Signatures of the sub-Rayleigh to supershear fracture transition in snow avalanche experiments

Domain: physics
Published: 2025-12-16
Editorial outcome: minor_revision

Abstract
Snow slab avalanches occur when a crack propagates within a highly porous weak snow layer buried beneath a cohesive snow slab. Here, we report direct observations of a supershear event in snow fracture experiments following the spontaneous transition from sub-Rayleigh to intersonic crack propagation. The experiments involve artificially triggered avalanches on a small slope with a natural snow pack, captured with high-speed cameras and analyzed using digital image correlation. Deformation fields reveal distinct signatures: slope-normal collapse of the weak layer and slab flexure drive sub-Rayleigh propagation, while supershear fracture is related to slope-parallel deformation and slab tension. These results are further reinforced by numerical simulations that replicate the experiment and provide strong supporting evidence that the Burridge-Andrews mechanism governs the transition to supershear propagation. Analogous to supershear strike-slip earthquakes linked with substantial magnitudes, our findings suggest that supershear avalanches relate to widespread crack propagation and large avalanche dimensions, holding significant implications for risk mitigation strategies.

Introduction
The release of a dry-snow slab avalanche (Fig. 1a) follows a sequence of failure and fracture processes1. An initial failure is initiated in a weak layer of snow buried beneath a cohesive snow slab. This leads to the formation of a crack which, under certain conditions, can propagate across the slope, leading to loss of support and sliding of the slab in steep terrain2. The propagation mechanism was initially understood in terms of strain-softening-induced shear fracture3,4. As this model did not explain crack propagation in low-angle terrain and remote avalanche triggering, Heierli et al.5 extended the anticrack concept – initially proposed to explain the failure process in deep earthquakes6, also suspected in firn and snowquakes7,8 – to the avalanche release process. Based on experimental evidence9–12, their model accounted for the volumetric collapse of the weak layer, which leads to closure of crack faces under normal slab loading. This enables the slab to flex, generating stress concentrations at the crack tip, thus facilitating propagation even in the absence of a driving shear force. In such a propagation mode, the crack speed should theoretically be limited by the Rayleigh wave speed13. The debate between the two proposed mechanisms, shear versus anticrack, has sparked intense discussion over the past three decades14–16.

In 2014, Hamre et al.17 reported, based on indirect measurements from avalanche videos, fracture speeds exceeding 200 ms−1, surpassing the Rayleigh wave speed of the slab, adding a new dimension to the discussion. These findings appeared contradictory, not only exceeding theoretical bounds but also all values obtained from contemporary state-of-the-art fracture tests which consistently yielded sub-Rayleigh speeds below 60 ms−1, typically10,16,18,19. Recently, Trottet et al.20 conducted numerical simulations indicating that the latter discrepancy could be explained by a sub-Rayleigh to supershear transition following the Burridge-Andrews mechanism21,22. They argue that current state-of-the-art fracture tests were too short to capture this transition towards intersonic fracture speeds. Yet, while this numerical study proposed an indirect validation based on analysis of avalanche videos23, direct evidence of intersonic – or supershear – fracture in snow avalanches remains non-existent. In particular, the detailed deformation mechanisms occurring around the weak layer are still unknown.

The supershear transition in slab avalanches may have crucial implications for the associated risk because it is thought to affect the avalanche release volume and thus its destructive potential24–26.

To investigate these processes, and inspired by experimental evidence of supershear fracture in laboratory earthquakes27 and friction experiments28,29, we designed controlled avalanche experiments on a small snow slope. This real-scale experimental setup allows us to explore the existence and characteristics of supershear fracture in avalanche release under safe and controlled conditions.

In this work, we report direct observations of a transition from sub-Rayleigh to supershear crack propagation. High-speed imaging combined with digital image correlation reveal clear kinematic signatures accompanying this transition, evolving from weak layer collapse and slab flexure during sub-Rayleigh propagation to slope-parallel tension during supershear rupture. Complementary numerical simulations reproduce these dynamics and provide strong evidence that the Burridge-Andrews mechanism governs the onset of supershear propagation.

Results
Snow avalanche experiments
Supershear experiments in snow are extremely challenging, as they require a highly unstable snow pack and a safe, accessible slope, conditions that rarely coincide. Even under favorable conditions, long propagation tests rarely succeed, especially for distances of 10 meters or more30. The successful experiments presented here followed years of unsuccessful attempts and extensive site preparation to preserve the weak layer and ensure favorable test conditions.

The snow fracture tests involved experimental avalanches triggered on a small snow slope, where a buried weak snow layer was identified in a manual snow profile31. The experimental setup adhered to the Propagation Saw Test (PST) standards defined by Greene et al.32, except using a 10 m sample length (Fig. 1b, c, and Supplementary Movie SM1). This, combined with the steep slope of 37∘, ensured that the conditions for supershear fracture as reported in Trottet et al.20 are met. An artificial crack of increasing size was introduced by sawing into the weak layer until unstable crack growth sets in (Fig. 1b, c). The open sidewall of the PST was speckled with black ink, and a high-speed camera was used to record the speckled wall (full-view perspective). In addition to the full-view, two close-ups focusing on the weak layer were recorded at a distance of about 2 and 8.5 m away from the trigger point (Fig. 1c). Digital Image Correlation (DIC) analysis and camera distortion correction of the high-speed recordings were performed as described by Bergfeld et al.33. This allowed us to compute the tangential and normal displacement fields u(t) and w(t), respectively, and to calculate crack propagation speeds extending the methodology outlined in Bergfeld et al.33, as detailed in the Methods Section.

Supershear fracture
Three side-by-side fracture experiments were performed at an interval of approximately 2.5 hours. In all tests, the crack propagated the first meters (x ≲ 5 m) at a speed of around 50 ms−1 (Fig. 2a, d, Supplementary Figs. 5 and 6), estimated from the three full-view perspectives as well as close-up #1. In PST #1 and PST #2, crack propagation in the weak layer was affected by slab fractures at distances of approximately 5.5 m, from the bottom of the test column (see Methods and Supplementary Fig. 5). In contrast, in PST #3, the absence of slab fractures allowed the crack in the weak layer to transition into a supershear propagation regime (Fig. 2). Based on tangential displacement analysis from the full-scale video, we observed a transition from a crack speed of approximately 50 ms−1 to around 130 ms−1 after a propagation distance of about 6 m. We conservatively estimated the slab shear wave speed by using the highest values of existing empirical formulations derived from snow acoustic measurements34 and dynamic loading35 (see Supplementary Fig. 7). With these estimates (65 and 78 ms−1) and after model-based verification of crack propagation speeds in layered slab systems (see Supplementary Note 3 and Supplementary Fig. 9 therein), we can confidently categorize the measured speed of approximately 50 ms−1 as sub-Rayleigh and the measured speed of about 130 ms−1 as intersonic, indicating supershear fracture. The analysis of the spatio-temporal evolution of displacement at the close-up location further strengthens this conclusion, offering detailed insights into the deformation mechanisms driving this transition. At the close-up #1, speed evaluations based on both tangential and normal displacements yielded similar crack speeds of around 40 ms−1. This supports the hypothesis that, in the sub-Rayleigh regime, the crack propagates as a mixed-mode anticrack16,20,36. However, at close-up #2, speed evaluation based on tangential and normal displacements differed. Only the analysis based on the tangential displacement led to intersonic speeds as also observed in the full-view (the w-based speed was around 80 m/s). This aligns with the assumption20 that, in this regime, the crack propagates as a shear (mode II) crack, in which volumetric collapse of the weak layer and normal displacement of the slab are uncoupled from the crack tip.

Deformation mechanisms
Through displacement analysis (Fig. 3), a distinct crack driving mechanism is discerned at each location along the column. At the lower part of the column (close-up #1 location), the predominant mechanism driving crack propagation is slab bending induced by volumetric collapse of the weak layer (see also Supplementary Movie SM2). This is evidenced by the downward-pointing displacement arrows in the full-view analysis (Fig. 3a, blue arrows), the spatial distribution of normal displacement (Fig. 3b), and localized strain patterns, which pinpoint the position of the crack tip (Fig. 3c). It is noteworthy that shear strain analysis did not reveal any discernible localization pattern, in contrast to the normal strain (see Supplementary Fig. 4d). This could be expected since crack propagation in this regime is driven by weak layer collapse and therefore slope-normal displacement.

Conversely, at the upper end of the column (location of close-up #2), both full-view and close-up displacement analyses indicate that the primary crack driving mechanism is slab tension resulting from shear failure of the weak layer (see also Supplementary Movie SM3). This inference is drawn from the slope-parallel orientation of the displacement vectors in the full-view analysis (Fig. 3a, red arrows), as well as the tangential displacement field (Fig. 3d) and the visualization of shear strain localization patterns, which delineate the crack in the weak layer (Fig. 3e). No distinct localization pattern was observed in the normal strain analysis (Supplementary Fig. 5i).

The preceding observations are further clarified by the relative displacement between the forming crack faces by analyzing the relative normal displacement (Δw) as function of the relative tangential displacement (Δu) at the close-up locations (Fig. 4). At close-up #1, the initial displacement predominantly aligns with the slope-normal direction, indicating a crack-closing mechanism (Fig. 4, blue trajectory). Conversely, at the location of close-up #2, the relative displacement primarily aligns with the slope-parallel direction, with a minor negative slope-normal component, indicative of slight dilation (Fig. 4, red trajectory). Note that weak layer collapse still occurred, but later in time, thus representing a secondary process subsequent to supershear fracture (Fig. 4, inset).

Numerical experiments
Further insights were gained through numerical simulations. Here, we employ the 2D Material Point Method (MPM) model presented by Gaume et al.37 (see Methods Section) to simulate crack propagation within an elastoplastic weak snow layer underlying a homogeneous, elastic snow slab (see also Supplementary Movie SM4) using field-measured parameters as model input in order to reproduce PST #3 (see Methods).

Our results (Fig. 5a) indicate that crack propagation occurs within the sub-Rayleigh regime, with speeds of approximately 50 m s−1 for propagation distances typically below 5 m. In this regime, crack propagation is primarily driven by the collapse and subsequent bending of the slab, as illustrated in Fig. 5b. At a propagation distance of approximately 5.5 m, we observe the nucleation of a secondary (daughter) crack around 0.5 m ahead of the primary crack (Fig. 5b and Supplementary Fig. 8), suggesting the presence of the Burridge-Andrews mechanism21,22. This phenomenon is further supported by stress distributions within the weak layer: a single stress peak is observed in both the sub-Rayleigh (Fig. 5h) and supershear (Fig. 5j) regimes, whereas two distinct peaks emerge at the onset of daughter crack nucleation (Fig. 5i).

To track the crack tip position in the weak layer (blue curve in Fig. 5a), we define it as the position of the farthest plasticized particle. This explains the apparent discontinuity in crack tip location at approximately 5.5 m of propagation, a feature that does not appear when tracking the crack tip through a threshold on the slab normal displacement (magenta curve). Beyond this transition, the crack enters a supershear propagation regime, reaching a speed of approximately 115 m s−1 at a propagation distance of 8 m, consistent with experimental observations (Fig. 5a). However, our simulations suggest that steady-state crack propagation is not yet achieved at this point. Instead, a steady-state regime is reached at around 15 m, where the formation of a Mach cone within the slab becomes evident (see Fig. 5d, 5g, as well as Supplementary Fig. 8 and Supplementary Movie SM4). Furthermore, we observe that the slab reaches a vertical displacement exceeding 50% of the total collapse height approximately 0.04 seconds after the crack tip has passed (Fig. 5a). Consequently, the crack speed evaluated using this vertical displacement threshold eventually reaches the supershear regime, but later than the crack itself, at a propagation distance of approximately 13 m. Finally, it is important to note that these findings are consistent with those obtained from entirely independent numerical simulations based on the Discrete Element Method (DEM), which models the weak layer as a highly porous, brittle thin layer using a simple bonded contact model (see Supplementary Note 4). Due to the simplicity of the contact laws in DEM simulations, they serve as true numerical experiments that, together with these continuum-based simulations, provide independent confirmation of our experimental findings and reinforce our conclusions.

Methods
Snow fracture experiments
On 19 January 2021, we conducted a series of avalanche experiments in a natural snow pack on a small slope near Davos, Switzerland. To prevent natural avalanche release, or remote triggering of our study slope, when the weak layer was still forming at the snow surface, we compacted the weak layer at the perimeter of the planned experiments. During the subsequent snowfalls that buried the weak layer and formed the snow slab, we used snow saws about 1.2 m long to isolate the test site from the rest of the field to ensure that we would not fracture the weak layer while we worked around it. The experimental setup is based on the Propagation Saw Test (PST), a standard fracture mechanical test for snow35,52. In the test, a column (width b = 30 cm) is isolated and an artificial cut is introduced into a pre-identified weak snow layer until a self-propagating crack starts when the critical cut length is reached. Our PSTs were around 10 m long, much longer than the 120 cm recommended standard length32. We performed three side-by-side PSTs within approximately 6 hours (PST #1, PST #2 and PST #3). The experiments were filmed with a Phantom VEO 710 high-speed (HS) camera. In PST #1, the HS camera was used to film the entire PST length (Fig. 1c, full-view), in PST #2 we filmed a close-up of the weak layer at a distance of 206–286 cm from the bottom edge of the PST, and in PST #3 we filmed a close-up at a distance of 861–919 cm. To also record the fracture event along the entire experiment, in PST #2 and PST #3 we filmed the entire PST with a standard digital camera (Sony RX100V, 250 fps) (Fig. 1c, full-view).

Snow properties
In close proximity to the PST experiment we characterized the snowpack by observing a traditional manual snow profile31. The weak layer consisted of buried surface hoar with a grain size of 10-15 mm below a 80 cm thick snow slab. The crack propagated at the bottom of this layer, at the transition to a 6 cm thick layer of faceted crystals resting above an ice crust (Supplementary Fig. 1). The slab above the weak layer mainly consisted of decomposing and fragmented precipitation particles and rounded grains, with a mean density of 102 ± 5 kg m−3. Density was measured every 5 cm using a 100 cm3 cylindrical density cutter (38 mm diameter). Spatial variability was documented with snow micro-penetrometer (SMP) measurements every 50 cm along the 10-meter-long PST. In general, the heterogeneity within the PST was relatively low (Supplementary Fig. 2), and the SMP measurements were in good agreement with the manual profile.

Instrumentation
The recorded sidewall of a PST was speckled with black ink (Indian Ink, Lefranc & Bourgeois) applied with a commercial garden pump sprayer. The HS camera recorded the experiments with a horizontal resolution of 1280 pixels and a frame rate of 5000 fps (full-view PST #1) or 10,000 fps (close-ups PST #2, PST #3). To avoid perspective distortion, we aligned the camera to be vertically and horizontally perpendicular to the wall and aimed the optical axis of the camera at the centre of the field of view, both horizontally and vertically. Correction for radial and tangential image distortion introduced by the camera lens was performed on the fly while correlating the images using the open source DIC engine53 software. Distortion coefficients k1, k2, k3, p1 and p2 as well as the camera model were estimated using chessboard calibration images and is based on Bradski and Kaehler54. For a more thorough description of camera calibration we refer to Bergfeld et al.33. In the close-ups we used, in addition to the HS camera, a Sony camera (RX100 V) to capture PST #2 and PST #3 also in the full-view perspective. Movies were recorded at 250 fps and an image resolution of 1920 by 1080 pixels. In contrast to the raw images of the HS camera, the Sony camera recorded compressed MP4 files (h264-codec). Extracting the individual images from the MP4 movie was done using FFMpeg55. Further camera settings can be found in Supplementary Table 1.

Digital image correlation
DIC engine was used for digital image correlation (DIC) analysis. It is an open source DIC tool to compute full-field displacements from sequences of digital images and it is provided by Sandia National Laboratories53. In the images, a region of interest (ROI) was selected encompassing the speckled PST wall. To derive displacement fields of the PST, the ROI was further subdivided into quadratic DIC subsets with a certain side length and step size (Supplementary Table 1). The position and deformation of each DIC subset was then tracked over time relative to an initial reference frame. In order to find the unique DIC subsets in all subsequent frames, the DIC subsets can translate, rotate and deform through both normal and shear components. For an arbitrary DIC subset i, we thus obtained the initial horizontal and vertical position (x, z; for the definition of the coordinate system see Fig. 1c) as well as the time-dependent horizontal and vertical displacement u(t) and w(t) relative to this initial position.

Postprocessing
Pixel-to-real-space conversion is done by manually picking the PST column length (or a ruler in the close-ups) as a reference to compute the pixel-to-meter conversion factor (Supplementary Table 1). To characterize the displacement behavior of the slab, we manually defined two time ranges: (i) a first time interval Δtrest when the slab-weak layer system was at rest, and (ii) the second time period Δtcrack when crack propagation took place (Supplementary Fig. 2a and 3a).

The displacement u(t) or w(t) of a subset within Δtrest was characterized by two distinct noise sources. First, the signal exhibits high-frequency noise (Supplementary Figs. 2b, 3b, transparent lines), which is attributed to the electronic shot noise inherent in a camera sensor56. As the frame rate of the recorded videos is high compared to crack propagation time, we used a third-order Savitzky-Golay filter57 to effectively reduce this high-frequency noise (Supplementary Figs. 2b, 3b, solid lines). Second, there were lower frequency sinusoidal oscillations in the HS-camera (not in the Sony camera). The sinusoidal noise comes from a built-in cooling fan and has a typical frequency of about 25 Hz. To remove this noise, the frequency and amplitude of the sinusoidal were fitted during Δtrest and suppressed for later times (Supplementary Fig. 2c, transparent and solid lines, respectively)).

In a second step, we applied tempo-spatial filtering to the displacement fields via Non-Local-Means (NLM58) video denoising (Supplementary Figs. 2d, e and 3c, d). In contrast to local filters, which smooth out sharp or discontinuous variations, the NLM method conserves fine detail while reducing additive noise59–62. The principle idea of NLM image denoising is to leverage the redundancy of information within a single image. Each image has small windows with similar patterns that repeat throughout the image. In NLM-denoising, a small window around a certain pixel is compared to all other windows within the neighborhood (or search domain Ω). The similarity of the windows provides the weight of the neighborhood, while averaging the pixel in the center of the initial window. Very similar to repetitive patterns in one image, a video exhibit pattern repetition between frames. Hence, NLM-denoising can directly be extended from 2D (spatial) to 3D (tempo-spatial) denoising63. All experiments in this study were NLM–denoised with a window size N = 33 pixel3, a search domain Ω = 213 pixel3, and a noise parameter h, which refers to the noise level of the signal of h = 3×σ(X(Δtrest)). Here, σ(X(Δtrest) represents the standard deviation of a signal X in the time interval Δtrest. Hence, the standard deviation of displacement u(t) or w(t) before cracking.

For the NLM algorithm, the measurement fields cannot have any voids. However, due to the threshold-based subset initialization routine in DIC engine, measurement points may be missing in areas of the ROI where speckling was poor (Supplementary Fig. 2d). As this poses a problem for the NLM algorithm, the voids were first filled using a contextual-recovery algorithm64,65 based on the biharmonic equation.

From the filtered displacements of a subset, u(t) and w(t), velocity and acceleration were derived. This was done using the same Savietzky-Golay algorithm as for removing the high-frequency noise, but the first and second derivatives of the polynomial were considered. Strain fields were computed from differentiation of u(t) and w(t) with the second order accurate central differences66.

In the last post-processing step, the ROI was divided into three subregions: (i) slab, (ii) weak layer and (iii) substrate. To define these subregions, we picked a time step at which strain concentrations in the weak layer were clearly visible (Supplementary Fig. 2f). The subregions were then defined using manually selected thresholds for the strain. All DIC subsets above the upper boundary were assigned to the slab (Supplementary Fig. 2f, pink colored), all those below the lower boundary to the substrate (turquoise colored), and all in between to the weak layer.

The subset size in the full-view measurements was between 9.7 cm and 26 cm, in the close-ups it was 1.1 cm (close-up #1 performed in PST #2) and 1.7 cm (close-up #2 performed in PST #3). As weak layer thickness ranged from 4 to 8 mm, the subsets assigned to the weak layer also contain parts of slab and/or substratum. Measured strain is therefore always an effective quantity across the presumably smaller strain concentration within the weak layer. True measurements of strain in the weak layer are therefore not feasible with our video recordings.

Crack speed evaluation
Measuring crack speed is essentially a question of identifying and following the crack tip with time. Compared to opening cracks when the material separates, for anticracks, this is more difficult as crack faces are not well defined49. In addition to snow crystal bond fracturing, new contacts also form during inter-penetration25. Hence, there is no clear state in which the crack flanks are clearly separated. Bergfeld et al.33 compared three different methods to estimate crack speed. The first two were based on displacement and strain thresholds to estimate crack tip positions67. The third method tried to avoid the localization of the crack tip by considering the slab-weak layer system as translation invariant regarding the crack tip. In this method, the slab acceleration signal w€(t) was cross-correlated along the crack propagation path (x-direction in Fig. 1c) to estimate the time lag in which two locations along the column undergo a similar acceleration pattern. In summary, Bergfeld et al.33 concluded that the correlation method is very sensitive to any dynamics during crack propagation. Crack speeds obtained with the correlation method were different compared to crack speeds derived via thresholds on the displacement or strain fields. However, the methodological similarity between the latter two methods was also reflected in comparable crack speeds and trends along the propagation distance. Another work49 applied a discrete micro-mechanical model to reproduce crack propagation dynamics and measure anticrack speed in numerical PSTs. Their analysis of weak layer stresses and bond breaking allowed them to monitor and track stress concentrations and the fracture process zone, also in transient crack propagation regimes. They applied four different methods to estimate crack speed. Besides threshold methods on slab displacement and acceleration, they also had access to stress and bond breakage in the weak layer. While their different methods yielded different crack tip locations, all methods provided very similar measures of crack speed. This finding further supports that displacement-based methods on experimental data to derive crack speed are appropriate. In this study, we largely followed the displacement-based idea from 33,67 to determine crack speed.

The deformation of the slab provides the energy to drive the crack in the weak layer, hence the deformation field of the slab is closely linked to the location and evolution of the crack tip. Different crack propagation modes result in different deformations of the slab. In mixed-mode anticrack the initial deformation is primarily driven by vertical displacement w(t), while in the supershear regime, slab deformation is dominated by horizontal displacement u(t) (see Fig. 4). Hence, for crack speed estimation of a mixed-mode anticrack the w(t)-field (Supplementary Fig. 6) and for cracks propagating in supershear mode the u(t)-field (Fig. 2) seem to be appropriate. This is particularly true when the fracture is not in a steady-state49. Since, the crack speed can therefore be based on u(t) or w(t), we now use X as a placeholder: The displacement X of a subset i located at the point j=x, k=z and at a video frame l=t is given by Xj,k,t. To determine crack speed, the displacement field Xj,k,t was first averaged over the thickness of the slab.

X = 1/Nz * Σ Xj,l

where Nz is the number of vertically aligned subsets in the slab. From the displacement field Xj,l, tempo-spatial plots can be drawn and give a good overview of the dynamics during crack propagation (Fig. 2d and Supplementary Fig. 5). At times (y-axis) and locations (x-axis) where the slab is not yet deformed we see a black region. For larger x-locations the black region extends to later time t, hence there is consecutive slab deformation due to cracking in positive x-direction.

For each location along the test column (x-axis), the time of cracking tc, indicated by the accompanying deformation of the slab (transition from black to yellow), can be assessed. To identify the time of cracking tc(x) for each location x, we used a threshold of 25 times the standard deviation of displacement measured before cracking started (t ∈ Δtrest). Crack speed was then measured as the slope of the linear regression in a least square manner of the data pairs (tc(x), x) where f(tc(x), x) | t ∈ Δtcrack, xmin ≤ x ≤ xmax and xmin, xmax providing the range in which the crack speed is to be measured. In the example of Fig. 2d, the linear regression is the green line for the spatial sub-range 1 m ≤ x ≤ 6 m and its slope represents the crack speed measure. To measure the evolution of crack speed along the PST column (see Fig. 2c) we used overlapping sub-ranges of twenty horizontally aligned subsets (≈1.2 m) with an overlap of 95%. The uncertainty in crack speed values was obtained by repeating the computation but changing thresholds to 50% and 150% of the nominal value, respectively.

Elastic wave speed
Snow is a fragile, porous material whose mechanical behavior defies simple characterization. Due to the complex microstructure and the existence close to its melting point, measurements of the elastic modulus of snow presents numerous challenges (e.g., see Mellor68), resulting in published values that exhibit scatter over several orders of magnitudes. The observed scatter can be attributed to a number of factors, including differences in snow density as well as rate- and temperature-dependency. Moreover, snow microstructure as well as its anisotropy alters the elastic behavior69,70.

In addition, crack propagation preceding an avalanche necessitates a layered snow pack, as the crack generally propagates in thin, weak layers. The energy required to create new crack surfaces (in the weak layer) originates from the slab layers above the weak layer, and the energy must be transferred towards the weak layer to further advance the crack. Consequently, there is an energy flux from the stress-strain field in the slab to the moving crack tip in the weak layer71. The relative magnitude of the strain components in the slab define the fracture mode and gives rise to the associated wave, which transfers the energy towards the crack tip. Knowledge about elastic wave speeds is therefore crucial as these are bounding limits for the speed of the crack72. All wave velocity metrics directly depend on the material’s elastic modulus, which poses two challenges: 1) which elastic modulus empirical parameterization should be used? and 2) what is the elastic wave speed in a layered slab? In this section, we address point 1) while point 2) is addressed in the Supplementary Note 3 by means of numerical modeling.

The difficulty in measuring the modulus of elasticity directly leads to uncertainty if an empirical density-to-modulus parametrization is used to compute wave velocities. Therefore, shear wave speeds were computed based on five different published parametrizations25,34,73–75 (Supplementary Fig. 7). For the measured mean slab density of ρ=102 ± 5 kg m−3, this results in a Young’s modulus ranging from 0.4 MPa to 1.5 MPa, and thus in shear wave speeds between 40 ms−1 and 79 ms−1. Despite the large spread in shear wave speed, measured supershear crack speeds were higher than the highest values estimated from the slab density. In particular, the largest estimates from Sigrist75 (dynamic cyclic loading at large strain rates) and Gerling et al.34 (acoustic wave propagation measurements) were used as reference in the main text. The associated parameterizations are given by

Eger = 25.4 MPa * (ρ/ρice)^4.6 (1)

and

Esig = 967 MPa * (ρ/ρice)^2.94 (2)

where ρice = 917 kg m−3 is the density of ice76.

Numerical simulations
Numerical simulations where performed using the Material Point Method along with the cohesive Modified Cam-Clay (MCC) plastic model as introduced for snow slab avalanches by Gaume et al.37 and used by Trottet et al.20 to provide the first numerical evidence of supershear crack propagation in such avalanches. The snow is modeled with Hencky’s finite strain elastic model to handle large deformations. The MCC plastic yield function and associated flow rule are added to the weak layer to account for fracture and allow crack onset and propagation. The elliptic yield function of the MCC allows for both volumetric and shear rupture, as well as strain softening and hardening. The slab is modeled using a hyperelastic model. The substratum is modeled as rigid. We refer to the previously cited articles20,37 for more information about MPM and the cohesive MCC plastic model. The simulated slab is homogeneous and has a density of ρ=102 kg m−3 according to the measured mean density in the experiment. The Poisson’s ratio is ν=0.3. The Young’s modulus was then derived using a dynamic experimental formula appropriate for the simulated dynamic regime. Sigrist’s formula was preferred over Gerling’s as it led to the highest elastic modulus prediction for our given density value (conservative estimate to ensure that supershear fracture occurred experimentally, see Fig. 2). All the simulation parameters are available in Supplementary Table 2).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Let me start by saying that I find this work very interesting and exciting. This manuscript reports about the occurrence of supershear avalanches through experiments performed in a controlled outdoor environment. The experiments are conducted employing a well-established protocol, the propagation saw test (PST), where samples are obtained by sections cut in natural snowpack containing a weak layer buried under a cohesive thicker slab. By employing longer samples than in the standard PST, which allow for longer propagation distances, and by tracking the crack tip speed, the authors have been able to produce and observe supershear rupture propagation of the simulated snow avalanches, something that has never been observed before in laboratory experiments. I think this work provides an outstanding contribution to the field and I believe it would also be of interest to a broad readership.\nI have the following questions and comments:\n1. The authors claim that supershear transition occurs by the Burridge-Andrews transition mechanism but what evidence do they have for that? There are other mechanisms, including transition induced by asperities or barriers (e.g. Page, Dunham, Carlson, JGR 2005). How can they be sure that the Burridge-Andrews is the operating mechanism?\n2. According to the Burridge-Andrews supershear transition mechanism, a secondary (supershear) crack nucleates ahead of the main (sub-Rayleigh) crack. After transition, there is still a trailing Rayleigh signature, a remnant of the main crack. Is this something that can be detected behind the supershear crack in the experimental measurements presented here?\n3. Another way to demonstrate that the Burridge-Andrews is the operating mechanism would be to show that the supershear transition distance is consistent with the analytical estimate proposed by Trottet el al. Nature Physics 2022. If that is the case, does the supercritical crack length scale with the square root of the slab depth, consistent with Eq. 1 of Trottet et al.?\n4. Conclusions are really based on a single test (PST#3). I am aware of the complexity involved in these tests. However, I think it would be important to check repeatability to add robustness to the findings. Also, I think it would be valuable to perform more experiments with slabs of different depths, which would allow to demonstrate, as mentioned above, the dependance of the supercritical crack length on the square root of the slab depth through the parameter Λ derived in Trottet el al. Nature Physics 2022.\n5. PST#1 and PST#2 did not propagate to supershear speed because of slab fractures. It is interesting that a slab fracture also occurred in PST#3 but since the rupture had already transitioned to supershear speed, the slab fracture did not arrest it. How frequent are slab fractures that result in hindering supershear propagation? In other words, supershear avalanches are possible but how likely are they due to the effect of slab fractures? In the tests presented in this manuscript only 1 in 3. Again, I think that more tests would be helpful to answer this.\n6. One of the main observations in this manuscript is that sub-Rayleigh propagation is characterized by a slope-normal collapse of the weak layer, whereas supershear propagation by predominantly slope-parallel motion. This observation is broadly consistent with the analytical solutions of Freund (1990) (and experimental measurements of Mello et al. Tectonophysics, 2011; Mello et al. JMPS, 2016; Rubino et al. JGR, 2020) for mode II cracks, where the interface-normal particle velocity predominates for sub-Rayleigh ruptures and interface-parallel predominates for supershear ruptures. To what point do these similarities hold and what differences are there between these analytical solutions and experimental measurements and those presented here?\nSome additional comments:\n7. How are subset sizes chosen in the DIC analysis? Has a convergence study and error analysis been conducted to pick the right subset size? What is the error estimate on the presented measurements?\n8. Material and Methods, page 16 (7th line from the bottom): is it not possible to select a smaller subset size so that the subset entirely falls within the weak layer and doesn’t contain part of the slab and/or substratum.\n9. What strategy has it been used to resolve discontinuous displacement fields across the interface of the weak layer with slab and substratum?\n10. Material and Methods, page 16 (9th line from the top): the displacement field is averaged over the thickness of the slab. This averaging would be legitimate if the slab moved as a rigid body but is this the case? I would think that there are important displacement gradients through the thickness of the slab. In fact, there need to be such gradients to produce the heterogeneous strain rate fields of Extended Data Fig. 7.\n11. What high-speed camera was used and what was the exposure time at 5000 and 10000 fps, respectively? Is it natural light enough even at the exposure time of 10000 fps or was any other light source employed?\n12. Fig.3 and Extended Data Fig. 4: I would suggest adding and marking regions of close up # 1 and close up # 2 in panel (a), similar to supplementary movies 1 and 3.\n13. Extended Data Fig.1: What are grain types a-e-d-g? Why not using the two-letter code indicated in “The international classification for seasonal snow on the ground” (ref. 30)? The grain type should be more clearly explained anyways.\nVito Rubino", "reviewer_2": "According to this manuscript by Bergfeld et al., confirmation of supershear cracks in snow was limited to avalanche videos (Hamre et al., 2014) and numerical simulations by the authors (Trottet et al., 2022). This next manuscript aims to re-confirm that their concept is valid, arguing that their earlier published experimental confirmation of supershear via full-scale measurements should be repeated in a controlled setting.\nThe manuscript describes that on 19 January 2021, a supershear crack was caught on camera in one fracture experiment. Since slab fracture did not arrest this shear crack, the authors also suggest it may lead to widespread propagation and believe this has “significant implications for risk mitigation strategies.”\nI don't know how experts in the Propagation Saw Test reject statistical flukes in a convincing way for statisticians. However, I can not encourage the publication of claims based on n=1. Currently, the evidence remains at a quite preliminary stage. From my own interaction with Nature Communications editors, I learned that they require a significantly more in-depth investigation for consideration as a full research article, and the journal does not have a brief communications format available as an alternative.\nMoreover, even assuming that the experimental evidence is reproducible, I do not feel there are convincing analogies to earthquake phenomena (currently, even basic references to supershear strike-slip are confusing and not consistent with the authors’ previous paper - Trottet et al., 2022 - as I explain below).\nAlso, the title is excellent clickbait but misleading: (i) no snowquake was studied in the paper, and (ii) I do not think avalanche professionals call the Propagation Saw Test a laboratory avalanche. According to my reading of the manuscript, the title is more like: “One propagation saw test reveals supershear crack.”\nFinally, some sections of the manuscript read like a remake of Trottet et al., 2022, but while some sentences feel like Deja Vu, others contradict the author's previous words, and some alignment of arguments and the storyline seem necessary.\n***\nDETAILED REMARKS (in the absence of line numbers, I just copy-paste text segments):\np. 2, suspected in firn and snowquakes [7,8,9]\nI am unsure how thermal contraction cracks studied in [8] relate to the anticrack.\np. 2, limited by the Rayleigh wave speed\nA reference would be helpful.\np. 3-4, laboratory (here and elsewhere)\nTo me, this is an “outdoor experimental site”, because a laboratory is “a room or building with equipment for doing scientific tests\" (https://dictionary.cambridge.org/).\np. 4, supershear snowquake (here and elsewhere)\nI do not see any rationale to make “intersonic fracture” a synonym of “supershear snowquake” except just making it sound cool. Mars quake, ice quake, Moon quake, etc., refer to a seismic event or vibration produced by something, like a crack. No vibrations were recorded to declare there was any kind of quake, and I am not even sure we know if it is detectable for different phases of the rupture.\np. 5, Fig. 2\nI understand the rationale later appearing in Methods on p. 18 about w(t), u(t). Nevertheless, I find [a-Normal, close-up #1] next to [b-Tangential, close-up #2] confusing. Comparison of the same components seems more straightforward. It makes the figure speak for itself without needing a reader to pause and figure out what is going on and whether the normal component drifts to subpanel (c) and how it falls into the context of shear in (d). This figure is introduced with a discussion of tangential displacement analysis, and only half a page later, the reader is informed that normal tangential displacements in close-up #1 yield similar speeds without these shown on the plots. Perhaps, caption could clarify that c = 41 m/s in (a) was also obtained from u(t).\nc^sig, c^ger could be explained in the caption.\np. 6, normal displacements differed.\nWhat was the speed based on the latter?\np. 6, aligns with the assumption\nIt was not introduced earlier and might need a reference.\np. 8, It is noteworthy\nIt could be helpful to explain why it is so because it may not be evident to the reader.\np. 9, “providing direct evidence of a supershear snowquake during snow avalanche release”\nI do not think any evidence related to the snowquake was provided; i.e., no measurements showed that the supershear crack is a seismic event.\nThe present study does not provide evidence “during snow avalanche release,” only “during one Propagation Saw Test.”\np. 10\nIt is unclear how the unlikely crack arrest by slab fracture “parallels findings in earthquake science,” and should be supported with references.\nI could not follow the discussion about the crack arrest. On the one hand, Fig. 4 shows that normal collapse is uncoupled from tangential and arrives with a delay to the 8.5 m camera position. If so, vertical slab fracture is unlikely to take place before arrival of normal displacement, which lags behind the supershear, and thus cannot arrest the crack. Moreover, I am confused about how Fig. 4 aligns with the numerical findings of Trottet et al. 2022, suggesting that anticrack propagation is not relevant at distances larger than 3-5 m (Trottet et al., 2022).\n“supershear earthquakes are generally associated with large magnitudes” needs a reference and specification of which magnitude. Moreover, in Trottet et al. (2022), the same authors claim the opposite: “supershear propagation has rarely been reported in large strike-slip earthquakes.”\nAnyway, I do not understand the “Just as” comparison of earthquakes to “supershear snowquakes” and wider release areas. Do supershear earthquakes have a large size because they rupture fast? Also, in Trottet et al. (2022), the larger avalanche size was associated with higher slab density, making fracture arrest unlikely, and I am unsure how the two effects coexist.\n“Our findings reinforce the analogy between avalanches and earthquakes.” I could not understand which analogy was reinforced? The anticrack concept inspired by deep-earthquakes, which can be completely neglected by using simple shear models (Trottet et al. 2022)?\nI could not follow the logic here: “firn and snowquakes, like deep earthquakes, are often associated with the volumetric collapse … and anticrack propagation. In this context, supershear snowquakes are analogous to supershear strike-slip (mode II) earthquakes.” First, are supershear strike-slip earthquakes often associated with volumetric collapse and anticracks? I do not think so. Second, are supershear strike-slip earthquakes represent deep earthquakes? Again, I do not think so, because strike-slip earthquakes are usually shallow.\nIn Discussion, I missed implications for the debate motivating the paper.\np. 30, crust arrow\nIm not sure where does it point vertically.\np. 32, typo\nof of the crack tip.\np. 36\nI think h-a-b layerings labels should be explained,\nSimilarly, \\rho_{m,-,+} could be associated to ED table 2\np. 39, _\ntheclose-up", "reviewer_3": "The authors report results of an experimental study of slab avalanche release. The experiments are denoted as laboratory experiments which is an interesting terminology given that they were conducted on a natural slope in natural snowpack (I can understand why the authors prefer not to speak of field experiments). They adhere to the protocol of the propagation saw test, a standard testing method designed to probe anticrack-type failure of collapsible weak layers in snow. The performed high-speed imaging and DIC analysis yield high-quality data for the displacement fields which indicate, albeit only in one of three samples, a transition from subshear anticrack to supershear crack propagation. In two other samples the same transition was not observed because of slab rupture after an otherwise similar anticrack propagation stage.\nWe find this study, which seems to partly confirm an earlier theoretical prediction by some of the authors, to be interesting and potentially important in view of our understanding of the various factors that decide the size of slab avalanches. The fact that not only the fracture propagation speed but also the slip pattern changes in the late stage of crack propagation adds to the evidence for a qualitative transition in propagation mechanism. The authors provide a good but not exhausting documentation of their data. In particular, we would have liked to see more data and clarification on the following points:\n1) The authors mention that crack speeds determined from u and w coincide for the 'anticrack' stage but differ for the 'supershear' stage. Can you give the respective c_u and c_w vs x curves or equivalent information?\n2) Displacement fields are shown for the 'successful' experiment but not for its two 'unsuccessful' counterparts. It would be important to know whether in these two experiments the 'anticrack' type displacement pattern persists throughout the propagation. Can you show these fields either in form of supplementary videos or graphics?\n3) In extended figure 7, it might be good to see a 'zoom' at about 10 m, i.e. near the location of the experimental window, to clearer assess the expected existence or not of a mach cone at that location. Also, despite strenuous efforts we could not find a definition for the abbreviation 'FOV'. This has not helped our understanding.\n4) The authors propose that a critical crack length is required for a transition into the supershear regime. A related definition of critical crack length for supershear was provided in a prior study using numerical simulations (Trottet, B., Simenhois, R., Bobillier, G. et al., Nat. Phys. 18, 1094–1098 (2022)), which is also cited in this paper. It would be interesting to see if this critical crack length can be linked to the observations.\n5) The Burridge–Andrews mechanism is alluded to multiple times in the paper. While the analogy with strike-slip earthquakes is clear, how exactly the Burridge-Andrews mechanism (initiation of a daughter crack in front of the main crack) takes place in this case (especially PST #3) is unclear in the paper.\nOverall, we think that this paper may make an important contribution of general interest, if the mentioned points can be fully addressed.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewer’s comments\nManuscriptNCOMMS-24-62974-T\nSignaturesofthesub-Rayleightosupershearfracturetransitioninsnowavalancheexperiments\nbyBergfeld,Gaume,Pellet,Bobillier,Schweizer,vanHerwijnen\nIn the following, we provide (in blue) detailed point-by-point answers to the comments raised by the re- viewers (in black, italic). In addition, modifications made to the manuscript are highlighted in a separate file withtracked-changes.\nResponse to Referee #1 – Prof. Dr. Vito Rubino\nLetmestartbysayingthatIfindthisworkveryinterestingandexciting. Thismanuscriptreportsaboutthe\noccurrenceofsupershearavalanchesthroughexperimentsperformedinacontrolledoutdoorenvironment. The\nexperimentsareconductedemployingawell-establishedprotocol,thepropagationsawtest(PST),wheresam- plesareobtainedbysectionscutinnaturalsnowpackcontainingaweaklayerburiedunderacohesivethicker\nslab. By employing longer samples than in the standard PST, which allow for longer propagation distances,\nand by tracking the crack tip speed, the authors have been able to produce and observe supershear rupture\npropagation of the simulated snow avalanches, something that has never been observed before in laboratory\nexperiments. IthinkthisworkprovidesanoutstandingcontributiontothefieldandIbelieveitwouldalsobeof\ninteresttoabroadreadership.\nWe thank Referee #1 for his positive comments on our manuscript and for very constructive suggestions\nthathelpedustoimprovethereadabilityandqualityofourpaper(seedetailedrepliesbelow).\nIhavethefollowingquestionsandcomments:\n1. TheauthorsclaimthatsupersheartransitionoccursbytheBurridge-Andrewstransitionmechanismbut\nwhat evidence do they have for that? There are other mechanisms, including transition induced by asperities\norbarriers (e.g. Page, Dunham, Carlson, JGR 2005). Howcan theybesure thatthe Burridge-Andrewsisthe\noperatingmechanism?\nThank you for this interesting reference. It is particularly relevant for cases where crack propagation is\ninfluencedbymacroasperities,suchasinforestedterrainorinthepresenceoflargeobstacles. However,inour\nstudy, no such asperities were present. We verified through SMP (Snow Micro Penetrometer) measurements\nalong the beam that we had homogeneous conditions (See Extended Data Figure 1). Given this, we assumed\nthat the supershear transition follows the Burridge-Andrews mechanism. While we do not have high-speed\nimagery at the precise location of the transition, our numerical simulations using DEM and MPM closely re- produce the experimental conditions and provide strong evidence for the Burridge-Andrews mechanism (see\nFigures1,7and5inthisrebuttalletterwhichwerealsoaddedtoourmanuscriptandsupplement). Specifically,\nwe observe the nucleation of a daughter shear crack ahead of the main crack at a location consistent with the\nobservedsupersheartransition,supportingthisinterpretation(Fig. 1). Wehaveclarifiedinthemanuscriptthat\nour interpretation is based on this assumption, supported by numerical experiments (and directly by the field\ntests)thatarenowalsopresentedinthemaintext.\n2. According to the Burridge-Andrews supershear transition mechanism, a secondary (supershear) crack\nnucleates ahead of the main (sub-Rayleigh) crack. After transition, there is still atrailing Rayleigh signature,\naremnantofthemaincrack. Isthissomethingthatcanbedetectedbehindthesupershearcrackintheexperi-\nmentalmeasurementspresentedhere?\nAfterthesupersheartransition,Rubinoetal. (2020)haveindeedreportedthepresenceofatrailingRayleigh\nsignature,reminiscentofthemaincrack(theanticrackinourcase). However,wedidnotobservesuchatrail-\ning wave in either our experiments or numerical simulations. Our simulations suggest that this signature is\nvisibleforafewmilisecondsafterthetransitionbutthatitisquicklydissipatedwithintheoverallmotion(see\nFigure 1: Numerical experiment of weak layer crack propagation beneath a homogeneous snow slab using\na 2D Material Point Method. (a) Time according to crack tip position, de´termined by following either the\nplasticized particles in the weak layer (blue) or the slab’s slope perpendicular collapse (magenta). The grey\ndotted line represents a virtual crack tip moving at the speed of the shear waves. (b to g) Zoomed views of\nthe slab (colormapped) and the weak layer (white and red) around the crack tip. In the weak layer, the white\nand redrepresent theunplasticized andplasticized particlesrespectively. b-d: magnitude ofspeed (colormap\nandarrowlengths)anddirectionofmovement(arrowdirections). e-g: slopeparallelstrainrate. (htoj)Shear\nstressintheweaklayernormalizedbythegravityinducedverticalstressτ ,forallthreezoomviewsabove.\ncizedparticlesintheweaklayerarecoloredinredandtheslabiscoloredbythelongitudinalrateofdeformation\n(left)andshearrateofdeformation(right). Thesupersheartransitionoccursatapproximately5.5m,consistent\nwith the transition reported in PST 3. A Mach cone begins to form beyond this point, becoming distinctly\nvisibleonceasteadystateisachieved. SeeFigure1.\nFigure2inthisletterandSupplementaryMovie4). Yet, weobservethattheslabflexuralwave, whichcanbe\napproximately tracked using a vertical displacement threshold (Fig. 1) [1], lags behind the supershear crack.\nInitially,inthesub-Rayleighregime,slabflexureplayedakeyroleindrivingcrackpropagation. However,after\nthe supershear transition, fracture propagation became governed by slope-parallel tension. As a result, what\nwasinitially aflexural waveevolvedinto asecondary effectofshear failureand eventuallyreachedintersonic\nspeeds, though at a later stage. We have added a discussion about this interesting aspect in the Discussion\nsection.\n3. Another way to demonstrate that the Burridge-Andrews is the operating mechanism would be to show\nthatthesupersheartransitiondistanceisconsistentwiththeanalyticalestimateproposedbyTrottetelal. Nature\nPhysics2022. Ifthatisthecase,doesthesupercriticalcracklengthscalewiththesquarerootoftheslabdepth,\nconsistentwithEq. 1ofTrottetetal.?\nThank you for this suggestion. Let us recall the formula for the supercritical crack length from Trottet et\nal.:\nτ −τ\np g\na = Λ (1)\nsc\nτ −τ\ng r\n(cid:112)\nwhere Λ = E′DD /G (E′ is the plane strain elastic modulus, D and D are the slab and weak layer\nwl wl wl\nthicknesses, respectively, and G is the shear modulus of the weak layer). Additionally, τ = ρgDsinψ\nwl g\nrepresentstheshearstressinducedbytheslabload,τ = µ ρgDcosψ istheresidualfrictionalresistance,and\nr d\nτ = c+µσ denotestheshearstrengthoftheweaklayer. Notethatµ representsthedynamic(orcrackface)\np n d\nfriction,whileµistheinternalfrictioncoefficient. Thecohesionoftheweaklayercanthereforebecalculated\nusingthefollowingequation:\na\nc\nc = (τ −τ )+τ −µσ (2)\ng r g n\nΛ\nThis calculation is particularly useful as it allows for a direct comparison between values obtained through\nback-calculationandshearframemeasurementdatafromtheliterature. Assumingµ = 0.6[2],µ = 0.36[3],\nd\nF = D /G = 10−7 m/Pa [4], and using the elastic modulus evaluation from [5], we obtain a cohesion\nwl wl wl\nvalue of c = 1.4 kPa (assuming a = 5.5 m). This falls within the range of values measured by [6] and [7].\nsc\nConversely, performing the inverse calculation by assuming a realistic range of cohesion values between 0.5\nand 2 kPa yields a range of supercritical crack lengths between 1.25 and 7.3 m. While this analysis does not\nprovideasingledefinitivepredictionduetouncertaintiesinweaklayerstrength,itstrengthenstheconclusions\nofourpaper. ItwasincludedintheDiscussionsectionandaSupplementaryNote. Furthermore,weconducted\nan additional analysis using two independent numerical methods (MPM and DEM, see Figures 1 and 5) to\nverify whether the field experiments could be reproduced in terms of crack speed regimes, deformation pat-\nterns,andsupercriticalcracklength. Theresultsofthisanalysis,presentedinthisletterandalsointherevised\nmanuscript,alsohelpaddressotherquestionsraisedbythisreviewerandotherreviewers.\n4. Conclusions are really based on a single test (PST#3). I am aware of the complexity involved in these\ntests. However, I think it would be important to check repeatability to add robustness to the findings. Also, I\nthink it would be valuable to perform more experiments with slabs of different depths, which would allow to\ndemonstrate, as mentioned above, the dependance of the supercritical crack length on the square root of the\nslabdepththroughtheparameterΛderivedinTrottetelal. NaturePhysics2022.\nWe appreciate the reviewer’s comment highlighting the need for repeatability and additional experiments.\nWhileweagreeontheimportanceofrobustnessinscientificfindings,weemphasizetheexceptionalchallenges\ninvolvedinconductingsupershearexperimentsinsnow. PerforminglongPropagationSawTests(PSTs)under\nthenecessaryconditionsisextremelyrareandlogisticallycomplex. Theseexperimentsrequireaveryunstable\nsnowpack—specifically, a persistent weak layer prone to crack propagation—combined with a safe slope for\ntesting. Suchconditionsareseldommet simultaneously, butinJanuary2021, weidentifiedarareopportunity\natalow-elevationsiteinDavoswheretheseconditionswerefinallysatisfied.\nOnesignificantchallengeisthattheweaklayermustbecarefullypreservedduringsetuptoavoidpre-triggering\nor natural/remote avalanche triggering. To prepare the slope, we removed the weak layer at the perimeter of\nthe planned experiments while it was still forming at the snow surface. After a subsequent large snowfall, we\nensured the weak layer remained intact and stable until the experiments were conducted. This combination\nof factors is exceedingly rare, as highly unstable snowpacks are typically found at higher-altitude alpine sites,\nwhere the logistical and safety challenges of performing such experiments are significantly greater. Over sev-\neral years, we have made repeated attempts to conduct long PST experiments, but these efforts were largely\nunsuccessfulduetotheseconstraints.\nInJanuary2021,however,wesuccessfullyperformedseverallongPSTexperimentsattheDavossite. Inoneof\nthese tests, we observed supershear fracture, conclusively supported by high-resolution, full-field Digital Im-\nage Correlation (DIC) analysis and close-up views, which independently confirmed the same behavior. These\nmultiple data points, including continuous speed estimates and localized measurements, provide robust, high-\nqualityevidencefortheoccurrenceofsupershearfracture,leavingnoambiguityabouttheresults.\nRegardingthesuggestiontoinvestigatethedependenceofthesupercriticalcracklengthonslabdepth,weagree\nthisisaninterestingdirectionforfutureresearch. However,performingadditionalfieldexperimentstovalidate\nthisdependencewouldlikelyrequirenumerousyearsduetotherarityoffavorableconditions. Toaddressthis,\nwe have complemented our fieldwork with numerical experiments, such as simulations using Material Point\nMethod (MPM) and Discrete Element Method (DEM), which offer a more practical and feasible approach to\nexplorethisphenomenon.\nFinally, we note that many foundational studies on supershear fractures, such as those examining supershear\nearthquakes, are based on single-event observations. While repeatability is ideal, our findings are supported\nby comprehensive, high-resolution data as well as additional numerical experiments and contribute valuable\ninsightsintothedynamicsofsupershearfractures.\n5. PST#1andPST#2didnotpropagatetosupershearspeedbecauseofslabfractures. Itisinterestingthat\na slab fracture also occurred in PST#3 but since the rupture had already transitioned to supershear speed, the\nslabfracturedidnotarrestit. Howfrequentareslabfracturesthatresultinhinderingsupershearpropagation?\nIn other words, supershear avalanches are possible but how likely are they due to the effect of slab fractures?\nInthetestspresentedinthismanuscriptonly1in3. Again,Ithinkthatmoretestswouldbehelpfultoanswer\nthis.\nThisisanimportantpoint. WehaveshowninTrottetetal. andinFig. 3belowthattensilefailureofsoftslab\ncan prevent the supershear transition (panels a and c). In this case, the volumetric collapse of the weak layer\ndrivesanticrackpropagationbecausetheslabcannotaccumulatethenecessaryamountofstrainenergyrequired\nforsupershearfracture. Thisleadstotheso-called‘en-echelon’typeoffracture. Thecrackpropagationspeed\ninthiscaseisvaryingwithinasub-Rayleighrange(panele). Thepropagationspeeddecreaseslocallyafterthe\nonset of slab tensile failure. Eventually, crack propagation may be stopped after an ultimate slab fracture. In\ncontrast, for harder slabs, slab tensile failure may occur but does not influences crack propagation dynamics\nand supershear fracture (panels b and d). In that case, the crack propagation speed is almost identical to that\nobtain for an elastic slab. Therefore, we assume that supershear fracture requires relatively hard slabs. In the\ncontextofourexperiments,inPST1andPST2,wearguethattheslabwasstilltoosofttoallowthetransition\ntowards supershear fracture. In PST 3, the absence of the slab fracture was very likely caused by the natural\nstrengthening of snow which is known to occur over very short timescales [8]. This strengthening effect is\nparticularlyefficientandfastinthecaseofnewsnowwithlowinitialdensity.\nFigure 3: Effects of slab fracture on the supershear transition (for a slope angle of 30 degrees). From MPM\nsimulations [9] in which we account for slab tensile failure for a relatively soft (σ = 6 kPa, left) and hard\nt\n(σ = 11kPa,right)slab. a,b: PSTcoloredbynormalizedverticaldisplacement(slab)andweaklayerfracture\nt\n(in red) at two different times. c, d: spatio-temporal evolution of h/Dwl and crack tip location. Black solid\nlines: crack tip position; empty circles: critical crack length ac; filled circles: super critical crack length asc.\nRedlines: slabfracture. e,f: normalizedpropagationspeed.\n6. One of the main observations in this manuscript is that sub-Rayleigh propagation is characterized by\na slope-normal collapse of the weak layer, whereas supershear propagation by predominantly slope-parallel\nmotion. ThisobservationisbroadlyconsistentwiththeanalyticalsolutionsofFreund(1990)(andexperimen-\ntal measurements of Mello et al. Tectonophysics, 2011; Mello et al. JMPS, 2016; Rubino et al. JGR, 2020)\nfor mode II cracks, where the interface-normal particle velocity predominates for sub-Rayleigh ruptures and\ninterface-parallelpredominatesforsupershearruptures. Towhatpointdothesesimilaritiesholdandwhatdif-\nferencesaretherebetweenTheseanalyticalsolutionsandexperimentalmeasurementsandthosepresentedhere?\nWe thank the reviewer for these insightful references. While our findings are qualitatively consistent with\ntheresultsofthesestudies, specifically, thatslope-normalvelocitiesdominateinthesub-Rayleighregimeand\nslope-parallelvelocitiesinthesupershearregime(Figure4),quantitativelyapplyingthetheoreticalframework\nproposed by Mello et al. to our setup is more complex. There are significant differences between our experi-\nmentalconfigurationandthelaboratoryearthquakeexperiments. First,ourmodelinvolvesanadditionalfactor:\ngravity,whichinfluencesbothslope-normalandslope-parallelvelocities. Thesevelocitiesaredirectlyaffected\nby gravitational acceleration, as well as by weak layer compaction energy, collapse height, and friction. This\ncombination of factors prevents a direct application of the theoretical framework developed by Mello et al.\nFurthermore,anotherkeydifferenceliesintheexperimentalsetupitself. Inlaboratoryearthquakeexperiments,\nhomogeneoussamplesaretypicallypre-stressedunderapredefinedloadingangle,allowingprecisecontrolover\ncrackfrontspeedandenablingmoretargetedandrepeatablemeasurements. Incontrast,weareobservingcrack\npropagationinanaturallylayeredsnowpack,wheresupershearpropagationarisesfromacontinuouslyevolving\nstressstatewithinthesystem,ratherthanfromexternallyimposedconditions. Wehaveaddedsentencestothe\nDiscussionsectiontoelaborateonthesedifferencesandincludethereferencedstudies.\nFigure4: Velocitydirectionofsupershear(red)andsubrayleigh(blue)closeup.\nSomeadditionalcomments:\n7. How are subset sizes chosen in the DIC analysis? Has a convergence study and error analysis been\nconductedtopicktherightsubsetsize? Whatistheerrorestimateonthepresentedmeasurements?\nAutomated subset initialization was performed using feature matching. This process required specifying\nthesubsetsize,stepsize,andathresholdfortheSumofSquareofSubsetIntensityGradients(SSSIG).SSSIG,\nwhich quantifies speckle pattern quality, guided the selection of subset size by starting with a small value and\nprogressively increasing it until exceeding the predefined threshold (see Extended Data Table 1). This proce-\ndurewasappliedseparatelyforeachcameraandfieldofview. Toestimateerrors(uncertainties),wepredicted\nthe standard deviation of the displacement solution based on the estimated image noise and SSSIG [10]. For\nthe high-speed camera capturing close-up views, typical values were around 0.01 mm. Since this uncertainty\nis specific to the DIC analysis, we also computed an alternative displacement uncertainty after postprocessing\n(see Materials and Methods). This was determined by calculating the standard deviation of displacements at\ntimesidentifiedasprecedingcracking(ExtendedDataFigure2a,lightgraybackground). Botherrorestimates\nareincludedinExtendedDataTable1.\n8. Material and Methods, page 16 (7th line from the bottom): is it not possible to select a smaller subset\nsizesothatthesubsetentirelyfallswithintheweaklayeranddoesn’tcontainpartoftheslaband/orsubstratum.\nUnfortunatelynot. Theblack-whitecontrastofthespecklepatternisinsufficientforthispurpose. Applying\nandoptimizationofspecklepatternpresentsseveralchallenges. Snowisaporousmaterial,anditscrystalsize\nis comparable to that of the weak layer thickness. Individual crystals reflect light, complicating the process.\nAdditionally,thesnowisjustbelowfreezingpoint,whiletheblackpaintisaboveitwhenitisapplied. Different\nspeckle patterns must be applied for varying fields of view and camera setups. Even seemingly minor issues,\nsuchasfrozenspraynozzles,cancausesignificantproblemsinthefield. Furthermore,theblackinkdrastically\naltersenergyinputfromsolarradiation. Afterapplication,theblackenedsnowsurfacerapidlyheatsup,melts,\nanddestroysthespecklepatternwithinminutes. Sincetestsamplesarenaturallyformedandsnowpackstability\nchangesquickly(withinhourstodays),testrepetitionisimpossible. Whilstweagreethatitwouldbebeneficial\nif thesize of thesubset was smaller than the thickness ofthe weak layer, wehave experienced greatdifficulty\ningettingthesubsetssmallerinfieldexperiements.\n9. What strategy has it been used to resolve discontinuous displacement fields across the interface of the\nweaklayerwithslabandsubstratum?\nEssentially, we removed regions around the discontinuity after analysis by attributing them to the weak\nlayer, thereby keeping the slab and substratum free from larger errors and poor correlation quality. To further\npreventthesmearingofthediscontinuity(beyondtheinherentsmoothingeffectofthesubsetsize)weapplied\nNon-Local Means (NLM) filtering, which provides excellent denoising while preserving edges and features,\nsuchascrackdiscontinuities.\nWe assume your comment was more focused on specialized strategies for handling crack-induced disconti-\nnuities in DIC (e.g., Regularized Finite Element-Based DIC or subset splitting). While we did not apply such\ntechniques,werecognizespecificchallengesforanticracks: (i)SubsetInterference(Inanticracks,displacement\nfieldsconverge, leadingtooverlappingsubsetsinsteadofacleanseparation); (ii)LossofSpeckleUniqueness\n(In an anticrack, speckles may move toward each other or overlap, breaking tracking consistency); (iii) Strain\nField Ambiguities (For anticracks, strain fields often appear as high compression zones, making it harder to\ndefineaclean”split”forsubsetsplitting).\n10. Material and Methods, page 16 (9th line from the top): the displacement field is averaged over the\nthicknessoftheslab. Thisaveragingwouldbelegitimateiftheslabmovedasarigidbodybutisthisthecase?\nI would think that there are important displacement gradients through the thickness of the slab. In fact, there\nneedtobesuchgradientstoproducetheheterogeneousstrainratefieldsofExtendedDataFig. 7.\nExtended Data Fig. 7 (c, e, f) presents strain rate fields derived from MPM simulations. The red-outlined\nregion marks the field of view of the close-up recordings from the experiments. Within this region, minimal\nheterogeneityisobservedinthez-direction,whichisalsoreflectedintheexperimentalresults(ExtendedData\nFig. 2d,e;ExtendedDataFig. 3c,d;ExtendedDataFig. 4d).\nIn Extended Data Fig. 4e, the bending-induced tangential displacement of the anticrack exhibits a displace-\nmentgradient. However,fromtheperspectiveofthestaticcamera,thedisplacementfieldshiftsuniformlywith\ncrack propagation. Therefore, we consider averaging over the thickness to be a valid approach for assessing\ncrack speed, as long as the deformation field remains largely self-similar with increasing crack propagation\ndistance,hencestrongdynamicsarenotpresent. Thatsaid,weacknowledgethatthisassumptiondoesnothold\natthesupercriticalcracklength,wherethetransitiontosupershearpropagationoccurs. However,atevenlarger\ncrack propagation distances, the deformation fields appear to regain self-similarity with propagation distance.\nIn this context, we would like to draw attention to Supplementary Movies SM2 and SM3, which show the\ntime-resolveddisplacementfieldsandtheirhomogeneity.\n11. Whathigh-speedcamerawasusedandwhatwastheexposuretimeat5000and10000fps,respectively?\nIsitnaturallightenoughevenattheexposuretimeof10000fpsorwasanyotherlightsourceemployed?\nA Phantom VEO 710 high-speed camera was used (see Materials and Methods). Exposure times were\n199.6µs and 99.0µs, respectively. The frame rate was limited by the available daylight. Consequently, the\nfirst experiment was filmed at 5000 fps, as direct sunlight was not available at that time of day. We added the\nmissinginformationinthetextandExtendedDataTable1.\n12. Fig.3andExtendedDataFig. 4: Iwouldsuggestaddingandmarkingregionsofcloseup#1andclose\nup#2inpanel(a),similartosupplementarymovies1and3.\nThankyouforthissuggestion,wehavechangedthefiguresaccordingly.\n13. Extended Data Fig.1: What are grain types a-e-d-g? Why not using the two-letter code indicated in\n“The international classification for seasonal snow on the ground” (ref. 30)? The grain type should be more\nclearlyexplainedanyways.\nThanks for pointing this out. We have switched to the two-letter code and provided further explanation in\nthecaption\nResponse to Referee #2\nWethankReferee#2fortheirinsightfulcommentsandsuggestions,whichhelpedidentifyareaswhereour\ndiscussionlackedclarityandcouldbemisinterpreted. Theseremarkshavebeenveryhelpfulinimprovingboth\nthe readability and overall quality of our manuscript. Below, we provide detailed responses addressing each\npointraised.\nAccording to this manuscript by Bergfeld et al., confirmation of supershear cracks in snow was limited to\navalanche videos (Hamre et al., 2014) and numerical simulations by the authors (Trottet et al., 2022). This\nnextmanuscriptaimstore-confirmthattheirconceptisvalid,arguingthattheirearlierpublishedexperimental\nconfirmationofsupershearviafull-scalemeasurementsshouldberepeatedinacontrolledsetting.\nThemanuscriptdescribesthaton19January2021,asupershearcrackwascaughtoncamerainonefrac-\nture experiment. Since slab fracture did not arrest this shear crack, the authors also suggest it may lead to\nwidespreadpropagationandbelievethishas“significantimplicationsforriskmitigationstrategies.”\nIdon’tknowhowexpertsinthePropagationSawTestrejectstatisticalflukesinaconvincingwayforstatis-\nticians. However,Icannotencouragethepublicationofclaimsbasedonn=1. Currently,theevidenceremains\nataquitepreliminarystage. FrommyowninteractionwithNatureCommunicationseditors,Ilearnedthatthey\nrequire a significantly more in-depth investigation for consideration as a full research article, and the journal\ndoesnothaveabriefcommunicationsformatavailableasanalternative.\nWe respectfully disagree with the assessment that our evidence is preliminary. Our conclusions are sup-\nportedbyhigh-resolution,full-fieldDigitalImageCorrelation(DIC)analysisandclose-upviews,bothofwhich\nindependentlyconfirmthesamebehavior. Thesemultipledatasources—includingcontinuousspeedestimates\nandlocalizedmeasurements—providerobust,high-qualityevidencefortheoccurrenceofsupershearfracture,\nleavingnoambiguityinourresults.\nWefullyrecognizetheimportanceofrobustscientificfindingsandareconfidentintherobustnessofourresults.\nRegardingadditionalpotentialmeasurements,weemphasizetheexceptionalchallengesofconductingsupers-\nhear experiments in snow. Long Propagation Saw Tests (PSTs) under the required conditions are exceedingly\nrare due to the necessity of a highly unstable snowpack with a persistent weak layer prone to crack propaga-\ntion, combined with a safe testing environment. Such conditions are seldom met simultaneously. In January\n2021, we identified a unique opportunity at a low-elevation site in Davos, where these conditions were finally\nsatisfied.\nA major difficulty is preserving the weak layer during setup to avoid premature triggering. To mitigate this,\nwe carefully removed the weak layer at the perimeter of the planned experiments while it was still forming at\nthe snow surface. After a subsequent large snowfall, we ensured it was intact and stable until testing. These\nconditions are exceptionally rare, as highly unstable snowpacks are typically found at higher altitudes, where\nlogisticalandsafetyconstraintsmakecontrolledexperimentsnearlyimpossible. Despiterepeatedattemptsover\nseveralyears,wewereunabletoconductsuccessfullongPSTsuntilthisrareopportunityarose.\nDuring this campaign, we successfully performed several long PST experiments, and in one of them, we\nobserved supershear fracture, conclusively validated by high-resolution, full-field DIC analysis and close-up\nviews. Theconsistencyacrossmultiplemeasurementsreinforcesthereliabilityofourfindings.\nItisalsoworthnotingthatmanyfoundationalstudiesonsupershearfractures—includingthoseexaminingsu-\npershear earthquakes—are based on single-event observations. While repeatability is ideal, our findings are\nunderpinnedbycomprehensive,high-resolutiondatathatoffernewinsightsintosupershearfracturedynamics.\nTo further substantiate our results, we conducted two independent numerical simulations using the Discrete\nElement Method (DEM, discrete approach) and the Material Point Method (MPM, continuum approach) to\nreplicate the experimental setup. Both approaches independently led to the exact same conclusions, further\nstrengthening the robustness of our experimental findings. The results of the MPM simulation are presented\ninFigure1, whichhasbeenincludedinthemainpaper. TheresultsofnumericalexperimentsusingDEMare\nshownbelowbutwerenotincludedinthemainpaper,astheyproduceessentiallythesameoutputastheMPM\nsimulations while requiring a significantly expanded Methods section. It was thus included as Supplementary\nNote instead. Both simulations were conducted using the same geometric setup for the weak layer-slab sys-\ntem,withhomogeneousslabproperties(densityandelasticmodulus)matchingtheexperimentaldata(average\nvalues)andaweaklayershearstrengthwithinthemeasuredrangereportedbyJamiesonandJohnson[6](ad-\njustableparameter). Bothsimulationswereablenotonlytoreproducetheobservedexperimentaldataverywell\nin terms of sub-Rayleigh speed, supershear speed, supercritical crack length as well as deformation patterns.\nTheyalsooffereddetailedinsightsintothetransitionmechanism,following,accordingtobothDEMandMPM\nsimualtionstheBurridgeAndrewsmechanism(seeFigures1c,1i,5c.2). Thesenumericalexperimentsfurther\nsubstantiate our experimental findings and reinforce the robustness of our conclusions. In particular, it fully\nrules out any potential ”statistical flukes” as suggested by the reviewer, as our observations are supported not\nonlybyhigh-resolutiondatafrommultiplesourcesbutalsobyresultsfromtwoindependentnumericalmodels.\nIn addition, we recognize (see also below) that the term ”Laboratory avalanche” in the title was not the\nbestchoice,asitmighthavemisleadinglysuggestthattheseexperimentsareeasilyreproducible. Inreality,our\nstudy represents a rare and exceptional achievement, capturing unprecedented insights into avalanche release\nandfracturemechanics. Thetitlewasthereforemodifiedaccordingly.\nMoreover,evenassumingthattheexperimentalevidenceisreproducible,Idonotfeelthereareconvincing\nanalogies to earthquake phenomena (currently, even basic references to supershear strike-slip are confusing\nandnotconsistentwiththeauthors’previouspaper-Trottetetal.,2022-asIexplainbelow).\nAs explained below, we acknowledge that a particular sentence in Trottet et al. may be open to misinter-\npretation. We will clarify our intended meaning and make the necessary revisions in the text of the present\npaper to prevent any misunderstandings. In addition, following the comments of Reviewer 1, we explored\ntheearthquakeliteratureingreaterdepthandidentifiedfurthersimilaritieswithourexperimentalresults,most\nnotably, the observation that sub-Rayleigh crack propagation is dominated by slope- or fault-normal displace-\nment,whereassupershearpropagationischaracterizedbydominantslope-orfault-paralleldisplacement(Fre-\nund(1990),Melloetal. Tectonophysics,2011;Melloetal. JMPS,2016;Rubinoetal. JGR,2020).\nAlso, the title is excellent clickbait but misleading: (i) no snowquake was studied in the paper, and (ii) I\ndo not think avalanche professionals call the Propagation Saw Test a laboratory avalanche. According to my\nreadingofthemanuscript,thetitleismorelike: “Onepropagationsawtestrevealssupershearcrack.”\nWe initially used the term ”snowquake,” which is often associated with anticrack-related ”whumpfs,” to\nemphasize the dynamic nature of the fracture process observed in our experiments, which shares similarities\nwithseismiceventsintermsofenergyreleaseandrapidcrackpropagation. However,weagreethatthiscould\nbemisleading,andwehaveremovedtheterm”snowquake”fromboththetitleandabstract. Wehaveretained\nits use in the introduction, where we describe the ”whumpf” sounds as well as anticracks and snowquakes\nreported in the snow literature. Regarding ”laboratory avalanches”, in seismology, it is common to refer to\nbasicsheartestsas”laboratoryearthquakes”duetotheirabilitytoreplicatecertainfeaturesofnaturalseismic\nevents. Similarly, a Propagation Saw Test (PST) reproduces several key ingredients of a snow slab avalanche,\nmakingitreasonabletorefertoitasa”laboratoryavalanche.”Nevertheless,weunderstandthatthistermmay\nbe misleading, especially in the context of the previous remark, and we are happy to modify the title to better\nreflectthescopeandfocusofourstudy.\nFinally,somesectionsofthemanuscriptreadlikearemakeofTrottetetal.,2022,butwhilesomesentences\nfeel like Deja Vu, others contradict the author’s previous words, and some alignment of arguments and the\nstorylineseemnecessary.\nIfanysentencesappeartocontradictthefindingsofTrottetetal.,thismusthavebeenanunintendedover-\nsightonourpartandwewillmakethenecessarychangestoimprovethestorylinehere(seebelow).\nFigure5: DEMsimulationofthePST3experiment. ThefirstrowshowstimeversuscracktiplocationinDEM\nand MPM simulations. The second row presents slab displacement fields, where color indicates displacement\nnorm(maximumvalue6mm)andarrowsrepresentdisplacementdirection,showninthesub-Rayleighregime,\nattheonsetofthesupersheartransition,andduringsteady-statesupershearpropagation. Thethirdrowdisplays\nbroken bonds, while the fourth and fifth rows show normal and shear stress distributions in the weak layer,\nrespectively. Resultsindicatethatinthesub-Rayleighregime,thecracktipremainssharplydefinedandaligns\nclosely with the peak in normal stress. At the transition, new bond breakage occurs significantly ahead of the\nmainanticrack,whichisstilldrivenbynormalstress,whereasthesenewlybrokenbondscorrespondtothepeak\ninshearstress. Inthesupershearregime,thecracktipsharpensagainandalignswiththeshearstresspeak.\nDETAILEDREMARKS(intheabsenceoflinenumbers,Ijustcopy-pastetextsegments):\np. 2, suspected in firn and snowquakes [7,8,9] I am unsure how thermal contraction cracks studied in [8]\nrelatetotheanticrack.\nThankyouforpointingthisout. Thereference[8]wasindeednotrelevanttotheanticrackdiscussion,and\nwehaveremoveditfromthemanuscript.\np. 2,limitedbytheRayleighwavespeedAreferencewouldbehelpful.\nWeaddedreferencetoSironetal. [11].\np. 3-4,laboratory(hereandelsewhere)Tome,thisisan“outdoorexperimentalsite”,becausealaboratory\nis“aroomorbuildingwithequipmentfordoingscientifictests”(https://dictionary.cambridge.org/).\nFor geologists and environment engineers, the field is often referred to as the laboratory (in Latin, combi-\nnation between laboratore: to work and -orium: place associated with a specific activity). We used this term\nto distinguish between ”classical” avalanche dynamics experiments performed at a very large scale. Despite\nthis, we modified the terminology accordingly throughout the whole paper and now refer to snow avalanche\nexperiments.\np. 4,supershearsnowquake(hereandelsewhere)Idonotseeanyrationaletomake“intersonicfracture”\nasynonymof“supershearsnowquake”exceptjustmakingitsoundcool. Marsquake,icequake,Moonquake,\netc.,refertoaseismiceventorvibrationproducedbysomething,likeacrack. Novibrationswererecordedto\ndeclare there was any kind of quake, and I am not even sure we know if it is detectable for different phases of\ntherupture.\nAlthoughitiscertainthatsuchafracturephenomenongeneratesaseismicsignal,weagreethattheseismic\nanalysiswasnotthetopicofthestudy. Thetermsnowquakewasremovedfromthetitle,abstractandfrommost\nofthepaper(exceptintheintroduction/stateoftheartsectionwherewebelieveitisimportanttomention).\np. 5,Fig. 2IunderstandtherationalelaterappearinginMethodsonp. 18aboutw(t),u(t). Nevertheless,\nIfind(a-Normal, close-up#1)nextto (b-Tangential, close-up#2)confusing. Comparisonofthesamecompo-\nnents seems more straightforward. It makes the figure speak for itself without needing a reader to pause and\nfigure out what is going on and whether the normal component drifts to subpanel (c) and how it falls into the\ncontext of shear in (d). This figure is introduced with a discussion of tangential displacement analysis, and\nonlyhalfapagelater,thereaderisinformedthatnormaltangentialdisplacementsinclose-up#1yieldsimilar\nspeedswithouttheseshownontheplots. Perhaps,captioncouldclarifythatc=41m/sin(a)wasalsoobtained\nfromu(t).\nWe followed the reviewer’s suggestion and added more details to the figure caption to highlight how the\nspeedsindicatedineachsubplotwereevaluated.\ncsig,cger couldbeexplainedinthecaption.\nThiswasaddedassuggested.\np. 6,normaldisplacementsdiffered. Whatwasthespeedbasedonthelatter?\nThe w-based speed was about 80 m/s. This can also now be seen more clearly in the new plot Extended\nDataFig. 5(seeFigure6below)inwhichthespeedevaluationinthefull-viewisshownbothbasedonw and\nu displacements. A similar analysis was also added to the main text in a new figure incorporating the results\nfromnumericalexperimentsreproducingthefieldtests.\nFigure 6: Slope-normal displacement with time for the three PST experiments (a,b,c). The slope-parallel\ndisplacement ˘is shown at the bottom (e,f,g). Slab fractures are shown with red arrows pointing at x-location\ntheyappeared.\np. 6,alignswiththeassumption\nItwasnotintroducedearlierandmightneedareference.\nWehaveaddedareferencetoTrottetetal. [9]\np. 8,ItisnoteworthyItcouldbehelpfultoexplainwhyitissobecauseitmaynotbeevidenttothereader.\nWe added to the text that this observation could be expected since fracture propagation in this regime is\ndrivenbyweaklayercollapseandthereforeslope-normaldisplacement.\np. 9,“providingdirectevidenceofasupershearsnowquakeduringsnowavalancherelease”Idonotthink\nanyevidencerelatedtothesnowquakewasprovided; i.e., nomeasurementsshowedthatthesupershearcrack\nisaseismicevent.\nThiswasrewordedasdiscussedabove.\nThepresentstudydoesnotprovideevidence“duringsnowavalancherelease,”only“duringonePropaga-\ntionSawTest.”\nWehavereformulatedthatweobtainedevidenceofasupersheartransitionduringasnowfracturetestrepli-\ncatingavalanchereleaseconditions.\np. 10\nIt is unclear how the unlikely crack arrest by slab fracture “parallels findings in earthquake science,” and\nshouldbesupportedwithreferences.\nThemisunderstandinglikelyarosefromthefollowingsentenceonourpart,whichweagreeisunclear: ’The\nobservationthatsupershearcrackpropagationislesslikelystoppedbyslabfractures,whichparallelsfindings\nin earthquake science, may have crucial practical implications. Just as supershear earthquakes are generally\nassociated with large magnitudes, supershear snowquakes resulting in slab avalanches are expected to come\nalongwithwidespreadcrackpropagationandextensiveavalanchereleaseareas,withsignificantconsequences\nfor risk management and mitigation.’ What we meant is that, in avalanches with supershear fractures, crack\npropagation is less likely to be halted by slab fractures. This suggeststhat supershear avalanches may involve\nwidespread crack propagation, leading to large avalanche dimensions. The association between supershear\navalanchesandlargedimensionsparallelsfindingsinearthquakescience,assupershearearthquakeshaveonly\nbeenobservedincasesoflarge-magnitudeevents. Wehaveclarifiedthisimportantaspectintherevisedtext.\nI could not follow the discussion about the crack arrest. On the one hand, Fig. 4 shows that normal col-\nlapse is uncoupled from tangential and arrives with a delay to the 8.5 m camera position. If so, vertical slab\nfractureisunlikelytotakeplacebeforearrivalofnormaldisplacement,whichlagsbehindthesupershear,\nand\nthus cannot arrest the crack. Moreover, I am confused about how Fig. 4 aligns with the numerical findings of\nTrottetetal. 2022,suggestingthatanticrackpropagationisnotrelevantatdistanceslargerthan3-5m(Trottet\netal.,2022).\nThisisnotentirelyaccurateandweclarifybelow. Slabfracturesarenotonlyduetonormalcollapse. Slab\nfractures can be caused by slab bending (induced by normal collapse) indeed or by slab tension (induced by\nslope parallel deformation) or a combination of the two. What we say here is that in the supershear shear\nregime, slab tension is building up and may lead to slab fracture. However, it was shown by Trottet that these\nslab fractures (purely induced by slab tension and not by collapse) could not stop the supershear fracture. We\nwouldliketopointthereviewertowardsthesupplementinTrottetetal. andFigure3inthisdocumentshowing\nsimulations including slab fractures. We recall them below for clarity. In the supershear regime, the collapse\nwavestillexistsbutislaggingbehindevenforlargepropagationdistances. Howeveritrepresentsasecondary\nprocessanddoesnotdrivethecrackanymore. Inaddition,pleasenotethatwhiletheslabtensilestressinduced\nby bending is limited by the extent of collapse, the tensile stress induced by slope-parallel tension continues\nto increase as the supershear crack propagates. Consequently, slab fracture will eventually occur; however,\nournumericalresults,supportedbytheexperiment,demonstratethatthisslabfracturedoesnothaltsupershear\ncrackpropagationintheweaklayer. Therefore, inthisregime, weexpecttopographytobetheprimaryfactor\ndeterminingwherethecrackstopsand,consequently,thesizeoftheavalancherelease.\n“supershearearthquakesaregenerallyassociatedwithlargemagnitudes”needsareferenceandspecifica-\ntionofwhichmagnitude. Moreover,inTrottetetal. (2022),thesameauthorsclaimtheopposite: “supershear\npropagationhasrarelybeenreportedinlargestrike-slipearthquakes.”\nWehaveaddedareferencetoBaoetal. [12]. RegardingTrottet,itseemsthesentencewasmisinterpreted,\nand we apologize if the main message was not clear. What we intended to convey is that supershear fractures\nhave only been reported in very large and therefore rare cases of strike-slip earthquakes. Thus, both claims\nalignandpointinthesamedirection. Wewillensurethisisclarifiedintherevisedtext.\nAnyway, I do not understand the “Just as” comparison of earthquakes to “supershear snowquakes” and\nwiderreleaseareas. Dosupershearearthquakeshavealargesizebecausetheyrupturefast? Also,inTrottetet\nal. (2022), the larger avalanche size was associated with higher slab density, making fracture arrest unlikely,\nandIamunsurehowthetwoeffectscoexist.\nThesentencehasbeenmodifiedandclarified. Regardingavalanches,yes,manyobervations[13]showthat\nthe thicker and denser the slab is, the larger is the avalanche release zone. Here, we assume that denser slabs\nhave a larger tensile strength and are therefore less likely to break before the supershear transition is met. For\ninstanceafter5metersofpropagation,ona35degreesslope,thetensioninaslabofdensity250kgm-3would\nbe around 2 kPa whereas the tensile strength would be around 3 - 3.5 kPa according to Jamieson [14]. There-\nfore,enoughtensioncanbuildupwithoutslabfracturefortheonsetofsupershearfracture. Wehaveexpanded\nthediscussiononthisimportantaspect.\n“Our findings reinforce the analogy between avalanches and earthquakes.” I could not understand which\nanalogy was reinforced? The anticrack concept inspired by deep-earthquakes, which can be completely ne-\nglectedbyusingsimpleshearmodels(Trottetetal. 2022)?\nAlthough we report that on a steep slope the anticrack mechanism is no longer the main driver of the\npropagation, it is still an important concepts that explains ”whumps” as well as remote avalanche triggering.\nThe concept of anticrack was first identified in earthquake science and later recognized as relevant to snow\navalanches. Similarly, supershear fracture has been extensively studied in earthquake science to explain the\nexceptionallyhighfracturespeedsobservedinsomelarge-magnitudestrike-slipearthquakes. Inthispaper,we\npresentdirectobservationsofsupershearfracturesinsnowavalancheexperiments,whichwebelievestrengthen\ntheanalogybetweenearthquakesandsnowslabavalanches. Furthermore,whileanticrackbehavioristypically\nassociatedwithdeepearthquakesandsupershearfractureswithstrike-slipearthquakes,ourfindingsshowthat\nboth mechanisms can coexist and transition within a single slab avalanche. Although the coexistence of these\nregimes within a single event has not been reported in earthquake science, it is remarkable that these two\nprocesses(anticrackandsupershearfracture), discoveredindependentlyinseismology, canoccursequentially\nduringavalancherelease. Thiswasclarified.\nI could not follow the logic here: “firn and snowquakes, like deep earthquakes, are often associated with\nthevolumetriccollapse... andanticrackpropagation. Inthiscontext,supershearsnowquakesareanalogous\ntosupershearstrike-slip(modeII)earthquakes.”First,aresupershearstrike-slipearthquakesoftenassociated\nwith volumetric collapse and anticracks? I do not think so. Second, are supershear strike-slip earthquakes\nrepresentdeepearthquakes? Again,Idonotthinkso,becausestrike-slipearthquakesareusuallyshallow.\nWe trust that the revisions to the text have clarified this point, as the reviewer’s interpretation does not ac-\ncuratelyreflectourintendedmeaning.\nInDiscussion,Imissedimplicationsforthedebatemotivatingthepaper.\nThisaspectwasbraughtbacktotheDiscussion.\np. 30,crustarrowImnotsurewheredoesitpointvertically.\nItpointstowardstheverythinkcrustlayerwhichappearsasahigh-densitylinejustbelowthearrow.\np. 32,typoofofthecracktip.\nThankyou. Thiswascorrected.\np. 36Ithinkh-a-blayeringslabelsshouldbeexplained,Similarly,ρ couldbeassociatedtoEDtable2\nm,−,+\nThisisnowbetterexplainedinthefigurecaption.\np. 39,theclose-up\nThanksforcatchingthiserror.\nResponse to Referees #3 and #4\nTheauthorsreportresultsofanexperimentalstudyofslabavalancherelease. Theexperimentsaredenoted\naslaboratoryexperimentswhichisaninterestingterminologygiventhattheywereconductedonanaturalslope\ninnatural snowpack(I canunderstandwhy theauthorsprefer notto speakoffield experiments). Theyadhere\ntotheprotocolofthepropagationsawtest,astandardtestingmethoddesignedtoprobeanticrack-typefailure\nof collapsible weak layers in snow. The performed high-speed imaging and DIC analysis yield high-quality\ndataforthedisplacementfieldswhichindicate,albeitonlyinoneofthreesamples,atransitionfromsubshear\nanticracktosupershearcrackpropagation. Intwoothersamplesthesametransitionwasnotobservedbecause\nofslabruptureafteranotherwisesimilaranticrackpropagationstage.\nWefindthisstudy,whichseemstopartlyconfirmanearliertheoreticalpredictionbysomeoftheauthors,\nto\nbeinterestingandpotentiallyimportantinviewofourunderstandingofthevariousfactorsthatdecidethesize\nof slab avalanches. The fact that not only the fracture propagation speed but also the slip pattern changes in\nthelatestageofcrackpropagationaddstotheevidenceforaqualitativetransitioninpropagationmechanism.\nTheauthorsprovideagoodbutnotexhaustingdocumentationoftheirdata. Inparticular,wewouldhaveliked\ntoseemoredataandclarificationonthefollowingpoints:\nWethankReferees#3and#4fortheirpositivecommentsonourmanuscriptandforveryconstructivesug-\ngestionsthathelpedustoimprovethereadabilityandqualityofourpaper(seedetailedrepliesbelow).\n1) The authors mention that crack speeds determined from u and w coincide for the ’anticrack’ stage but\ndifferforthe’supershear’stage. Canyougivetherespectivec andc vsxcurvesorequivalentinformation?\nu w\nWe have now included several additional plots in the suggested direction. Figure 6 in this document has\nbeen added as an Extended Data Figure, illustrating crack speed evaluation for the three PST cases based on\nboth u and w displacements. Additionally, we compare the crack tip position to the location where the slab\nreaches 50% of the collapse height—a metric often used to characterize the flexural wave in previous studies\n(e.g.,Gaumeetal. [15]). Thiscomparisonispresentedinboththeexperimentsandthenumericalsimulations\ninFig. 5ofthepaper,aswellasinFigure1ofthisdocument.\n2) Displacement fields are shown for the ’successful’ experiment but not for its two ’unsuccessful’ coun-\nterparts. It would be important to know whether in these two experiments the ’anticrack’ type displacement\npatternpersiststhroughoutthepropagation. Canyoushowthesefieldseitherinformofsupplementaryvideos\norgraphics?\nThankyouforthissuggestion. Torespondtothiscommentandotherremarksbyanotherreviewer,wehave\naddedExtendedDataFigure5whichcomparesthefull-viewanalysisforthethreePSTsincludingthecasesin\nwhich supershear fracture did not occur. When it comes to the displacement fields in the other tests, we have\nnow included 3 new supplementary videos showing the displacement vectors for the three PSTs based on the\nfull-viewanalysis,assuggested.\n3)Inextendedfigure7,itmightbegoodtoseea’zoom’atabout10m,i.e. nearthelocationoftheexper-\nimental window, to clearer assess the expected existence or not of a mach cone at that location. Also, despite\nstrenuouseffortswecouldnotfindadefinitionfortheabbreviation’FOV’.Thishasnothelpedourunderstand-\ning.\nTo address this comment, we have added a new plot to the paper as Extended Data Figure 8 which serves\nthepurposeofillustratingboththeBurridge-AndrewsmechanismandtheapparitionoftheMachcone. Atthe\nlocationofclose-up2,theMachconeisvisiblebutnotperfectlyclearyetbecausethesteadystatehasnotbeen\nreachedyet. WehavealsorewordedFOVfieldofview.\n4) The authors propose that a critical crack length is required for a transition into the supershear regime.\nArelateddefinitionofcriticalcracklengthforsupershearwasprovidedinapriorstudyusingnumericalsimu-\nlations(Trottet,B.,Simenhois,R.,Bobillier,G.etal.,Nat. Phys. 18,1094–1098(2022)),whichisalsocitedin\nthispaper. Itwouldbeinterestingtoseeifthiscriticalcracklengthcanbelinkedtotheobservations.\nFigure 7: Evidence of the Burridge Andrews mechanism in MPM simulation of the experiment. Plasticized\nparticles in the weak layer are colored in red and the slab is colored by the longitudinal rate of deformation\n(left)andshearrateofdeformation(right). Thesupersheartransitionoccursatapproximately5.5m,consistent\nwith the transition reported in . A Mach cone begins to form beyond this point, becoming distinctly visible\nonceasteadystateisachieved.\nInanewSupplementaryNote(#4),weestimatetheexpectedsupercriticalcracklengthbasedonweaklayer\nshear strength measurements by Jamieson and Johnson [6]. Using a realistic shear strength range of 0.5 to 2\nkPa, we obtain supercritical crack lengths between 1.25 and 7.3 m, aligning with experimental results. While\nthis analysis does not yield a single definitive prediction due to uncertainties in snow property measurements,\nit reinforces the conclusions of our study. This discussion has been incorporated into the main text. Further-\nmore, we conducted an additional analysis using two independent numerical methods (continuum: MPM and\ndiscrete: DEM) to verify whether the field experiments could be reproduced in terms of crack speed regimes,\ndeformation patterns, and supercritical crack length. The results of this analysis, presented in Figure 1, also\nhelpaddressotherquestionsrelatedtotheBurridge-AndrewsmechanismandMachconeexistence.\n5) The Burridge–Andrews mechanism is alluded to multiple times in the paper. While the analogy with\nstrike-slip earthquakes is clear, how exactly the Burridge-Andrews mechanism (initiation of a daughter crack\ninfrontofthemaincrack)takesplaceinthiscase(especiallyPST#3)isunclearinthepaper.\nWe hope that the inclusion of numerical simulations, additional illustrations, and text modifications will\nhelpclarifythisimportantpoint. Notably,wehaveaddedadedicatedsectionintheResultsdiscussingtheeval-\nuationandinterpretationoftheBurridge-AndrewsmechanisminPST3,basedonanumericalreproductionof\nthistest.\nOverall,wethinkthatthispapermaymakeanimportantcontributionofgeneralinterest,ifthementioned\npointscanbelullyaddressed.\nThankyouverymuch.\nReferences\n[1] Gre´goireBobillier,BastianBergfeld,Ju¨rgDual,JohanGaume,AlecvanHerwijnen,andJu¨rgSchweizer.\nMicro-mechanicalinsightsintothedynamicsofcrackpropagationinsnowfractureexperiments.Scientific\nReports,11(11711),2021.\n[2] A van Herwijnen and J. Heierli. Measurements of crack-face friction in collapsed weak snow layers.\nGeophys.Res.Lett.,36,2009. L23502.\n[3] I.Reiweger,J.Gaume,andJ.Schweizer. Anewmixed-modefailurecriterionforweaksnowpacklayers.\nGeophys.Res.Lett.,42(5):1427–1432,2015.\n[4] Bettina Richter, Ju¨rg Schweizer, Mathias Rotach, and Alec van Herwijnen. Validating modeled critical\ncracklengthforcrackpropagationinthesnowcovermodelSNOWPACK. TheCryosphereDiscussions,\n(June):1–21,2019.\n[5] C Sigrist. Measurements of fracture mechanical properties of snow and application to dry snow slab\navalancherelease. PhDthesis,ETHZu¨rich,2006.\n[6] J.B Jamieson and C.D Johnston. Evaluation of the shear frame test for weak snowpack layers. Ann.\nGlaciol.,32:59–69,2001.\n[7] B. Jamieson and J. Schweizer. Texture and strength changes of buried surface-hoar layers with implica-\ntionsfordrysnow-slabavalancherelease. J.Glaciol.,46:151–160,2000.\n[8] DenesSzaboandMartinSchneebeli. Subsecondsinteringofice. AppliedPhysicsLetters,90(15):151916,\n2007.\n[9] Bertil Trottet, Ron Simenhois, Gregoire Bobillier, Bastian Bergfeld, Alec van Herwijnen, Chenfanfu\nJiang, and Johan Gaume. Transition from sub-Rayleigh anticrack to supershear crack propagation in\nsnowavalanches. NaturePhysics,18(9):1094–1098,72022.\n[10] BingPan,HuiminXie,ZhaoyangWang,KemaoQian,andZhiyongWang. Studyonsubsetsizeselection\nindigitalimagecorrelationforspecklepatterns. Opt.Express,16(10):7037–7048,May2008.\n[11] MarinSiron,BertilTrottet,andJohanGaume. Atheoreticalframeworkfordynamicanticrackandsuper-\nshearpropagationinsnowslabavalanches. JournaloftheMechanicsandPhysicsofSolids,181:105428,\n2023.\n[12] HanBao,LiuweiXu,LingsenMeng,Jean-PaulAmpuero,LeiGao,andHaijiangZhang.Globalfrequency\nofoceanicandcontinentalsupershearearthquakes. NatureGeoscience,15(11):942–949,2022.\n[13] JohanGaume,G.Chambon,N.Eckert,M.Naaim,andJ.Schweizer.Influenceofweaklayerheterogeneity\nand slab properties on slab tensile failure propensity and avalanche release area. Cryosphere, 9(2):795–\n804,2015.\n[14] BJamiesonandCJohnston. In-situtensiletestsofsnowpacklayers. J.Glaciol.,36(122):102–106,1990.\n[15] J.Gaume,A.vanHerwijnen,G.Chambon,K.Birkeland,andJ.Schweizer.Modelingofcrackpropagation\ninweaksnowpacklayersusingthediscreteelementmethod. TheCryosphere,9:1915–1932,2015."}, {"round": 2, "reviewer_comments": {"reviewer_1": "I think the authors have done a great job at revising the manuscript. The remaining weak point is the repeatability of the supershear event. While reporting a single supershear event falls short of providing evidence for repeatability, I also think that due to the challenges involved in performing such “field experiments”, it is not realistic to produce experiments at the same rate as in a fully controlled laboratory environment.\nThe authors explain this very clearly in their response letter, discussing the requirement for a “very unstable snowpack – specifically, a persistent weak layer prone to crack propagation – combined with a safe slope for testing” and saying that “such conditions are seldom met simultaneously”. However, I think that these complexities are not emphasized as much in the manuscript. I believe it may be valuable to add some of this description somewhere in the method’s section, in order to illustrate the specific challenges of these field experiments that set them apart from tests conducted on lab specimens, especially considering the broader readership of this article.\nThe numerical simulations with the material point method and the discrete element method well complement the experimental observations and add robustness to the study. Overall, I think this manuscript can provide a very valuable contribution and I recommend it for publication.\nI also have some minor suggestions:\n- The term “Supershear fracture” is often used in the manuscript. It is general in nature and it probably fine. At the same time, I may also suggest perhaps a slightly more appropriate term: “supershear rupture” as this is typically used in the literature to refer to (frictional) ruptures along pre-defined interfaces, as it is the case for those propagating along the weak layer. This is opposed to fractures breaking bonds in a material or along a cohesive interface.\n- Fig. 3a: add label with close up#1 and #2 to connect the text more easily with the figure, without having to refer to Fig. 1.\n- Fig. 5: Magnitude of speed -> Velocity magnitude", "reviewer_2": "This is my second review of the study, and I find it in much better shape than the initial submission. The conclusions are still based on n=1 tests, but considering the demanding nature of such tests and in combination with numerical simulations, are convincing enough for publication.\nI have little to add, except some technical remarks:\nAbstract:\n\"Here, we report direct observations of a supershear fracture in snow avalanche release following the spontaneous transition from sub-Rayleigh to intersonic crack propagation.\" - This is an overstatement as no direct observations were made in snow avalanche release, only in a propagation saw test (or snow fracture test).\nFig. 5b-d:\n\"IVI\" could be defined in the caption.", "reviewer_3": "We are happy that the authors have comprehensively addressed the issues raised in our previous review. The added material is significant and has clarified our questions. Also the changes made in the title have helped to clarify the scope and content of the manuscript. Finally we note that the somewhat misleading term 'laboratory experiment' has been changed. We think that the paper provides an important contribution to the general literature, notably in view of the existence of similar phenomena in earthquakes. Also the obtained data are of very high quality and will be of wide interest to researchers involved in avalanche modelling. We recommend the paper for publication.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to reviewer’s comments\nManuscriptNCOMMS-24-62974-T\nSignaturesofthesub-Rayleightosupershearfracturetransitioninsnowavalancheexperiments\nbyBergfeld,Gaume,Pellet,Bobillier,Schweizer,vanHerwijnen\nIn the following, we provide (in blue) detailed point-by-point answers to the comments raised by the re- viewers (in black, italic). In addition, modifications made to the manuscript are highlighted in a separate file withtracked-changes.\nResponse to Referee #1 – Prof. Dr. Vito Rubino\nI think the authors have done a great job at revising the manuscript. The remaining weak point is the repeatabilityofthesupershearevent. Whilereportingasinglesupersheareventfallsshortofprovidingevidence\nfor repeatability, I also think that due to the challenges involved in performing such “field experiments”, it is\nnotrealistictoproduceexperimentsatthesamerateasinafullycontrolledlaboratoryenvironment.\nTheauthorsexplainthisveryclearlyintheirresponseletter,discussingtherequirementfora“veryunstable\nsnowpack – specifically, a persistent weak layer prone to crack propagation – combined with a safe slope\nfor testing” and saying that “such conditions are seldom met simultaneously”. However, I think that these\ncomplexities are not emphasized as much in the manuscript. I believe it may be valuable to add some of\nthis description somewhere in the method’s section, in order to illustrate the specific challenges of these field\nexperiments that set them apart from tests conducted on lab specimens, especially considering the broader\nreadershipofthisarticle.\nThenumericalsimulationswiththematerialpointmethodandthediscreteelementmethodwellcomplement\nthe experimental observations and add robustness to the study. Overall, I think this manuscript can provide a\nveryvaluablecontributionandIrecommenditforpublication.\nWethankReferee#1forhispositivecommentsonourmanuscriptandhisrecommendationforpublication.\nToaddressthecommentabouttheexperimentalsetupdifficulties,wehaveaddedthefollowingsentenceatthe\nbeginning of the results section, as suggested: Supershear experiments in snow are extremely challenging, as\nthey require a highly unstable snowpack and a safe, accessible slope, conditions that rarely coincide. Even\nunder favorable conditions, long propagation tests rarely succeed, especially for distances of 10 meters or\nmore[1]. Thesuccessfulexperimentspresentedherefollowedyearsofunsuccessfulattemptsandextensivesite\npreparationtopreservetheweaklayerandensurefavorabletestconditions.\nIalsohavesomeminorsuggestions:\n- The term “Supershear fracture” is often used in the manuscript. It is general in nature and it probably\nfine. Atthesametime,Imayalsosuggestperhapsaslightlymoreappropriateterm: “supershearrupture”as\nthis is typically used in the literature to refer to (frictional) ruptures along pre-defined interfaces, as it is the\ncaseforthosepropagatingalongtheweaklayer. Thisisopposedtofracturesbreakingbondsinamaterialor\nalongacohesiveinterface.\nWethanktherefereeforthiscomment. Whileweunderstandthat“supershearrupture”iscommonlyusedin\nthecontextoffrictionalrupturesalongpre-existinginterfaces,ourstudyinvolvescohesive-frictionalinterfaces,\nwhere bond breaking at the grain scale (modeled here through plasticity theory) plays a central role. As such,\ntheprocesswedescribeisnotpurelyfrictionalbutinvolvesthepropagationofafracturefrontthroughaweak\nlayer. For this reason, we believe that the term “supershear fracture” remains appropriate in our context and\nbetterreflectstheunderlyingphysicalprocesses.\n-Fig. 3a: addlabelwithcloseup1and2toconnectthetextmoreeasilywiththefigure,withouthavingto\nrefertoFig. 1.\nThiswasmodifiedassuggested.\n-Fig. 5: Magnitudeofspeed− >Velocitymagnitude\nThiswascorrectedassuggested.\nResponse to Referee #2\nThis is my second review of the study, and I find it in much better shape than the initial submission. The\nconclusionsarestillbasedonn = 1tests,butconsideringthedemandingnatureofsuchtestsandincombina-\ntionwithnumericalsimulations,areconvincingenoughforpublication.\nWethankReferee2fortheirpositivefeedbackandarepleasedthatourrevisionswereconvincingenough\ntowarranttheirrecommendationforpublication.\nI have little to add, except some technical remarks: Abstract: ”Here, we report direct observations of a\nsupershear fracture in snow avalanche release following the spontaneous transition from sub-Rayleigh to in-\ntersoniccrackpropagation.”-Thisisanoverstatementasnodirectobservationsweremadeinsnowavalanche\nrelease,onlyinapropagationsawtest(orsnowfracturetest).\nWemodifiedthissentenceandnowrefertosnowfractureexperiments.\nFig. 5b-d: ”IVI”couldbedefinedinthecaption.\nThisisnowdefinedinthecaption.\nResponse to Referee #3\nWe are happy that the authors have comprehensively addressed the issues raised in our previous review.\nThe added material is significant and has clarified our questions. Also the changes made in the title have\nhelpedtoclarifythescopeandcontentofthemanuscript. Finallywenotethatthesomewhatmisleadingterm\n’laboratory experiment’ has been changed. We think that the paper provides an important contribution to the\ngeneral literature, notably in view of the existence of similar phenomena in earthquakes. Also the obtained\ndata are of very high quality and will be of wide interest to researchers involved in avalanche modelling. We\nrecommendthepaperforpublication.\nWethankReferee#3fortheirpositivecommentsonourmanuscriptandtheirrecommendationforpublica-\ntion.\nResponse to Referee #4\nIco-reviewedthismanuscriptwithoneofthereviewerswhoprovidedthelistedreports. Thisispartofthe\nNature Communications initiative to facilitate training in peer review and to provide appropriate recognition\nforEarlyCareerResearcherswhoco-reviewmanuscripts.\nWethankReferee#4fortheirpositivecommentsonourmanuscriptandtheirrecommendationforpublica-\ntion.\nReferences\n[1] EHBair, RSimenhois, AvanHerwijnen, andKBirkeland. Theinfluenceofedgeeffectsoncrackpropa-\ngationinsnowstabilitytests. TheCryosphere,8:1407–1418,2014."}]

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