:\nFeldspar makes fault frictional healing less efficient\nOr\nFeldspar reduces the efficiency of fault frictional healing\nWe remain open to suggestions from the reviewer if he/she has alternative titles that\nbetter capture the core of our study.\nMy second concern regards the stick-slips that appear in the data. These are interesting\nin there own right, and maybe something may come out of analyzing these data.\nHowever, they obscure the healing and I do not think that you can obtain an accurate\nmeasure of healing in a material that is stick-slipping. The Δµ measurement requires an\ninitial steady state friction value, which is clearly never the case for a stick-slipping\nmaterial. The cyclic nature of stick-slips induces inaccuracies in the measurement,\nwhich is illustrated in the lower inset of Figure 1; the Δµ value is clearly affected by\nwhere in the loading curve the hold happens to start. Using the stick-slip peaks cannot\nbe substituted for a steady-state friction rate, because you do not know what the steady\nstate value would be without the stick-slips.\nAnother point related to the stick-slips, is that they in themselves indicate some degree\n6\nof healing. Assuming repetitive stick-slips are laboratory repeating earthquakes, healing\nneeds to occur in order for the stick-slips to repeat in the laboratory. Going back to the\n400 °C experiment in Figure 1, there is no way that the 1000 3000 s holds can be\nclassified as “weakening” or have negative healing, because the stick-slips indicate that\nhealing has to be occurring.\n1) Ideally, the slide-hold-slide (SHS) experiments should reproduce the interseismic\nperiod in the laboratory, during which the experimental fault heals after a slip event.\nThe slip event could be seismic (unstable stick-slip) or aseismic (i.e., steady or non-\nsteady creep). As correctly stated by the reviewer, frictional healing was originally (and\nusually) determined in experiments in which the friction coefficient is stable during\nsliding and the pre- and post-hold friction coefficient can be clearly determined (see\nreview in Marone, 1998). Moreover, the presence of a “steady-state” regime allows us\nto interpret the frictional healing with the Rate-and-State friction law and a simple\ndefinition of the friction state variable (Ruina, 1983). However, frictional healing after\na simulated and spontaneous seismic slip event (stick slip) with associated stress drop\nis perhaps the most realistic simulation of natural healing of a seismic fault\n(seismogenic faults can be locked for most of the time).\nAs correctly stated by the reviewer, the stick-slip behavior indeed introduces some\nuncertainty in determination of “steady-state” friction. Because of this, some authors\n(Chen et al., 2015) considered taking the average friction between the peak and\nminimum friction during stick-slip as the nominal steady-state friction (dashed red line\nin Rebuttal Fig. 3a). In this approach, Δμ is defined as the difference between the peak\nfriction after the hold and the average friction during stick-slip before the hold (referred\nto as peak-ave method hereafter). In our study, Δμ is measured as the difference\nbetween the peak friction after the hold and the peak friction during stick-slips before\nthe hold (referred to as peak-peak method). To figure out whether different approaches\ninfluence the robustness of frictional healing behavior determined in our experiments,\nwe re-analyzed the datasets using the peak-ave method (shown as hollow symbols in\nRebuttal Fig. 3b), and compared the results with those obtained using the peak-peak\nmethod (solid symbols in Rebuttal Fig. 3b). We find that while the absolute Δμ values\nslightly differ, the overall pattern of frictional healing remains almost unchanged. This\nindicates that the key observation of this study, namely a reduced/suppressed frictional\nhealing after long holds, is robust regardless of the determination of pre-hold friction\nof the stick slip.\nThe reviewer may question whether “steady-state” friction during stick-slip can be\nestimated by fitting the data with the Rate-and-State friction (RSF) law using a spring-\nslider model. To address this, we attempted to fit the stick-slip curves by introducing a\nsmall velocity perturbation (an increase of 1.01 times) to a spring-slider system\ngoverned by the RSF law (the aging law was used and the damping effect was\nconsidered). The rate-state parameters a, b, L and the reference “steady-state” friction\nwere determined through fitting, and a representative result (stick slip events from the\nexperiment LHV2760 performed on gabbro at T= 200 °C) is shown in Rebuttal Fig. 4.\nHowever, due to the variations in the period of stick-slip events in our experimental\ndata, the model predictions cannot closely match every stick-slip cycle. Therefore, we\ndecided not to determine the steady-state friction by data fitting, given the presence of\nlarge errors and uncertainties.\nRebuttal Fig. 4\n2) Then a related question is which approach, peak-ave or peak-peak method, is more\n8\nsuitable for determining frictional healing in the presence of stick slip? There is likely\nno consensus on this point. In our study, we prefer the peak-peak method because:\na) the peak friction during the stick phase is well determined. During this phase, friction\nincreases slowly relative to fast friction drop during the slip phase. Once the slip occurs,\nthe fault slip velocity can be large (> 1 m/s). Because the machine may vibrate, the\nmeasurement of the lowest friction during the coseismic stress drop with the 100 Hz\nacquisition rate used may be overestimated;\nb) during the stick phase, the machine and the columns accumulate elastic strain energy\nthat will be released during the slip event: the minimum friction coefficient during the\nslip phase is related to both the stiffness of the machine and dynamic coseismic\nfrictional weakening.\nFor these reasons, we consider the peak friction during the stick phase as the reference\nvalue for pre-hold friction, which represents the short-living spontaneous healing of the\nexperiments fault and is also well measured. To avoid confusion, we refer to the\nfrictional healing and healing rate determined in this method as “equivalent healing”\nand “equivalent healing rate” in the revised manuscript, as compared to the\nconventional determination associated with SHS experiment involving stable slip.\n3) As stated by the reviewer, the occurrence of a stick phase is due to healing of the\nexperimental fault. However, this spontaneous frictional healing is relatively small and\noccurs in a short lifetime (typically ~ 1 to 2 s in our experiments). Instead, the peak\nfriction at the end of the imposed hold is the static strength of the simulated fault at the\nend of a longer “interseismic” period. The reviewer also raised a critical concern:\n“the Δµ value is clearly affected by where in the loading curve the hold happens to\nstart.”\nTo better assess the impact of the initiation of the hold during the stick phase on the\nmeasured healing values, here, we present an experiment that was performed on a gouge\nlayer of orthoclase at temperature of 400° C, pore fluid pressure of 30 MPa and effective\nnormal stress of 50 MPa. This experiment was designed to investigate the displacement\ndependence of frictional healing, and included eight SHS sequences, each with hold\ntimes ranging from 10 s to 1000 s (Rebuttal Fig. 5). The right panel of the figure shows\nΔμ determined from 1000 s holds using peak-peak method for SHS sequences\nconducted at increasing displacements. We take the 3rd and 5th sequence data as an\nexample for comparison. The hold in these two sequences were initiated at different\npoints of the stick phases, i.e., near the peak and trough of the stick-slip curves,\nrespectively. The measured Δμ values were very similar, indicating that the timing of\n9\nthe imposed hold during the stick phase has a relatively minor (i.e., negligible) effect.\nRebuttal Fig. 5\nIn summary, we acknowledge that there is currently no consensus, well-accepted\nmethod for determining frictional healing in the case of stick slip sliding. To address\nthis point, we have chosen to calculate the frictional healing using peak friction values\nduring the stick phase as a reference point, and have referred to the resulting Δμ as\n“equivalent Δμ” to distinguish it from the traditional Δμ. However, we would like to\nemphasize that our main findings refer to the phenomenon of healing rate transitioning\nfrom positive to negative, not necessarily negative values of frictional healing. This\nkey\nobservation of this study is robust regardless of the determination of Δμ (discussed\nabove), as long as a consistent approach is applied to all datasets.\nNow we have added additional description in the main text:\nLines 75-80: “Typically, frictional healing Δμ is measured as the difference between\npeak μ value (μ) upon re-sliding and steady state friction (μ) before the hold. However,\np ss\nin the cases of stick-slip, a well-defined μ cannot obtained. We adopt the peak friction\nss\nduring the stick phase before the hold as a reference point. The equivalent Δμ is then\nmeasured as the difference between the peak μ value of the stick-slip events after and\nbefore the holds (see Methods; hereafter both referred to as Δμ for simplicity, Fig. 1a\ninset).”\nLined 398-403: “Frictional healing Δμ is defined as the difference between μ peak\nvalue (μ) upon re-sliding and steady state friction (μ) before the hold. For stable\np ss\nsliding, μ is calculated as the average values of μ measured over the last 0.2 mm of\nss\ndisplacement before the hold. In the cases of stick-slip, a μ counterpart is determined\nss\nby averaging μ peak values during the stick phases within the last 0.2 mm of\n10\ndisplacement before the hold (normally 6-8 cycles).”\nThe AFM measurements are a good idea, however I am not sure that they support the\nrotary experiments. The authors have clearly attributed the lower healing rates to\nfeldspar under hydrothermal conditions. It is unclear to me how the AFM measurements\nunder room temperature and humid air are controlled by the same mechanisms. The\nauthors then speculate that the process is “thermally activated, water assisted and\nchemical”, which sounds like pressure solution to me. Is this even realistic in the AFM\nmeasurements conducted here?\nWe thank the reviewer for raising this concern. As recognized by the reviewer, the\nreduced frictional healing behavior exhibited at the macroscale is attributed to feldspar\nunder hydrothermal conditions. Although we did not have direct proofs to confirm that\nthe frictional behavior observed in the AFM measurements was governed by the same\nmechanisms as the rotary shear experiments, the characteristic features reflected by the\nnanoscale tests strongly suggest that these two experiments are closely related as\nelaborated below.\nBoth the rotary and AFM experiments show that suppressed frictional healing rate could\noccur under long holds. More importantly, in both systems, the occurrence of this\nabnormal behavior requires the same conditions: i.e. (a) feldspar must be involved in\nthe contact interface, and (b) water has to be exist in the environment. Although we\ncould not conduct AFM friction experiments in liquid water due to instrumental\nlimitations, previous studies have demonstrated that a water bridge between the AFM\ntip and the substrate was likely to form under relatively high humidity (Szoszkiewicz\nand Riedo, 2005; Bocquet et al., 1998). Therefore, the similar suppressed frictional\nhealing behavior exclusively observed under the similar conditions strongly suggests\nthat the underlying physical processes at these two length scales are closely related, if\nnot identical.\nWe understand that the reviewer has a concern regarding the discrepancy in the\nconditions between rotary and AFM experiments. In particular, the temperature used in\nthe AFM tests is significantly lower (room temperature) than that (≥ 200 °C) to observe\nsuppressed frictional healing in the rotary shear experiments. However, due to\nlimitations of the AFM instrument, we were not unable to conduct experiments fully\nimmerged in liquid water while heating up far beyond its boiling point. Nevertheless,\nwe were lucky to find that, in the nanoscale experiments, the suppressed frictional\nhealing could have already been observed for feldspar even at room temperature.\nsurprising. As evident in the previous AFM experiments (Li et al., 2011; Li et al., 2020),\nthe frictional healing effect and other interfacial phenomena are commonly much more\npronounced, by orders of magnitude, than those observed at the macroscale. This high\nsensitivity is possibly due to the high local stress and good chemical uniformity for the\nnanoscale asperity contact. Because the temperature in our AFM experiments was\nsignificantly lower than one in the rotary shear experiments, as a consequence, the\nsuppressed frictional healing could only be observed occasionally (with certain\nprobability) at the nanoscale despite the high interfacial sensitivity. We anticipate that\nif the nanoscale tests could be extended to elevated temperature as rotary shear\nexperiments, the suppressed frictional healing would be observed more frequently (with\nhigher probability).\nAs shown in Figure 4c, the contact stiffness remains constant during the holds in AFM\nexperiments, which suggests that the contact area does not appreciably change. This\nobservation rules out the mechanical contribution to healing in our experimental setup\nand we conclude that the frictional healing behavior observed in AFM experiments is\nprimarily attributable to chemical processes.\nIn summary, although the experimental conditions differ between AFM experiments\nand hydrothermal rotary shear experiments, the consistent observation of suppressed\nfrictional healing after long holds in feldspar across nano- to macro-scale suggests the\ninvolvement of water-assisted and thermally-activated weakening processes. Most\nimportantly, the AFM experiments underpin our key hypothesis that the decreased\nfrictional healing is attributed to a reduction in contact strength (i.e., contact quality).\nWe have revised the main text correspondingly (Lines 225-236).\nSpecific Comments:\nLine 44: healing *usually* increases log-linearly – there are healing studies showing\nexamples where this is not the case\nWe agree with the reviewer, “usually” has been added (Line 45).\nLine 46: what is meant by “minerals”?\nWhat we intended to state was that log-linear healing behavior is valid for a variety of\nmineral types. To clarify this point, we have revised the text to read “a variety of\nminerals” (Line 48), as also suggested by reviewer #2.\nLine 67: please explain the experiment protocols for the SHS sequences here\n“The SHS procedure involves: (1) sliding at V = 10 μm/s for 1 mm of slip displacement,\n(2) holding at zero load-point velocity for hold times (t ) ranging from 3 s to 10,000 s,\nh\nand (3) resuming the sliding at the original velocity (V = 10 μm/s).”\nLine 75-76: this is the “standard” definition of healing rate that is usually applied to\nmaterials with a constant healing rate, in other word materials where multiple healing\npeaks can be fit with a single log-linear regression. Since your data deviates from this,\nit may be advisable to use a “local” or “individual” healing rate, or a β calculated\nbetween two individual hold times.\nWe thank the reviewer for this comment. We have revised the main text accordingly to\nclarify this point (Lines 81-84).\n“Since some of the datasets in our study cannot be adequately described by a single\nlog-linear relation, we adopt individual healing rates determined within a monotonic\ninterval to explicitly capture the time-dependent evolution of β.”\nLine 91: Δµ does not appear to always increase monotonically in this figure, there looks\nlike a clear rollover for the 300 and 400 °C experiments at the longer hold times.\nThank you for finding this mistake, and it has been revised in the main text (Lines 99-\n101).\n“Under dry conditions, Δμ increases monotonically with log t and β is positive in most\n10 h\ncase, except for a slight deviation from the log-linear trend with a decreased, but still\npositive β at T =300 ℃ and 400 ℃ at longer hold times (≥ 1000 s) (Fig. 1b).”\nLine 132: it would be helpful here to provide a brief classification (K, Na, Al) of the\nfeldspars chosen here.”\nThank you for the suggestions. We have added the compositional classification of the\nfeldspar used in this study, indicating that they include albite (Na-rich), orthoclase (K-\nrich) and andesine (Na-Ca rich) (Lines 145-146).\nLine 240-247: as in the main comments, I do not see time-dependent weakening in this\nstudy, and the data do not indicate reduced stress levels.\nAs discussed above, a decreased healing or a lower healing rate indicates that the faults\nregain their strength more slowly after coseismic slip. Assuming that tectonic loading\nis constant, this may lead to a possible scenario in which the shear stress acting on the\nfault overcomes fault strength at a lower failure threshold compared to a fully healed\n13\ncases, thereby triggering failure earlier in the loading cycle, as illustrated in Rebuttal\nFig. 6.\nRebuttal Fig. 6\nLine 247-249: This text on spectrum of slip modes comes out of nowhere and has no\ncontext, I suggest deleting.\n(Line 262) We have deleted the text on “spectrum of slip modes”.\nLines 262-264: this text also comes out of nowhere and rate-and-state friction is not\nintroduced in the text, I suggest deleting this as well.\nThanks, we have deleted the text related to rate-state friction (Lines 275-276).\nReferences\nBocquet, L., Charlaix, E., Ciliberto, S. et al. Moisture-induced ageing in granular media and the\nkinetics of capillary condensation. Nature 396, 735–737 (1998). https://doi.org/10.1038/25492\nChen, J., Verberne, B. & Spiers, C. Effects of healing on the seismogenic potential of carbonate fault\nrocks: Experiments on samples from the Longmenshan Fault, Sichuan, China. Journal of\nGeophysical Research: Solid Earth 120, 5479-5506 (2015).\nhttps://doi.org/10.1002/2015JB012051\nLi, Q., Tullis, T. E., Goldsby, D. & Carpick, R. W. Frictional ageing from interfacial bonding and\nthe origins of rate and state friction. Nature 480, 233-236 (2011).\nhttps://doi.org/10.1038/nature10589\nLi, S., Zhang, S., Chen, Z., Feng, X.-Q. & Li, Q. Length Scale Effect in Frictional Aging of Silica\nContacts. Physical Review Letters 125 (2020).\nhttps://doi.org/10.1103/PhysRevLett.125.215502\nEarth Planet. Sci. 26, 643-696 (1998). https://doi:10.1146/annurev.earth.26.1.643\nRuina, A. Slip instability and state variable friction laws. Journal of Geophysical Research: Solid\nEarth 88, 10359-10370 (1983). https://doi.org/10.1029/JB088iB12p10359.\nSzoszkiewicz, R. & Riedo, E. Nucleation Time of Nanoscale Water Bridges. Physical Review\nLetters 95, 135502 (2005). https://doi.org/10.1103/PhysRevLett.95.135502\nReviewer #2 (Remarks to the Author):\nThe manuscript discusses an interesting study regarding the evolution of friction\nstrength in different materials at elevated temperatures. The observation of frictional\nweakening under geothermal conditions is still new and these results help to validate\nthe very limited previously published work. Additionally, these results come from a\nrotary shear apparatus showing that time-dependent frictional weakening doesn’t not\ndepend on measurement apparatus. The project is well designed and interesting. A\nsignificant amount of work went into verify the behavior and isolating factors that could\ninfluence the observed weakening. The results are potentially very significant with\nrelevance to earthquakes and geothermal systems. However, there are details that are\neither missing or need clarification, figures need to be reviewed for accuracy, and there\nare some grammatical improvements needed.\nWe would like to express our gratitude to the reviewer for his/her positive assessment\nand the constructive comments. We have tried our best to address the comments as\nshown below.\nThe AFM results are interesting but did raise some questions regarding uncertainties\nassociated with this type of measurement. Admittedly I am not very familiar with AFM\nmethods, but my understanding is that friction determined with AFM is based on\ncomparison of forces measured in both the forward and reverse directions as the lateral\nforce depends on if you are moving up or down a slope. I gather from the manuscript\nthat the change in force reported is based on measurements in one direction. I would\nthink that surface roughness and location on the surface could then have a significant\nimpact on the measured healing. How did the roughness of the different mineral\nsubstrates compare? Also how did the tips evolve during the tests? Did the pyroxene or\nfeldspar tip experience more wear? How would this affect your measurement.\ndetermined as the difference between the lateral forces measured during the forward\nand backward scanning. Rebuttal Fig. 7 shows a representative lateral force loop\nobtained after a 700 s hold in feldspar-feldspar contact tests. To illustrate this point, we\nhave added the following description to the Methods section (Lines 433-435).\n“The tip is first slid 1 μm, then held stationary for a set time, followed by sliding another\n1 μm at the original velocity. After this, the tip is slid backward to the original position.\nThe friction was determined by calculating the difference between the forward and\nbackward lateral forces.”\nThe polished substrate used for the AFM experiments has a root-mean-square\nroughness of ~3 nm, measured within an area of 4 × 4 μm2. As shown in Rebuttal Fig.\n7, the lateral force remains relatively stable during sliding (variation of the lateral force\nis much smaller compared with the magnitude of healing), suggesting a minimal effect\ndue to the substrate surface roughness.\nAs for tip wear, although we did not symmetrically measure the apex of the tips before\nand after test, we found that the steady-state friction before and after tests stayed more\nor less the same, suggesting minimal tip wear. To further minimized the potential effect\ndue to tip blunting, we normalizing the frictional healing by the steady-state friction\nusing F /ΔF as adopted in previous literature (Li et al., 2011; Li et al., 2020).\nss\nRebuttal Fig. 7\nI would like to see some discussion of uncertainty in the measurements. There are never\nany error bars or uncertainty estimates in the figures or text, but we know there is\n16\nuncertainty associated with measurements. I think this is especially important as the\nauthors acknowledge system stiffness and seal friction are not accounted for.\nWe thank the reviewer for this valuable suggestion, and fully agree that quantifying\nuncertainties is essential to appropriately interpretation of the experimental results. In\nthe revised manuscript, we have included uncertainty estimates for rotary shear\nexperiments and AFM experiments. The figures 1-4 have been updated to include error\nbars, and the corresponding descriptions have been added in the Methods section (Lines\n398-403; Lines 439-447):\n“Frictional healing Δμ is defined as the difference between μ peak value (μ) upon re-\np\nsliding and steady state friction (μ) before the hold. For stable sliding, μ is calculated\nss ss\nas the average values of μ (with standard error) measured over the last 0.2 mm of\ndisplacement before the hold. In the cases of stick-slip, a μ counterpart is determined\nss\nby averaging peak μ values (with standard error) during the stick phases within the\nlast 0.2 mm of displacement (normally 6-8 cycles).”\n“To estimate the uncertainty of ΔF/F for each hold event, we first determined the peak\nss\nforce value (F ) and subtracted from it the individual force values (F) measured during\np i\nthe post-hold steady-state sliding, thereby obtaining a dataset of differences ΔF = F -\ni p\nF. Based on the ΔF and F datasets, we then calculated the mean value (𝛥(cid:3364)(cid:3364)(cid:3364)𝐹(cid:3364) and 𝐹(cid:3364)(cid:3364)(cid:3364)(cid:3364))\ni i i (cid:3046)(cid:3046)\nand standard deviation (𝜎 and 𝜎 ) of ΔF and F , respectively. Finally, the standard\n(cid:3057)(cid:3007) (cid:3007)(cid:3294)(cid:3294) ss\ndeviation of ΔF/F was estimated using the error propagation formula:\nss\n𝜎 =\n(cid:3057)(cid:3364)(cid:3364)(cid:3364)(cid:3007)(cid:3364)\n(cid:3495)(\n(cid:3097)(cid:3305)(cid:3255))(cid:2870)+( (cid:3097)(cid:3255)(cid:3294)(cid:3294))(cid:2870)”\n(cid:3019) (cid:3007)(cid:3364)(cid:3364)\n(cid:3294)\n(cid:3364)\n(cid:3294)\n(cid:3364) (cid:3057)(cid:3364)(cid:3364)(cid:3364)(cid:3007)(cid:3364) (cid:3007)(cid:3364)(cid:3364)\n(cid:3294)\n(cid:3364)\n(cid:3294)\n(cid:3364)\nLine 25: The use of “particularly” here implies that negative friction healing rates were\nobserved in some tests at temperatures ≥200 °C when pressurized water was not present.\nHowever, supplementary figure 3 shows the results of the dry test and negative healing\nrate are not observed at any temperature.\nThank you for pointing out this unclarity.\nLine 43 and 46: remove excess punctuation\nThank you for finding this mistake, change made.\nLine 46: “… including Westerly granite and [a variety of] minerals.”\nDone (Line 48).\nWording issue here – doesn’t make sense\nThis point was also raised by Reviewer#3, this sentence has now been revised to read\n“…may have negative healing rates, which will alter the seismic cycle” (line 52).\nLine 60: Experiments aren’t being performed in the manuscript, but the results are being\npresented.\nThe sentence has been revised (Lines 62-64):\n“Here we present results of 30 SHS experiments performed in the presence of hot and\npressurized water using a rotary shear machine (LHV-Beijing) equipped with a\ndedicated hydrothermal pressure vessel.”\nLine 103: We already know that 𝛥μ depends on the duration of the hold period prior to\nresliding, I think you mean to specify that depends only on the prior hold and the\ndependent on previous history of healing.\nThank you for this helpful clarification. Our intention was to emphasize that the\ndecrease in Δμ is independent of the sequence in which different hold times are applied\nand the cumulative duration of the holds, and is controlled solely by the absolute\nduration of the individual hold event. Now we revised the main text in Lines 113-115:\n“This observation demonstrates that the decrease in Δμ at longer t is independent of\nh\nhold sequence, and depends solely on the absolute duration of individual hold prior to\nthe re-sliding.”\nFigure 3: This is a nice figure presenting initial work looking at the weakening of\nmultiple materials.\nThank you.\nLine 164: Were the samples used for FESEM removed from the experimental apparatus\nfollowing an extended hold period or immediately after completion of a sliding period?\nSecondary minerals maybe easier to see if examined at the end of a hold period while\nthey would likely be very disturbed in difficult to see in a sample taken at the conclusion\nof a sliding period.\nThe samples used for FESEM tests were collected after a final sliding period. We agree\nthat secondary minerals, if present, may be more easily detected if the sample is\nretrieved immediately following a prolonged hold, without additional shearing.\nMoreover, due to elevated temperatures applied in our experiment, the samples required\n18\na cooling period of approximately 1 hour for T = 200 °C before being removed. This\nprocess might have slightly modified the microstructures and also potentially affected\nthe visibility of any transient secondary phases.\nLine 200 & Figure 4b: Supplementary figure 10b shows the highest value of F/Fss (~\n4.8) for the 100 s hold but Figure 4b only shows values less than 4 for the 80% humidity\ncase. Why is does the supplementary data not match the data presented in the main text?\nThe data are consistent. In Figure 4b, Y-axial shows ΔF/F , which is defined as (F-\nss\nF )/F = F/F - 1. Thus, the F/F value of ~4.8 shown in supplementary Fig. 10\nss ss ss ss\ncorresponds precisely to an ΔF/F value of ~3.8 in Fig. 4b.\nss\nLine 206: wording here is awkward, the suppressed frictional healing is not observed\nat 5% humidity.\nWe have revised the main text, also suggested by the reviewer #3, as follows (Lines\n219-221):\n“We also perform SHS experiments for feldspar-feldspar contacts under relatively low\nhumidity conditions (RH~ 5%) and find that conventional frictional healing is more\napparent than in fluid-present SHS tests (Fig. 4b).”\nLine 208- 210: not clear what you mean by “with certain statistics” I found no\ndiscussion of statistical analysis in the text.\nThank you for pointing it out. We intended to express that the suppressed frictional\nhealing at 80% humidity only occurred occasionally with certain probability. We have\nrevised the sentence to clarify this point. It now reads (Lines 223-224): “we are still\nable to observe the suppressed frictional healing at room temperature occasionally with\ncertain probability if sufficient SHS tests are performed.”.\nLine 222: if contact area does not change for holds less than 300s what is causing the\ninitial strengthening?\nIn the nanoscale SHS tests, frictional strengthening could be primarily attributed to the\nformation of more or stronger chemical bonds at the contact interface. This hypothesis\nis consistent with the previous studies, where the importance of chemical bonding in\nfrictional healing has been highlighted (Li et al., 2011; Liu and Szlufarska, 2012; Li et\nal., 2020). Further investigation is needed to identify the specific types of chemical\nbonds formed in our feldspar-feldspar AFM experiments.\nLine 243-249: If the fault surface is becoming weaker with time the elastic strain energy\nstored would also be low because it would be creeping under constant stress loading.\nrecurrence interval. What is written here contradicts line 14-15.\nWe appreciate the reviewer’s thoughtful comment. While we agree that a weak fault\nwould not accumulate high elastic strain energy, we would like to clarify that this is not\nwhat we intended to imply.\nIn our context, “decreasing healing” or “negative healing rate” refers to a reduced rate\nof fault strength recovery during the interseismic period, not suggesting a transition in\nfault frictional stability from unstable to stable (creep) slip. As illustrated in Rebuttal\nFig. 6, a lower healing rate indicates that the faults regain their strength more slowly\nafter coseismic slip. Assuming that tectonic loading is constant, this may lead to a\npossible scenario in which the shear stress acting on the fault overcomes fault strength\nat a lower failure threshold compared to a fully healed cases, thereby triggering failure\nearlier in the loading cycle. Therefore, we hypothesize that reduced healing, or healing\nrate, may result in shorter earthquake recurrence intervals. We have slightly revised the\nmain text correspondingly.\nRebuttal Fig. 6\nLine 251: Not sure why you bring up the 40 wt% feldspar here as the composition\nrequired for negative healing. The presented data in figure 3 examined feldspar content\ndown to around 40 wt% and all wt% exhibited weakening so nothing can be said based\non the data available about the amount needed to observed weakening.\nWe have removed the original statement “~ 40 wt% of feldspar according to the data\nin Figure 3” in the revised manuscript.\nLine 252: “… behavior in common crustal rocks…”\nLine 391-392: I think there are some words missing. Sentence doesn’t quite make sense.\nThe setting of an appropriate threshold velocity for the servo motor is important for our\nexperiments because it ensures that the motor does not rotate during holds when the\nzero-voltage input signal is disturbed by electrical noise. The information has been\nadded in Lines 404-408:\n“To ensure that no rotation is applied by the machine to the gouge layer during holds,\nthe servo motor control system is set to stop once the imposed revolution rate is less\nthan 1 rpm (corresponding to a slip velocity ~ 1.3 μm/s for the sample assembly used).\nBy doing so, the servo motor remains stopped even if zero-voltage input signal is\ndisturbed by electric noise, and no angular rotation is detected during the holds.”\nLine 395: was displacement dependence discussed?\nThank you for pointing it out. Displacement dependence was not discussed in this\nmanuscript, as it would introduce additional complexities (e.g., evolution of gouge\ntexture, etc.) that may obscure the main focus of this study. In the revised manuscript,\nwe have deleted the relevant sentence “…to investigate the effect of slip displacement\non frictional healing and the healing rate”.\nExtended data figure 7 – I recommend plotting using the same scale for easier\ncomparison. Also, in caption the statement is made “these data show a great consistency,\nproving the reliability of the results obtained with the machine used.” I think you\nintended to indicate that the results are independent of the machine used.\nWe thank the reviewer for the suggestion regarding Extended Fig. 7. We have plotted\nthe data from Jeppson et al., (2023) and our data using the same scale for easier visual\ncomparison (see updated Supplementary Fig. 7 below).\nIt is worth noting that the two datasets were obtained under considerably different\nexperimental conditions: Jeppson et al., (2023) used bare surface under an effective\nnormal stress of 30 MPa and pore fluid pressure of 10 MPa, whereas our experiments\nwere conducted on gouge layer under an effective normal stress of 50 MPa and pore\nfluid pressure of 30 MPa. We intend to highlight that, despite the discrepancies in\nexperimental setups and conditions, the results show a remarkable consistency in the\npattern of frictional healing behavior of Westerly granite. This consistency indicates the\nreliability of our experimental results, independent of the machine used.\nReferences\nJeppson, T. N., Lockner, D. A., Beeler, N. M. & Moore, D. E. Time‐Dependent Weakening of\nGranite at Hydrothermal Conditions. Geophys. Res. Lett. 50, 1–9 (2023).\nLi, Q., Tullis, T. E., Goldsby, D. & Carpick, R. W. Frictional ageing from interfacial bonding and\nthe origins of rate and state friction. Nature 480, 233-236 (2011).\nhttps://doi.org/10.1038/nature10589\nLi, S., Zhang, S., Chen, Z., Feng, X.-Q. & Li, Q. Length Scale Effect in Frictional Aging of Silica\nContacts. Physical Review Letters 125 (2020). https://doi.org/10.1103/PhysRevLett.125.215502\nLiu, Y. & Szlufarska, I. Chemical Origins of Frictional Aging. Physical Review Letters 109 (2012).\nhttps://doi.org/10.1103/PhysRevLett.109.186102\nReviewer #3 (Remarks to the Author):\nThis manuscript presents new slide-hold-slide experiments to investigate “rate and state\nfriction” sliding of feldspars and feldspar-bearing rocks under hydrothermal conditions\nthat are very common in the Earth’s subsurface (including conditions of active\nseismogenic fault zones), and on feldspar individual contacts at different values of\nrelative humidity. The results presented are new and exciting, and this study deserves\nclose attention by those who study frictional properties and their significance for the\nearthquake cycle. It is well placed in a high-profile journal such as Nature\nCommunications. These novel results are closely linked to the current hydrothermal\nconditions of the experiments, and it is not even clear to me that I should use the terms\n“rate and state friction” given how different the results are from the large data set of\n22\nfrictional properties of rocks at varying rates and slide-hold-slide, which are mostly\nrestricted to ambient lab conditions.\n”Rate and state friction” of rocks at room temperature and relative dry atmospheric\nconditions typically shows a direct effect (during which frictional resistance to shear\ncorrelates with sliding rate) and an evolution effect, that is related to changes in strength\nthat occur during hold stages of slide-hold-slide experiments. Typically, frictional\nsliding stability and, by inference, fault sliding stability are governed by whether the\nevolution effect changes the frictional resistance to sliding more or less than the direct\neffect. More generally, as pointed out by the authors of this manuscript, the evolution\nof frictional strength in slide-hold-slide experiments is marked by an increase in\nfrictional resistance with hold time. This increase in frictional resistance is sometimes\ninterpreted to result from increases in true area of contact of the frictional surfaces with\nhold time, as normal stress across the frictional surfaces and local stress concentrations\nlead to plastic (or other) deformation of the rough surface of the adjacent rocks.\nAlternatively, increases in frictional resistance with hold time have been interpreted to\nrepresent increasing strengths of contacts, as might happen due to formation of van der\nWaals (or other chemical bonds) at points of contact. All of these characteristic\nfrictional responses are apparent for feldspar, diopside, and feldspar-bearing rocks\nunder room temperature and/or dry conditions shown in Figures 1a, 1b, 1c, 2a. The\nsingle mineral sliding experiments show similar (typical) frictional sliding results for\nfeldspars at low relative humidity and pyroxene of increasing strengths with hold time\nin Fig. 4b (top and bottom).\nThe new results of this study, which are so interesting, are the apparent frictional sliding\nreductions ascribed to reductions in frictional healing (where changes in friction\ncoefficients show negative slopes with hold time (Figs 1c, 2b, 2c, 3, and 4 (middle)).\nThe authors point out that the reductions in frictional healing correspond to the presence\nof feldspar at points of contact and water at high temperature and pressure. They also\nsuggest that the loss of frictional healing is thermally activated, water-assisted and\nchemical in origin. I am in broad agreement with this interpretation, and wonder, in fact,\nif this isn’t good evidence for the role of pressure solution to frictional properties. In\nparticular, it seems that dry surfaces in contact under compressive normal forces must\nshow increasing contact areas and/or bond strengths of contacts, unless a process such\nas pressure solution dissolves the solid (feldspar) contacts, reducing real areas of\ncontact (relative to the dry behavior) or replaces bonds across loaded solid contacts by\nnm-scale fluid films. That is, the results suggest that areas of contact are reduced with\n23\nhold time (relative to healing under dry conditions) or that contacts have reduced\nstrengths with hold time (relative to healing under dry conditions), contrary to most\nmicro-contact models of frictional sliding. And yet, the hydrothermal conditions of\nthese experiments are probably much more representative of conditions on real faults\nthan the many room temperature, dry experiments on rocks.\nWe are grateful to the reviewer for the positive comments on our manuscript, and also\nfor highlighting the importance of our work for the rock deformation and earthquake\nmechanics community. We also thank the reviewer for pointing out the possibility of\npressure solution as a potential interpretation of our observations. We agree that\npressure solution could be a plausible explanation for the observed reduction in\nfrictional healing. However, in the deformed samples (T=200 °C) that show decreased\nhealing, we did not find direct evidence for pressure solution features (e.g., dissolution\ntextures or precipitates). Instead, the dominant deformation process is pure cataclasis\n(i.e., grain size reduction, see Supplementary Fig. 8d). The P–T conditions (50 MPa &\n200 °C) applied in our SHS experiments may not be sufficient to drive significant\npressure solution of feldspar over the relatively short experimental duration. Although\nindentations of grains or possible neck grain boundaries were observed in our previous\nexperiments performed on the same gabbro gouges at T=400 °C (see red in color arrow\nin Figure 3g of Feng et al., 2024 reported below), such features were not observed in\nour SHS experiments conducted mostly at lower temperatures.\nMoreover, axial shortening data from the SHS experiments clearly show gouge\ncompaction during holds, which intuitively indicates an increase in contact area over\ntime. Combined with our AFM experiments which show a decreased healing in\nfeldspar-feldspar contact without changes in contact area, these observations lead us to\nfavor a chemically driven mechanism that reduces contact strength as a more likely\nexplanation for the observed reduction in frictional healing.\nμ\n1 m\nThe lack of evidence for product minerals due to reaction at feldspar surfaces by XRD\nand SEM is not surprising, as the amounts of reaction products and likely nm-scale\nclays and other new phases will be difficult to detect by these methods. It is worth trying,\nof course, but powder XRD typically requires %-level concentrations minerals and\nfeldspar reaction to clays and components in solution may be very slow (thus leading\nto only small new growths). I wonder if Raman spectroscopy of feldspar surfaces might\nallow detection of nm-scale films or precipitates. I am not suggesting that this is\nnecessary for publication but if the authors have access to a Raman lab, they may wish\nto confirm existence of clays on their clay gouge particles.\nWe agree with the reviewer that, even if clays may have formed locally, they would be\nextremely difficult to be detected due to their likely nanoscale size and low abundance.\nRegarding Raman spectroscopy tests, we have indeed conducted Raman measurements\non several deformed feldspar gouge samples collected from experiments that showed\ndecreased frictional healing behavior. However, no signals other than those\ncorresponding to feldspar were detected. We also performed Raman spectroscopy on\nthe feldspar substrates used in the AFM experiments, and the current dataset shows no\nclear evidence for clays. These results can be understandable, given that the spatial\nresolution of the Raman laser (1 μm) is likely much larger than the newly formed phases.\nevidence of clay formation in deformed samples (see red in color arrow in Fig. 3f of\nFeng et al., 2024 reported below). In our manuscript currently under review (Feng et\nal., submitted to EPSL), we collected diffraction patterns of these clay-like materials\nunder TEM and confirm that they are likely to be smectite. However, those samples\nwere collected from experiments performed at a higher normal stress (100 MPa),\nelevated temperature (400 °C), and a longer slip displacement, all of which are expected\nto enhance the kinetics of hydrothermal alteration. Therefore, the formation of similar\nproducts cannot be simply inferred for the short duration SHS experiments performed\nat 200 °C in this study.\nREDACTED\nWhatever the microscopic mechanisms, this behavior has major implications for long-\nterm fault strength, fault stability, and the aseismic periods of the earthquake cycle. I\nrecommend that this manuscript be accepted for publication in Nature Communications.\nThis manuscript is well written and my editorial suggestions are minor. I would like\nto suggest that the authors choose better headings for this manuscript. In particular, I note\nthe heading “Main” rather than something like “Introduction” or “Prior Studies of Fault\nHealing”. However, most of my editorial suggestions are just minor rewording; I will\ngive these line-by-line:\nWe are especially grateful to the reviewer for the detailed line-by-line rewording\nsuggestions, which have significantly helped us improve the clarity, precision and\noverall quality of our manuscript. We have carefully revised the text correspondingly.\nLine 29 – I suggest starting this line with “crystals on a feldspar substrate. …”\nDone\nLine 30 – I suggest “… variations in fault strength, chaing our”\nDone\nLine 37 – “etc., and in extreme cases, fault welding …”\nDone\nLine 38 – “… between asperity contacts at the sub-”\nDone\nLines 39-40 – “… and, (3) a combination of the two11. The underlying mechanism of\nfault frictional healing is generally attributed to …”\nDone\nLine 51 – “… healing rates which will alter the seismic”\nDone\nLine 52 – delete “here”\nDone\nLine 53 – “rate” rather than “rates”\nDone\nLines 54-55 – “we measured the frictional haling behavior of a wide range of common\ncrustal rocks at temperatures of hypocentral depth. We concluded …”\nDone\nLine 56 – “… feldspar in the process of decreasing fault”\nDone\nLines 57-58 – “… nanometer-scale friction experiments with a single feldspar micro-\ngrain translating along a feldspar substrate …”\nDone\nLine 64 – “We first report frictional healing data …”\nDone\nLine 69 – “… the evolution of the friction coefficient …”\nDone\nLine 74 – “… between the peak μ value of the stick-slip …”\nLine 78 – “… test and result of gabbro under hydrothermal”\nDone\nLine 81 – “… Hold times (th) are”\nDone\nLine 82 – what is meant by “bottom”? Do you mean the low stress value? Please reword.\nWhat we intended to convey is that the durations of hold (i.e., hold times) are labeled\nbeneath each corresponding hold event in the figure. We believe this helps readers\nclearly know the sequence of hold period. To clarify this, we have revised the caption\nas follows (Line 89):\n“Hold times (t ) are labeled below each corresponding hold.”\nh\nLine 95 – “… indicating that β has”\nDone\nLine 100 – “performed” rather than “perform”\nDone\nLine 102 – “… Extended Data (Figure 4). This observation”\nDone, now it has changed to Supplementary Fig. 4.\nLine 106 – “performed” rather than “perform”\nDone\nLine 111 – delete “both”\nDone\nLine 115 – “healing rate, we conducted an additional SHS experiment LHR3240 on\npure andesine gouge. The”\nDone\nLine 120 – “… rock-forming minerals of gabbro. …”\nDone\nLine 123 – “… at all temperatures investigated …”\nDone\nLine 135 “… time-dependent, negative …”\nDone\nLine 144 – “feldspars investigated (hollow symbols)… “\nDone\nLine 154 – “… water (Figures 1-3). Thus, the negative …”\nDone\nLine 155 – “… impact the seismic cycle. This frictional”\nDone\nLine 161 – “A similar negative β value has been reported recently for granite …,”\nLine 162 – delete “hypothetically”\nDone\nLine 171 – “formed clay at original silicate mineral surfaces may contribute to the\nnegative healing …”\nDone\nLine 172 – “Time-dependent fault …”\nDone\nLines 182- 183 – “long holds for felspar-rich rocks we must conclude that the frictional\nstrength Δτ decreases during long holds.”\na\nDone\nLine 186 – “difficult to determine the shear strength of individual nano- to micro-\ncontacts from the overall”\nDone\nLine 189-190 – “(AFM) and an individual mineral grain, either feldspar or pyroxene,\nmounted to the AFM cantilever, measuring the force as the individual mineral in contact\nwith the substrate of the same type of mineral is displaced at room temperature under”\nDone\nLine 198 – delete “although”\nDone\nLines 199-200 – “… (blue-shaded in color line) but many of the data have lower values,\nand in some cases ΔF/Fss (pink …”\nDone\nLines 201 -202 – “domain, Figure 4b) is negligible. Although the “usual” frictional\nincrease in healing with log (th) is detected occasionally, this healing is markedly\nsuppressed. Similar behavior is evident”\nDone\nLine 206 – “find that conventional frictional healing is more apparent than in flujid-\npresent SHS tests (Figure 4b). Due”\nDone\nLines 208-215 – “… pressurized water. However, we are still able to observe the\nsuppressed frictional healing at room temperature if sufficient SHS tests are performed\nto improve measurement statistics. Individual contact healing (ie. ΔF/Fss up to ~6, Fig.\n4b) is more pronounced than the equivalent quantity, Δμ/μss of ~0.1 for multiple-\ncontact macro-scale rotary shear experiments (Fig. 1) and values reported previously\nfor nano-scale SHS experiments16,38. Nevertheless, the negligible friction healing for\nthe feldspar-feldspar contacts after long hold times at high relative humidity is\n29\nconsistent with our results for macro-scale gouge …”\nThank you, change made.\nLine 234 – delete “in color”\nDone\nLine 235 – delete “in color”\nDone\nLine 236 – “… diagram …”\nDone\nLine 243 – “… healing may potentially influence the earthquake mechanism and”\nDone\nLine 250 – “Although the experiments discussed …”\nDone\nLines 253-254 – “… remains challenging. By comparison with conditions of natural\nfault systems, (1) the experiments presented here are of short duration and reaction\nkinetics are too slow to observe …”\nDone\nLine 260 – “may lead to a decrease in shear strength of faults …”\nDone\nLines 363 – 364 – “Etna basalt, diorite, and orthoclase and albite. The mineralogical\nmake up of the rocks are presented in Extended Data Table 1, based on XRD data. The\nsamples are crushed …”\nDone\nLine 366 – “… performed with a Low to High …”\nDone\nLine 367 – “… with a hydrothermal pressure vessel …”\nDone\nLine 370 – “…under nominally “dry” or with a pore fluid pressure …”\nDone\nLine 381 – “… is heated to target temperature. Heating normally take …”\nDone\nLine 388 – “The original rate (V = 10 μm./s). …”\nDone\nLine 391 – “… by the machine to the gouge layer …”\nDone\nLine 392 – “… revolution is less than 1 rpm …”\nDone\nLine 398 – “… using a LVDT (1 μm”\nLine 399 – “… column of the machine. No”\nDone\nLine 405 – “… feldspar/pyroxene particles, composed of the same materials as in rotary\nshear”\nThank you, our intention was to emphasize that the feldspar/pyroxene particles used in\nthe AFM experiments were directly extracted from the same samples used in the rotary\nshear experiments. To clarify this point, we have revised the sentence as follows (Lines\n419-420):\n“The feldspar/pyroxene particles, separated from the samples used in rotary shear\nexperiments, are attached to tipless AFM probes.”\nLine 407 – delete “approximately”\nDone\nLine 408 – “… solid gabbro substrate ins polished …”\nDone\nLine 410 – please specify what kind of plasma\nWe used air plasma, and it has been added in the context (Line 417).\nLine 412 – “installed at the Tsinghua …”\nDone\nLine 418 – “… lateral displacements). The slide-hold-slide procedure is then conducted …”\nDone\nLine 420 – “1 mm at the original velocity. The hold time is …”\nDone\nLines 424-425 – “… which is the ratio of the increment of frictional force during the\nhold stage, and the steady-state friction Fss after the hold, called ΔF.\nThank you for your suggestion. The ratio we describe is ΔF divided by the steady-state\nfriction force F after the hold. We have revised the sentence as follows (Lines 440-\nss\n441):\n“…, which is the ratio of the increment of friction force during the hold stage (ΔF) and\nthe steady-state friction (F ) after the hold.”\nss\nLine 431 – “holder that lacks any diffraction lines …”\nDone\nLine 435 – “… Solaris Field-Emission SEM of the Department”\nDone\nAndreas Kronenberg\nWe sincerely thank Prof. Kronenberg for his careful review and positive assessment on\nour manuscript.\nReference\nFeng, W., Yao, L., Gomila, R., Ma, S., Pennacchioni, G. & Di Toro, G. Slip-dependence of fault\nfrictional stability under hydrothermal conditions. Geophys. Res. Lett. 51, e2024GL108525 (2024).\nWe would like to express our gratitude to all three reviewers for their time and\nconstructive comments on our manuscript. Below, we provide our point-by-point\nresponse (in blue font) to the reviewers’ comments (in black font). Line numbers refer\nto those in the clean version of the revised manuscript.\nIn this revised paper the authors have taken significant steps to improve the manuscript.\nI do think that more revisions are necessary, but I do appreciate the authors efforts to\nthis point. I believe that with the proper attention all issues can be resolved.\nThank you for your constructive comments. We have tried our best to revise the\nmanuscript and we believe that all the issues have been addressed accordingly.\nIn general, because of the definition of healing as a positive increase in frictional\nstrength, even when the rate is decreasing, I think the authors have to be very careful to\navoid mentioning “weakening”, or language that could be interpreted as weakening (as\ndiscussed previously). There are still some instances in the text where this needs to be\nfixed, with more precise language (e.g. L244, L263, L267; there may be more). We are\nreally talking specifically about healing values that are lower relative to an expected\nextrapolated value under given conditions. “Reduced” or “suppressed” healing is better,\nbut maybe something like “reduced-rate” or “suppressed-rate” would be better? I say\nthis because the feldspar healing is reduced only relative to its own projection; Fig, 4\nshows that the feldspar healing values are not even reduced to be lower than the log-\nlinear pyroxene values. I suggest showing the shading from the pyroxene data also on\nthe feldspar plot (without the data points) so this comparison can be clearly made.\nWe have carefully reviewed the manuscript to ensure the use of the proper terminology\nthroughout the main text. As suggested, we have made corrections in Lines 263 and267.\nAs the meanings are clear and non-misleading in the context, we would like to keep our\nstatements as “reduced” and “suppressed” healing, instead of “reduced-rate” or\n“suppressed-rate”.\nWe would also like to keep the sentence in Line 244 “Therefore, the suppressed friction\nhealing effect during the longer hold times must be caused by a reduction in the shear\nstrength of the micro-grain contacts.” as it is. This description echoes the sentence in\nLine 207 “Since the increase in ΔAr cannot account for the decrease in Δμ during the\nlong holds for feldspar-rich rocks, we must conclude that the frictional strength Δτa\ndecreases during long holds.” Both of these sentences discuss the possible reduction in\nshear stress at the asperity contact (rather than in macroscopic shear strength), which\ncompetes with the increase in real area of contact acting as a mechanism for the\nsuppressed healing.\nWe thank the reviewer for the suggestion regarding the data presentation in Figure 4.\nrevised manuscript to more clearly describe how we define the “normal” healing (the\nblue-shaded area) and the “suppressed” healing (the pink-shaded area) in Figure 4.\nWe hope that the revised manuscript will better present our ideas with causing confusions\nto the readers (Lines 224-236).\n“To better quantify the healing behaviors of the feldspar-feldspar contacts, we\ncategorize the frictional healing data into two regimes, i.e., the “normal” healing and\nthe “suppressed” healing. To do that, we first estimate the scattering of the frictional\nhealing data of the pyroxene–pyroxene contacts as a reference scattering, as indicated\nby the width of the blue-shaded area in Fig. 4b. Then we find the trendline of the\n“normal” healing behavior for the feldspar-feldspar contacts by fitting the maximum\nfrictional healing values under different hold times using a log-linear fit. Finally, we\nobtain the normal healing regime (denoted by the blue-shaded area) by considering the\n“normal” trendline with a same scattering as the pyroxene–pyroxene contacts.\nEssentially, this “normal” healing regime for the feldspar-feldspar contacts is defined\nas the projection of the normal log-linear healing behavior while considering a\nscattering due to uncertainty of the measurement system. Clearly, for the frictional\nhealing measured in RH~ 80%, many of the data points do not fall into the “normal”\nhealing regime. For those data points that lie below the “normal” healing regime, we\nclassify as the “suppressed” healing regime (denoted by the pink-shaded area, Fig. 4b).”\nThe last two title suggestions are better (I don’t think the title with “weakens” should\nbe used), but could more accurately reflect the results of the study. Maybe something\nlike “Reduced healing rates in feldspar under hydrothermal conditions”. Since the\nreduced healing rates only occur under specific conditions, this is important to mention.\nI also suggest tightening up the parameter definitions with more precise language and\nterminology. In the introduction, I suggest that the authors clearly distinguish between\nthe conventional log-linear healing rate, a reduced rate but still positive healing (which\nis what the authors observe here), and true weakening, which is a rare and extreme case\nof reduced healing.\nWe respectfully acknowledge the reviewer’s suggestion regarding the title. We have\nchanged the title to “Feldspar reduces fault frictional healing rate under\nhydrothermal conditions”\nWe also revised the main text to introduce the three types of frictional healing behavior\n3\nin the Introduction section (Lines 49-53), as suggested.\n“However, the results of recent experiments also conducted on Westerly granite, but\nunder hydrothermal conditions, cast doubt on the occurrence of this log-linear\nfrictional behavior: at T = 200 °C, frictional healing decreases, rather than increases,\nwith log(t ) (i.e., negative healing rate), although the healing is still positive within the\nh\ninvestigated hold times. But, at T= 250 °C, the frictional healing becomes negative\nwhen hold time is longer than a threshold 26.”\nIt is good that the authors now discuss an “individual” healing rate, but it is confusing\nto still use the parameter β. I think it is better to leave β defined as a “conventional”\nhealing rate fit over multiple points, and to use a subscript, or better yet, a whole\nnew variable – I’ll call it “X” (but the authors can choose whatever they want), where\nX = an individual Δµ/Δ log t measurement in this paper. This is an advantage for the\nauthors because it highlights the “novel” aspect of the measurements, distinguishing them\nbetter from the conventional β used in previous work.\nThank you for the suggestions. We have revised the main text correspondingly (Lines\n83-90).\n“The rate of healing (β) is conventionally determined as the slope of Δ versus Δlog (t )\n10 h\nover the entire dataset, expressed as β=Δ / Δlog t 19. Since some of the datasets in\n10 h\nour study cannot be adequately described by a single log-linear relation, we adopt\nindividual healing rates (βloc), determined within intervals of monotonic change, to\nexplicitly capture the time-dependent evolution of β. Typically, two βloc values are\nsufficient to describe the change of Δ with hold time. A positive βloc indicates that\nfrictional healing increases with hold time, whereas a negative βloc indicates that\nfrictional healing decreases with hold time.”\nI respect the authors efforts in evaluating healing of stick-slipping material, which is\nof course not straightforward. Given the two choices (average vs peak friction), I agree\nwith the authors choice of using the peak friction values as some sort of strength\nmeasurement; the “average friction method” has no physical basis and is subject to\ninaccuracies via stiffness. The authors call the measurement an “equivalent Δµ” (I think\n“apparent Δµ” would be better), but then use “Δµ” for everything anyway, so that the\nreader still can’t distinguish Δµ picked from steady-state sliding, from Δµ picked from\nstick-slip sliding. I think the authors need to clearly separate these two measurements\n– I’ll call it “Y”, where “Y” is an individual healing point picked from stick-slipping\ndata (and “X” is from steady-state data). The plots then should use different symbols to\n4\nshow the “X” and “Y” data points to clearly distinguish them (maybe with Y being open\nsymbols, or circled symbols or something like that). This has the added advantage of\nshowing which experiments showed stick-slip and under what conditions, which is\ninformation that is lost in the current figures.\nWe have now revised the main text (Lines 79-83) and modified the Figures 1-3 to\nclearly distinguish the two measurements of frictional healing (Δ and Δpp with solid\nand open symbols, respectively).\n“Typically, frictional healing Δ is measured as the difference between peak  value\n() upon re-sliding and steady state friction ( ) before the hold. However, in the cases\np ss\nof stick-slip, a well-defined  cannot obtained. We adopt the peak friction during the\nss\nstick phase before the hold as a reference point. The apparent Δ is then measured as\nthe difference between the peak  value of the stick-slip events after and before the\nholds (see Method; referred to as Δpp, Fig. 1a inset).”\nI like the approach the authors use in the Rebuttal Fig. 5, but I think it should be taken\na step further. The authors have evaluated the 1000 second hold from 2/8 SHS\nsequences. Why not evaluate the 1000s hold from all 8 sequences? This is a trivial\namount of work. For 8 sequences, you should get a random distribution of starting\npoints within the loadup curve (where Sequence 3 appears to be something like 80% to\npeak, and Sequence 5 around 10%). If the distribution isn’t great, you can always do\nthe same thing with a different hold time. You can then plot Δµ as a function of position\nin the loadup curve; if it does not affect the data, there should be no trend and a linear\nregression will show an extremely low R2 value. This should be put into the supplement,\nand I think this will provide the necessary support for using the “peak picking” method.\nThank you for the suggestions. We have plotted (apparent) frictional healing as a\nfunction of hold-starting position within the loadup curve, showing that there is no clear\nrelation between Δand the position where the hold happens to start. This figure has\nbeen added in the supplementary material as Supplementary Figure 11.\nIn the last line of the abstract and in the last section of the paper, the authors touch on\nthe implications for the seismic cycle, but this is somewhat vague and can be expanded\non. The real problem is, without a consistently straight log-linear trend, you can no\nlonger extrapolate the curve from lab timescales to earthquake timescales as in previous\nwork (the Marone/Vidale papers from the ‘90s). This is important (in my opinion).\nWe agree with the reviewer that, without a strict log-linear or any other definite\nrelationship between frictional healing and hold time, extrapolation from the\nexperimental results to natural seismic timescales is not valid. Our dataset clearly\nsuggests that the presence of feldspar (~ 40 wt% of feldspar according to the data in\nFigure 3) may reduce the frictional healing rate under certain hydrothermal conditions.\nHowever, we are keenly aware that the actual conditions in natural fault zones are far\nmore complex than those simulated in our experiments. How other factors, such as fluid\ncomposition, fluid pressure, effective normal stress, may influence the healing\nprocesses under hydrothermal conditions remains unknown. Nevertheless, the\nsuppressed healing associated with feldspar that we observed may contribute to the\nvariations and complexities of healing processes in natural faults, and may even become\npredominant under certain conditions. As discussed in the first round of revision, a\nreduced frictional healing rate indicates that the faults regain their strength more slowly\nafter coseismic slip. Assuming that tectonic loading is constant, this may lead to a\npossible scenario in which the shear stress acting on the fault overcomes fault strength\n6\nat a lower failure threshold compared to a fully healed cases, thereby triggering failure\nearlier in the loading cycle. As a consequence, we interpret the results with lots of\ncaution and mainly emphasize the possible variations and complexities in fault strength\nrecovery processes based on our experimental results. We believe that these discussions\nand implications are measured and reasonable.\nWe have slightly revised the main text.\nLines 31-33: “Given the abundance of feldspars in the Earth’s crust, their peculiar\nhealing behavior under conditions resembling those in our experiments can lead to\nunexpected variations in fault strength, changing our understanding of the fault\nmechanics and the seismic cycle.”\nLines 287-291: “Therefore, this healing behavior may potentially influence the seismic\ncycle in feldspar-bearing faults. For instance, the decrease in frictional healing and\nhealing rate will 1) reduce the rate of the elastic strain energy stored by wall rocks and,\nif the stress loading rate remains constant, 2) the recurrence interval of earthquakes\nalong this fault may become shorter, and subsequent earthquakes may occur at a lower\nshear stress level44.”\nSpecific Comments:\nL23: does not hold *under certain (hydrothermal) conditions*\nChange made.\nL68: two *identical* SHS sequences (right?)\nDone\nL80: as discussed above, here you have to distinguish between the different types of\nhealing picks. Clarity is more important than simplicity here.\nWe have revised the main text and figures correspondingly (Lines 82-83).\n“The apparent Δ is then measured as the difference between the peak  value of the\nstick-slip events after and before the holds (see Methods; referred to Δpp, Fig. 1a inset).”\nL269: Is there any evidence of pressure solution in these AFM experiments?\nWe did not find any direct evidence of pressure solution in the AFM experiments and\nthe rotary shear experiments. We do not expect the pressure solution to occur at room\ntemperature for feldspars and feldspar-rich rocks.\nThe manuscript presents an interesting set of experiments and the authors have done a\ngood job of responding to my comments and those of other reviewers. However, there\nis some information the authors provide in their response that really needs to be\nincluded in the manuscript and there are some parts that still need minor clarification.\nLine 99-100: wording here is awkward. I would suggest: “Under dry conditions, 𝛽 is\npositive and 𝛥μ generally increases monotonically with log th except for a slight\n10\ndeviation…”\nThank you, we have revised the sentence as suggested (Lines 107-109).\n“Under dry conditions, β is positive and Δ generally increases monotonically with\nlog t except for a slight deviation from the log-linear trend at T =300 ℃ and 400 ℃\n10 h\nat longer hold times ( 1000 s) (Fig. 1b).”\nLine 227 & 235: Authors edits didn’t really improve this. The wording they are using\nneeds to come with some quantification. They say there is a certain probability of\nobserving suppressed friction but do not indicate what that probability is. The wording\nsuggests the authors have the numbers but they do not provide them. Is it more or less\nprobable to see suppressed healing than generally anomalous data? Suppressed healing\nisn’t consistently observed in every test but is it statistically significant?\nWe thank the reviewer for pointing out the need for more rigorous clarification. We\nhave added explanations to more clearly describe how we define the “normal” healing\n(the blue-shaded area) and the “suppressed” healing (the pink-shaded areas) in Figure\n4 (Lines 224-236). We have also provided quantitative information on the occurrence\nof the suppressed healing in the feldspar-feldspar contacts (Lines 237-240).\nLines 224-236: “To better quantify the healing behaviors of the feldspar-feldspar\ncontacts, we categorize the frictional healing data into two regimes, i.e., the “normal”\nhealing and the “suppressed” healing. To do that, we first estimate the scattering of the\nfrictional healing data of the pyroxene–pyroxene contacts as a reference scattering, as\nindicated by the width of the blue-shaded area in Fig. 4b. Then we find the trendline of\nthe “normal” healing behavior for the feldspar-feldspar contacts by fitting the\nmaximum frictional healing values under different hold times using a log-linear fit.\nFinally, we obtain the normal healing regime (denoted by the blue-shaded area) by\nconsidering the “normal” trendline with a same scattering as the pyroxene–pyroxene\n8\ncontacts. Essentially, this “normal” healing regime for the feldspar-feldspar contacts\nis defined as the projection of the normal log-linear healing behavior while considering\na scattering due to uncertainty of the measurement system. Clearly, for the frictional\nhealing measured in RH~ 80%, many of the data points do not fall into the “normal”\nhealing regime. For those data points that lie below the “normal” healing regime, we\nclassify as the “suppressed” healing regime (denoted by the pink-shaded area, Fig. 4b).”\nLines 237-240: “Taking the data set of 100 s hold time as an example (10 points in\ntotal), only one of the data points lie in the normal healing regime while the 9 data\npoints are in the suppressed healing regime. Similar behavior is more evident for the\ndata sets of 300 s and 700 s long holds, where 9 out of 10 data points and 10 out of 10\ndata points are in the suppressed healing regime, respectively.”\nLines 249-251: “However, we are still able to observe the suppressed frictional healing\nat room temperature in the feldspar-feldspar contacts, e.g. in 9 out of 10 data points for\nt = 300 s (Fig. 4b).”\nh\nDiscussion of possible water bridges forming in the AFM test (response to reviewer 1)\nshould be added to the text. As the authors are hypothesizing that the AFM data suggests\na water-assisted process they need to provide a reason why there would be liquid water\npresent. Also, I assume “nominally dry” in the rotary shear tests means room humidity.\nCan the authors provide an average or range in humidity this term encompasses?\nThis would be useful for comparison to the AFM tests.\nWe have added the description of water bridge in the main text (Lines 268-269).\n“...its close relation with humidity (a water bridge between the AFM tip and the\nsubstrate was likely to form under relatively high humidity41,42), we speculate that the\nprocess may be thermally activated, water-assisted and chemical in origin.”\nThe “nominally dry” experiments in the rotary shear tests were performed at\ntemperatures  100 °C (i.e., 100, 200, 300 and 400 °C). Since the pressure vessel is\nvented to the room atmosphere in these dry experiments, saturation vapor pressure\nwithin the vessel increases dramatically with temperature (e.g., 101.3 kPa at 100 °C and\n8581 kPa at 300 °C). Considering that the relative humidity at room temperature in\nBeijing, China is ~ 40-60% (water vapor partial pressure ≈1.27–1.90 kPa), the relative\nhumidity within vessel should decrease to less than 2% at T  100 °C. We have added\nthe corresponding description (Line 71).\n“Experiments are performed under an effective normal stress (σ = -P ) of 50 MPa,\neff n p\n9\na loading velocity of 10 m/s, temperature (T) ranging from 25 to 400 ℃ and pore fluid\npressure (P ) of 0 (nominally dry, relative humidity 2% at T 100 ℃) or 30 MPa.”\np\nInformation about when FESEM samples were collected (in response to reviewer 2)\nshould be added to the methods. This information is needed for understanding and\ninterpretation of the images.\nThank you for the suggestions, we have added corresponding description in the Method\nsection (Lines 438-441):\n“At the end of the experiment, the deformed samples are collected for micro-analytical\n(XRPD and FESEM) investigations. Note that the samples required a cooling period of\napproximately 1 hour for T = 200 °C before being removed, this process might have\npotentially modified the microstructures.”\nReviewer #3 (Remarks to the Author):\nThe authors have taken all of my comments into account in this revised manuscript, and\nI see that they have seriously addressed the concerns of the other reviewers. This is very\nimportant work, and should be of interest to a wide range of geoscientists studying fault\nslip and earthquakes. The manuscript presented the results of these experiments and\ntheir significance the first time I read it and it is improved here. I recommend acceptance\nfor publication in Nature Communications without hesitation.\nWe thank Prof. Kronenberg for the positive assessments on our manuscript.\nWe would like to express our gratitude to the reviewer for his/her time and constructive\ncomments on our manuscript. Below, we provide our point-by-point response (in blue\nfont) to her/his comments (in black font). Line numbers refer to those in the clean\nversion of the revised manuscript.\nReviewer #1 (Remarks to the Author):\nThe paper is nearly ready; only a few problems still remain. I am sorry to keep having\nto repeat the same thing, but some text still needs fixing.\nFrom the response letter:\nWe would also like to keep the sentence in Line 244 “Therefore, the suppressed friction\nhealing effect during the longer hold times must be caused by a reduction in the shear\nstrength of the micro-grain contacts.” as it is. This description echoes the sentence in Line\n207 “Since the increase in ΔAr cannot account for the decrease in Δµ during the long holds\nfor feldspar-rich rocks, we must conclude that the frictional strength Δτa decreases during\nlong holds.” Both of these sentences discuss the possible reduction in shear stress at the\nasperity contact (rather than in macroscopic shear strength), which competes with the increase\nin real area of contact acting as a mechanism for the suppressed healing.\nAll the “Δµ” values from the AFM measurements are positive (i.e. positive healing). If Δµ is\npositive, there cannot be a reduction in shear strength.\nThe argument on Line 207 only makes sense if the authors mean “contact quality” rather\nthan “shear strength”. As correctly noted earlier, contact strength depends on both the\nquantity and quality of the contact area. If the area increases and the healing does not,\nit means that the contact quality is not keeping up with the area increase (but note: it\nalso does not mean that the quality is necessarily decreasing). So the contact quality, or\nthe portion of the strength that depends on contact quality could decrease, but the\noverall shear strength cannot.\n(note: this is now on Line 266)\nWe thank the reviewer for this clear explanation. We agree that the suppressed healing\nbehavior observed must be related to changes in the growth rate of the contact quality\nrather than a reduction in the bulk shear strength. We have now changed the relevant\ntext correspondingly.\nr\nthe long holds for feldspar-rich rocks, we must conclude that the growth rate of the\ncontact quality Δτ decreases during long holds37.”\na\nLines 267-269: “Therefore, the suppressed friction healing effect during the longer hold\ntimes must be caused by a reduced growth rate in contact quality, relative to short holds,\nof the micro-grain contacts.”\nFurthermore, the text on Lines 264-267 doesn’t make sense to me. According to\nconventional (Dieterich-type) healing systematics, if the contact area does not change\nover time, there should be no healing. Isn’t this consistent with your observations of\nsuppressed healing? There is no reason why the shear strength of the contacts needs\nto decrease, considering that (1) the Δµ values are still positive, and (2) the contact\narea isn’t increasing anyway. Actually, in this case it seems that the shear strength needs\nto somehow increase via contact quality to get the still-positive healing values.\nWe apologize for the unclarity in the original text, which may have led to\nmisunderstanding. We fully agree that, in our AFM system, the contact quality between\nthe feldspar-feldspar contacts must to increase to get positive healing values during all\nholds, given that the contact area has negligible contributions to the healing. However,\nthe observation of suppressed healing values at long holds (pink-shaded domain, where\nlonger holds yield a relatively smaller healing) suggests that the contact quality does\nnot maintain a fixed growth rate with time during holds, but have to decrease to result\nin a lower healing at the end of long holds. We have revised the text accordingly.\nLines 267-269: “Therefore, the suppressed friction healing effect during the longer hold\ntimes must be caused by a reduced growth rate in contact quality, relative to short holds,\nof the micro-grain contacts.”\nI don’t think you should use the term “normal” healing. I get what you mean, but\n“normal” refers to something perpendicular to the shear surface. In the other sense of\n“normal” that implies that there is some sort of existing value that is considered\n“normal”, and that isn’t really the case. I suggest you use “predicted” or “projected”\nor “expected” healing.\nWe have replaced the word “normal” healing with “expected” healing.\nFrom the response letter:\nLines 31-33: “Given the abundance of feldspars in the Earth’s crust, their peculiar\nhealing behavior under conditions resembling those in our experiments can lead to\nunexpected variations in fault strength, changing our understanding of the fault\nmechanics and the seismic cycle.”\nLines 287-291: “Therefore, this healing behavior may potentially influence the seismic cycle\nin feldspar-bearing faults. For instance, the decrease in frictional healing and healing rate\nwill 1) reduce the rate of the elastic strain energy stored by wall rocks and, if the stress\nloading rate remains constant, 2) the recurrence interval of earthquakes along this fault may\nbecome shorter, and subsequent earthquakes may occur at a lower shear stress level44.”\nThese changes are so slight that they did not actually address the comment. I understand\nthat faults under real conditions are complex, and it is good to be conservative with\npredictions here. Previous studies (e.g. the aforementioned Marone/Vidale papers) have\nused lab-derived log-linear healing rates to “predict” recurrence rates on real faults\n(albeit with some very large assumptions). My point is, your data suggest that such\nprojections cannot be made for feldspar under hydrothermal conditions, which is\nactually consistent with your message of being conservative with the interpretations.\nThanks for the suggestions. We have revised the main text corresponding.\nLines 30-34: “Given the abundance of feldspars in the Earth’s crust, their peculiar\nhealing behavior under hydrothermal conditions can lead to unexpected variations in\nfault strength, changing our understanding of fault mechanics and the seismic cycle,\nand challenging the direct extrapolation of laboratory-derived healing parameters to\nthe prediction of earthquake recurrence intervals in nature.”\nLines 290-292: “Our results suggest that extrapolating laboratory-derived healing\nparameters to predict the recurrence interval of earthquakes in feldspar-rich fault zones\nunder hydrothermal conditions should be treated with caution, unlike the direct\napplications made in previous studies11.”\nLine 28: healing rate. Or delete “negative”\nDone, “negative” has been deleted.\nLine 87: do you mean one value of βloc?\nWe refer to the local healing rate (βloc) as the slope of Δμ versus Δlog (t ) determined\n10 h\nwithin a specific time interval where the data show a monotonic trend. Because most\ndatasets in this study exhibit a change from time-dependence increase to decrease of\nΔμ, we typically divide each dataset into two intervals and thus determine two βloc\nvalues per experiment.\nLine 86-89: “Since some of the datasets in our study cannot be adequately described\nby a single log-linear relation, we determine individual healing rates (βloc) within\n3\nintervals of monotonic change to explicitly capture the time-dependent evolution of β.\nTypically, two βloc values are sufficient to describe the change of Δ with hold time.”\nLine 177: only βloc, not β\nChange made\n4"}]

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