The authors would like to express our sincere gratitude to your thoughtful comments, which are very valuable for improving the quality of this work. We have carefully reflected on your concerns regarding the consistency among the field investigation, ring-shear tests, and numerical simulation. This issue was also raised by other reviewers, highlighting it as a critical aspect that needs to be addressed in the manuscript. In response, we have thoroughly re-examined each part and identified several areas where consistency can be strengthened. Please find our point-by-point responses below.

1. In our study, the phenomenon of impact induced large landslide runout is derived from the field investigation, which motivated us to conduct this study. Ring-shear test is a very powerful tool to investigate how the impact effect influences the large-displacement behaviour of the sliding zone materials from a mechanical perspective. However, this lab test can only represent mesoscopic behaviour within the shear zone, rather than the overall runout features of the landslide. Thus, we used numerical simulations to model the full landslide process.
   
   As suggested, an effective way to demonstrate the relevance of the modelling is to examine whether the simulations can reproduce the stress path we proposed in the physical tests. The difficulty is that the stress path derived from the ring-shear tests is highly idealised, whereas in the numerical simulations the rupture surface continuously evolves during the impact process. This leads to spatial variability in the stress paths within the shear zone, making them difficult to present with a single curve.
   
   Nevertheless, we agree that it is valuable to compare the stress path evolution. Therefore, we are going to plot the stress path of a typical point selected around the impact region to compare only the patterns of pore pressure evolution. We believe this addition could improve the coherence between the experimental and numerical results.

1. This is very good advice to improve the clarity of our study. In the initial manuscript, the test and simulation programs were only described in the text, which seems difficult to follow. We have added tables for both the ring-shear tests and the numerical simulations to clearly present their setups and organisation.
2. The MPM algorithm used in this study is based on an open-source software developed by the community. As we did not modify any algorithm, we initially provided only limited details. However, we agree that it is necessary to include a clearer description, particularly regarding the coupling between the solid and fluid phases, since pore pressure evolution is central to this study. Thus, we have added a brief description of the algorithm in the Methodology section.
   
   In our simulations, we aimed to study how the disintegration of the sliding mass and erodible sediment - controlled by material properties – affects the impact duration and consequently the pore pressure buildup. This is why we focused on the elastic parameters, which are used here to represent the material’s deformability and tendency to disintegrate. As you pointed out, other important factors also influence pore pressure response. To provide a more comprehensive analysis, we are going to add additional simulation scenarios to discuss the influence of these factors.

1. This is a very good suggestion. We considered maintaining the traditional structure with the methodology presented first, but we agree it is more intuitive for readers to understand the background and motivation before diving into the experiments. Therefore, we have reordered Section and Section 3 in the revised manuscript.
2. This is an important point. We agree that further clarification of the effective stress paths under different conditions is needed. Upon reconsideration, the total stress path represents the loading path imposed in the experiments and can be directly illustrated. The effective stress path is actually a response measured during the tests. Thus, it is not appropriate to present them together at the beginning of the manuscript. Instead, we are going to add a new figure in Discussion section to present three possible effective stress paths based on observations from the test results, along with corresponding explanations.
3. This sentence was originally intended to explain our observation that increasing impact load does not necessarily lead to faster liquefaction. However, we realised that the previous wording was unclear and even contained wrong ideas. As under undrained conditions, the stress increment is primarily carried by pore water pressure in saturated specimens. We apologise for this inappropriate expression. As noted in our response to Comment 5 and also to other reviewers, we have removed this sentence and will add more theoretical and phenomenological explanations in the Results section.
4. This expression was used in line 246 to interpret the numerical simulation results. What we intended to convey is that the formation of a low-resistance layer facilitating landslide motion in the numerical simulations is consistent with the behaviour observed in our ring-shear tests. To avoid confusion, we have replaced the phrase ‘previous physical experiments’ with ‘ring-shear tests’.
5. We apologise for the confusion. In each simulation, one parameter was varied while other was kept constant. Following your suggestions, we have now clearly documented all simulation scenarios and their control variables in the simulation program table (as mentioned in our response to Comment 2).

The authors would like to express our gratitude for your considerate comments. Detailed replies to each point are listed below.

1. It is very good advice for us to place more emphasis on the broader impact of the mechanical results from this study. The entire mechanical process of impact-induced liquefaction is one of the core findings of our work. To explicitly demonstrate how this understanding can be used for future hazard mitigation, we have now highlighted that attention to this impact effect is critical for analysing landslide mobility. Accordingly, we further discuss that developing mathematical models that more precisely reflect the impact effect would be important for the future improvement of landslide numerical simulations. In addition, recognising the significance of impact force levels and the properties of erodible materials can provide useful guidance for researchers and practitioners working on risk analysis and mobility estimation. In particular, incorporating parameters such as slope angle changes and the mechanical state of regional materials may be necessary when conducting hazard assessments. These points have been added to the Conclusion section to show the broader impact of our study.
2. We fully agree that it would be super interesting to conduct comprehensive experiments across multiple soil types and environmental conditions, which would help achieve a more complete understanding of the impact-liquefaction mechanism. However, we must admit that conducting a large number of tests is challenging and resource-intensive. In our current study, we are preliminarily stay at proposing this possible mechanism, which is most likely helpful in explaining the observations from the LSB landslide case. We acknowledge that the current study framework is limited in material scope, and we remain open to further discussion and future investigations that extend this work.

We also very much appreciate your careful corrections regarding the misspellings in the manuscript. The author’s name on the title page was generated by the submission system, and we will ensure it is corrected in the final version for consistency. The misspelled work ‘graduate’ has been revised to ‘gradual’.

The authors sincerely appreciate your thoughtful review of our manuscript. We originally intended to keep the manuscript concise, which led us to omit some of the details. However, we realise that this may have weakened the support for our findings. We fully agree with your comments and have now provided much more detailed descriptions of both the numerical and laboratory experiments. We believe these additions will help clarify and strengthen the support for our hypothesis. Detailed responses are listed below.

1. The angle $\alpha$ in Fig. 2 represents the angle between the impact direction and the surface of the basin sediment. Since the sliding surface is clearly exposed in the LSB landslide, this angle was defined as the angle of exposed sliding surface and the surface of the Maoyaba basin, based on the drone-derived digital elevation model. As suggested, we have added the input stress paths (as shown in Fig 2b into Fig 7), to better illustrate the experimental design. To improve consistency between laboratory and numerical results, we are also going to derive typical stress path trends from numerical simulations and compare them with those from the laboratory tests, as discussed in our response to Reviewer 1, Comment 1.
2. Relevant references, including studies using similar models and documentation of the adopted model, have now been added. In addition, the constitutive equations that explains how the solid and liquid phases are interacted have been added, as also suggested by Reviewer 1, Comment 3. Please find the details on this added description on the numerical modelling in the attached file to CC 1.
3. We have further discussed about this phenomenon and reached the following explanations based on the test results. According to the three different impact modes illustrated in Fig. 2, the tests produced three distinct responses. For $k_d = 1.7$, the impact peak (point B2 in Fig. 2) just reaches the failure line. The corresponding effective stress path moves rapidly leftward after impact and subsequently follows the failure line. This behaviour can be interpreted as structure-failure-induced liquefaction occurring at an early stage. During the unloading process, shearing continues, and pore pressure keeps increasing due to grain crushing, even though the controlled shear stress decreases. In contrast, for $k_d = 2.5$, the effective stress path moves upward toward the failure line. This indicates stress-induced failure at the initial stage rather than liquefaction induced failure. The subsequent pore water buildup that occurred in unloading process is mainly driven by grain crushing, which requires time or shear displacement to accumulate. This difference explains why the case $k_d = 1.7$ reaches complete liquefaction more rapidly. This discussion has been added to the manuscript.
4. The disintegration process is indeed an important factor influencing entrainment. In our current simulations, this process is not explicitly modelled (e.g., through fracture or breakage mechanics), but is instead indirectly represented through variations in material stiffness and compressibility. We acknowledge that this is a simplification. There are several additional processes not considered in the current model, such as variations in impact angle, degree of saturation, and other material heterogeneities. These limitations have now been discussed in the revised Discussion section.
5. We agree that our initial statement was imprecise, and we appreciate your correction. Impact velocity depends heavily on the initiation mechanism and the propagation process prior to impact, both of which are influenced by material properties. This is why we varied material properties in the numerical simulations. However, in the ring-shear tests, it is not possible to directly vary material properties without changing the soil type, nor can the impact velocity be independently controlled. Thus, we introduced the dynamic coefficient to quantify the impact load level. This parameter serves as a proxy for the combined effects of velocity and loading conditions associated with different material properties in real cases.
   
   To correct the misleading sentence, we have revised it to: ‘For a given landslide geometry, the mass at impact is fixed. However, the impact duration and velocity depend on the initiation mechanism and propagation processes, where are influenced by material properties. To investigate these effects, we assume a constant impact duration while varying the dynamic coefficient, which represents the level of impact load associated with different material conditions.’

1. We apologise for this overgeneralised statement. As discussed in our response to Comment 3, the efficiency of strength loss during impact depends on the relative contributions of structure-failure-induced liquefaction and shear-induced liquefaction. Therefore, the observed behaviour for $k_d = 1.7$ is specific to the conditions of this study and cannot be generalised. The sentence has been removed.
2. The difficulty in deriving physically-based erosion models has now been added to the manuscript, in agreement with your suggestion. Based on our own experience, we also acknowledge the challenges in ensuring energy conservation during material exchange and in model calibration.
3. The word has been corrected.
4. As described in lines 107-109 of the original manuscript, the stress states at point C and A in Fig. 2b correspond to stages C and A in Fig. 2a. These represent the conditions before and after the addition of an overlying material layer. Under the simplifying assumption that the surface of rapture evolves parallel to the sediment surface, both share the same slope angle $\theta$. The added layer increases both normal and shear stresses, following trigonometric relationships based on $\theta$, which correspond to the projections of line AC in Fig. 2b.
   
   However, in real cases, the rupture surface slope may vary spatially and differ from the sediment surface slope. Therefore, $\theta$ in the two figures is not necessarily identical. In this study, they are assumed to be the same for illustrative clarity. This clarification has been added to the figure caption. We have also revised the caption to define all notations, including $\phi_p$ (peak friction angle) and $\phi_m$ (mobilised friction angle).

1. Scales have been added to images (a), (b), and (c).
2. Poisson’s ratio and effective Young’s modulus were selected based on common value ranges for sandy soils (https://www.geotechdata.info/parameter/soil-young-s-modulus). The initial porosity values can vary strongly depending on the degree of weathering and the deposition state. In this study, they were chosen as representative values for highly weathered granite (sliding mass) and loosely deposited sandy sediments (erodible bed).
3. We fully agree that many other factors may influence the results. Based on the comments from all reviewers, we are going to conduct additional simulations to investigate the influence of porosity and permeability.
4. The X label has been added, and the abbreviations TSP and ESP have been replaced with their full terms.

We sincerely thank you for your thoughtful comments and for carefully pointing out the shortcomings in the writing. We fully acknowledge that the previous version required substantial improvement in terms of writing clarity and overall presentation. In response, the manuscript has been thoroughly revised, rephrased, and carefully proofread to improve the language. We have also incorporated more recent and relevant references to better position our study within the current state of knowledge.

In addition, A more detailed introduction to the numerical modelling framework has been added to the Methodology section, including clarification of the adopted model and its governing equations, as also suggested by Reviewer 1 and 3. Detailed responses to all specific comments can be found in the attached file.

Dear Hervé,

We are very grateful for your thoughtful and detailed comments. It is truly a privilege to receive feedback from the community with strong expertise in this field. Your remarks have helped us carefully re-examine the numerical modelling and clarify several important aspects.

We fully agree with your clear summary of recent studies demonstrating that dilatancy and permeability are two of the key factors that influences the generation of pore water pressure during landslide-induced shearing and impact. Your comments made us realise that the Introduction did not sufficiently review the current state of research in this area. Thus, we have modified a part of the Introduction to incorporate a more comprehensive discussion of these influential factors and the relevant literature. Please find the attached file.

In the original manuscript, we provided limited details on the numerical models because we used an open-source MPM software rather than developing a new algorithm. However, we recognise that this lack of detail may cause confusion regarding the underlying constitutive assumptions. In the revised manuscript, we have added a more complete description of the modelling framework, including the governing conservation equations, the phases coupling scheme, and the full list of parameter values.

The primary objective of the simulations, using an LSB-like landslide geometry, was to investigate how impact duration, which is linked to the disintegration and entrainment processes during impact and controlled by material properties, influences final mobility and runout patterns. We agree that elastic parameters such as Young’s modulus and Poisson’s ratio should ideally constrained by laboratory testing and treated as intrinsic material properties rather than tuning parameters. However, our intention was to explore how variations within a plausible range may influence impact duration and deformation behaviour. From a practical perspective, we aimed to show that once basic geotechnical parameters are obtained from laboratory tests, preliminary assessments of potential runout patterns could be performed without an extensive number of simulations.

In the previous simulations, the effective dilatancy angle of both materials was set to zero. We considered that the basin sediments were freshly deposited by fluvial processes and therefore loosely packed. Under this assumption, volumetric strain does not evolve after yielding, and volumetric behaviour is only determined by the Poisson’s ratio in the pre-yield regime.

Concerning permeability, the intrinsic permeability was fixed at 5×10^-11 m/s² for both materials. We agree that permeability critically controls the dissipation timescale of excess pore pressure and therefore influences erosion and runout. In the original study, we focused primarily on impact-induced pore-pressure generation rather than its dissipation. However, following your suggestion, we are extending the numerical analyses to include scenarios with different permeability values in order to evaluate their influence on pore-pressure persistence and mobility. The corresponding results and discussion are being incorporated into the revised manuscript.

For the assumed Young’s modulus values, we did not find the explicit reported value in your shared literature. Our previous selected values are common ranges selected from (https://www.geotechdata.info/parameter/soil-young-s-modulus). For Fig. 2b, thank you for pointing out the potential inconsistency. In the revised version, we have separated the figure into two: one showing the prescribed total stress path as the input loading condition, and another showing the measured effective stress path as the material response. Due to the rapid generated pore pressure, the strength of the specimen decreases, and the specimen may not sustain the imposed total stress level. This results in a mismatch in the imposed total stress and the responded effective stress. The true total and effective stresses in the specimens is recorded in the Results section as Fig. 7. Finally, we have revised the citation at line 27 to a more fundamental and physically based treatment of entrainment.

Once again, we sincerely thank you for your constructive and insightful comments. Please do not hesitate to let us know if any of our response remains unclear. It is truly our privilege to further consider and improve the manuscript based on your valuable suggestions.

With best regards,

The authors

We thank the reviewer for your positive comments and helpful suggestions for improving the quality of this work. Detailed responses are listed below:

1. The density of the soil specimens in the laboratory tests was controlled through consolidation under the same stress level in order to approximate field conditions, assuming a normally consolidated state. The densities were kept consistent across all tests. We acknowledge that direct extrapolation of the results to field-scale stress conditions is challenging, as the stress states varied strongly in the real case, whereas the laboratory tests represent an idealised situation.
2. In the numerical simulations, we fixed the impact load factor by using the same landslide geometry, and focused on the influence of impact duration on the runout pattern. This duration is related to the disintegration behaviour of materials, which is determined by the two elastic parameters. In contrast, the laboratory tests were designed to investigate the influence of different impact load levels. Together, these two approaches can provide complementary insights into the mechanism.
3. Indeed, it is quite difficult to distinguish them in the trench due to their similar mineral composition and long-term geological processes affecting this ancient landslide. Thus, the sampling was not conducted within the trench, but at the frontal boundary of the landslide as shown in Fig. 4, where the deposited landslide material is relatively thin and the alluvial material can be clearly identified in the active stream in front of the landslide.
4. The sentence has been revised to “Landslides are one of the most frequent and dangerous natural hazards in mountainous areas.”
5. Yes, this refers to the fact that basin sediments are typically deposited by fluvial processes without significant consolidation, resulting in loose deposits with a relatively high void ratio. However, as this sentence describes a general condition and the exact void ratio also depends on particle size distribution, we prefer to retain the original wording. We are open to revising this if the reviewer considers it necessary.
6. Thank you for this comment. In this sentence, we intended to refer to real landslide cases occurring in similar geomorphic settings to the LSB landslide, particularly the Oso landslide, rather than focusing on the specific model used in the cited study. To clarify this, we have added an additional reference related to Oso landslide (Yerro et al., 2019).
   
   Yerro, A., Soga, K., & Bray, J. (2019). Runout evaluation of Oso landslide with the material point method. Canadian Geotechnical Journal, 56(9), 1304-1317.

1. As our primary focus was on mechanical behaviour at critical states, we did not initially provide detailed initial density features. However, we agree that this information is important. The initial void ratio of each test has now been added, and B-Values were measured and confirmed to be greater than 0.95, indicating a high degree of saturation, as we clarified in the appendix.
2. We apologise that the previous section 2.2 did not what we did and how those parameters are considered. In response, we have now added tables to clearly present the design of the simulation program, along with a more detailed description of the methods used, including the governing equations. Please find the attached file to this.
3. Please see our response to Comment 3.
4. These sentences have been removed to the Method section.
5. We fully agree with this comment. As also noted in our response to Reviewer 1 Comment 6, we identified this as an incorrect interpretation of the laboratory observations and have thoroughly revised the explanation.
6. To improve consistency between the numerical simulations and laboratory tests, we have now included stress path in the simulations. These results may help illustrate how material properties influence the development and efficiency of path liquefaction.
7. This part has been revised following additional analyses on the influence of porosity and permeability. We agree with your suggestion and have moved this content to the Results section.