Previously hidden landslide processes revealed using distributed acoustic sensing with nanostrain-rate sensitivity

Domain: earth_science
Published: 2024-07-24
Editorial outcome: accept

Abstract
Landslides sometimes creep for decades before undergoing runaway acceleration and catastrophic failure. Observing and monitoring the evolution of strain in time and space is crucial to understand landslide processes, including the transition from slow to fast movement. However, the limited spatial or temporal resolution of existing landslide monitoring instrumentation limits the study of these processes. We employ distributed acoustic sensing strain data below 1 Hertz frequency during a three-day rainfall at the Hollin Hill landslide and quantify strain-rate changes at meter and sub-minute scales. We observe near-surface strain onset at the head scarp, strain acceleration at a developing rupture zone, retrogression towards the scarp, and flow-lobe activity. These processes with displacements of less than 0.5 mm are undetected using other methods. However, the millimeter processes over three days agree with previously observed seasonal landslide patterns. Here, we show landslide processes occurring with nanostrain-rate sensitivity at spatio-temporal resolution previously not possible.

Introduction
Landslides are a major global geohazard, resulting in the loss of thousands of lives1 and an estimated average of 20 billion USD in economic losses annually2. Slow-moving landslides are associated with movement rates ranging from millimeters to several meters per year3. However, these landslides may also accelerate without apparent warning3,4. These points highlight the unpredictability of slow-moving landslides and the importance of detecting the early onset of motion and understanding their movement patterns. Furthermore, landslide risk is expected to increase due to anthropogenic factors, including urbanization, deforestation, and the effects of climate change (e.g., increased precipitation, permafrost degradation, and wildfires)3,5–8. As settlements and infrastructure increasingly encroach upon areas with potential landslide hazards, effective monitoring systems are a critical component to support informed decisions for mitigation of landslide risk.

Landslide monitoring systems encompass a range of sensors and techniques to detect changes at the Earth’s surface (e.g., geodetic surveying, tiltmeters, and remote sensing) and at depth (e.g., inclinometers, acoustic emissions, and piezometers)9–13. Geophysical methods support landslide monitoring by inferring changes occurring at depth over broad spatial areas14. Satellite and ground-based remote sensing technologies excel in providing broad spatial coverage and enabling landslide monitoring at sites with difficult or hazardous access. However, they are limited in capturing sudden acceleration over shorter timescales and where seasonal effects may impede measurements3,15,16. In comparison, distributed fiber optic sensing technologies provide broad spatial coverage, but usually require more installation efforts versus remote sensing. Once installed, distributed fiber optic sensing can provide ongoing near real-time measurements17,18. Their combined high spatial and temporal resolution make them an attractive option to consider for landslide monitoring applications19,20.

Distributed acoustic sensing (DAS), also referred to as phase-sensitive optical time-domain reflectometry, is a fiber optic sensing technology relying on the phenomena of Rayleigh backscattering and is sensitive to axial strain and temperature perturbations in the fiber19. By injecting pulses of coherent laser light into an optical fiber, an optical phase change is recorded, resulting from the backscattered light between two sections of fiber21. As DAS permits a fiber optic cable to be repurposed into an array of broadband seismic sensors, it has effectively spurred a burgeoning field of research known as fiber-optic seismology22–28. DAS capabilities to monitor changes at low frequencies (i.e., toward 0 Hz) are less studied. Low-frequency DAS was used to characterize hydraulic fracturing geometry, demonstrating the value in the low-frequency domain29. Other studies, referring to a DC-coupled DAS as a distributed Rayleigh sensing system, demonstrate the capability of low-frequency DAS to measure changes in strain associated with the movement of slow-moving shallow landslides30,31.

We reveal the kinematics of a slow-moving landslide at the Hollin Hill Landslide Observatory in England by employing low-frequency (<1 Hz) DAS data over a 3-day period. The data are acquired with nanostrain-rate sensitivity, 1 Hz temporal sampling, and a 4-m spatial resolution over 925 m of optical fiber. This method provides insights into rainfall-driven near-surface landslide motion and quantifies the spatiotemporal landslide sequence, including the onset of slope movement, retrogression of the landslide and flow-lobe activity near the toe. The processes of this sequence occur at timescales of minutes and exhibit spatial patterns of meter scale. We compare the results with collocated geotechnical instrumentation to demonstrate the unparalleled spatiotemporal resolution of low-frequency DAS. Existing monitoring methods cannot resolve the observations we present. For example, monitoring methods such as ground-based interferometry and automated inclinometers (i.e., ShapeArrays) can resolve up to sub-millimeter displacements over minute timescales, but here we resolve changes in the order of nanometers with 1 Hz sampling frequency15,32.

The Hollin Hill Landslide Observatory, located in North Yorkshire, UK32, is one of the most studied slow-moving landslides in the world and has been monitored by the British Geological Survey since 20083,33. The landslide is classified as a very slow to slow-moving composite multiple earthslide–earthflow9,34, with average velocities typically ranging from 0.5 to 3.5 m/year9,35. Earlier studies9,31,32,35–37 divide the landslide into three major domains: the rotation-dominant domain (above mid-slope), the translation-dominant domain (mid-slope), and the flow-dominant domain (below mid-slope). Slope failure is due to the presence of very weak and highly weathered mud rocks within the landslide-prone Whitby Mudstone Formation (WMF), which outcrops on the valley side. Translational deformation of the WMF leads to rotational deformation above from unloading and oversteepening of the toe along the upper slope. The progressive displacement of the translational deformation occurs as a flow-like behavior over the Staithes Sandstone Formation (SSF) below the mid-section9. Landslide processes at Hollin Hill have been studied with multiple site characterization and monitoring methods, including seismic refraction tomography, geoelectrical resistivity, self-potential, inclinometers, piezometers, lidar change detection, interferometric synthetic aperture radar (inSAR), and cone penetration testing9,32,33,35–38. Earlier studies integrated multiple methods to reveal different controls on landslide movement. For example, 3D time-lapse imaging of inferred slope moisture content (from inverted resistivity models) demonstrate moisture accumulation in the upper slope over wintertime correlating with landslide reactivation and drainage from the mudstone to sandstone formation in the summertime19,35.

A single-mode fiber-optic cable of 925 m length is buried ~10 cm below the ground surface at the site (Fig. 1). Most of the fiber-optic cable installed is of tight-buffered construction, but 140 m of loose-tube, gel-filled cable was placed alongside a similar length of the tight-buffered cable for comparison. Our analysis focuses exclusively on data acquired from the tight-buffered cable due to its improved strain transfer properties. The British Geological Survey acquired data along the entire cable length using an OptaSense ODH-F interrogator unit with a spatial sampling interval of 1 m (i.e., channel) and a spatial resolution of 4 m (i.e., gauge length) over a three-day period in January 2021. The interrogator unit was housed in a nearby barn, located ~750 m from the site. The cable was buried in a shallow trench using a small backhoe and shovel, running from the interrogator housing along a farm track to the site30. Along the farm track, the cable was encased in protective housing and was laid bare at the site for improved coupling with the surrounding formation (Fig. S1). The processed cable sections (Fig. 1a) are parallel to the direction of slope movement covering an area of ~135 m by 50 m. The approximate slope gradient in the instrumented area is 15 degrees. Collocated instrumentation includes piezometers, automated inclinometers (ShapeArrays), and a weather station (rain gauge, air temperature, barometric pressure)9. The locations of the fiber optic installation and collocated instrumentation are shown in Fig. 1. Terrestrial lidar data were acquired in November 2020.

Strain perturbations along the cable result in an optical phase change Δϕ of the Rayleigh-backscattered light over the gauge length (LG) of the measurement, which is mapped along the axis of the fiber using principles of optical time-domain reflectometry, and has a linear relationship with strain ε19,20

ε = λΔϕ / 4πζnLG, (1)

where n corresponds to the refractive index of the fiber, λ corresponds to the wavelength of the coherent laser pulse and ζ corresponds to a scalar multiplicative factor to account for changes in the index of refraction (Methods). The optical phase changes and corresponding strain measurements represent a change from a baseline measurement occurring at the start of data acquisition. All references to strain ε and strain-rate ε̇ herein represent a relative measurement from the baseline measurement on January 12, 2021 at 11:38 UTC.

In this work, we apply a 1-Hz low-pass filter, then convert the optical phase data to units of strain and strain rate. To support interpretation, we create spatiotemporal images of the cable strain-rate data along select cable sections extending from the slope crest to toe. Cable sections are numbered from west to east in increasing order. The resulting spatiotemporal images of strain rate reveal complex patterns of accelerating and decelerating strain at the site. Landslide displacements are inferred from strain and velocities are inferred from strain-rate (Methods). These are quantitatively compared with collocated geotechnical instrumentation. We characterize landslide features at the Hollin Hill site by identifying distinct patterns in the spatiotemporal images, including retrogression towards the scarp and a flow surge event near the toe. Our results highlight landslide processes at scales previously unresolved, enabling insights into the kinematic evolution of slow-moving landslides.

Results
With 1-min temporal resolution (based on the median filter, see Methods), 4-m spatial resolution (akin to the gauge length), and nanostrain-rate sensitivity, we develop strain-rate spatiotemporal images (Fig. 2) with an unprecedented level of detail to assess evolving changes in the near-surface compared to remote sensing methods15. Our results highlight the strain-rate patterns observed at the four easternmost cable sections. Spatiotemporal strain-rate images for all six cable sections are included in Fig S2. Strain spatiotemporal images are also available for comparison (Fig. S3).

Positive and negative strain and strain-rate observations correspond to cable extension and compression, respectively, and we use the extensional or compressional descriptors herein. Thanks to the spatiotemporal characteristics of the DAS data, velocity features of the landslide are obtained per the following approaches: (1) the velocity occurring at a single location (i.e., DAS channel), (2) the velocity of a strain or strain-rate front propagating over multiple DAS channels with time. We guide our analysis using a conceptual framework to interpret key strain and strain-rate patterns, as follows:

(1) Extensional strain-rate observations that are upslope of compressional strain-rate occurring over the same time period are interpreted as a shallow slip surface intersecting with the cable (Fig. 3a). (2) Propagation of the extensional strain-rate processes upslope with time are interpreted as slope retrogression. We characterize the retrogressive behavior by analyzing the slope of the strain and strain-rate fronts with time (Fig. 3b). (3) A paired extensional and compressional strain-rate observation propagating downslope is interpreted as a surge of saturated materials (i.e., a flow surge) propagating over the cable (Fig. 3c). (4) A sequence of paired extensional and compressional strain-rate observations are attributed to topographic and material strength variations (Fig. 3d).

Rainfall occurred over a 1.5-day period (January 13 at noon to January 14 at 18:30) with a cumulative precipitation of 28 mm. Following our conceptual framework of strain-rate patterns (Fig. 3), we interpret the five event sequences which are ordered chronologically: (1) initiation of strain at the head scarp, (2) subsequent triggering of a rupture zone, (3) retrogressive strain towards the scarp, (4) a flow-lobe surge near the toe, and (5) stabilization of the rupture zone with a gradual increase in strain at the scarp. Sequences 1 through 5 are numbered and highlighted by dashed rectangles in Fig. 2b–e. The unparalleled spatiotemporal resolution of our findings reveals complex patterns of landslide movement at previously unresolved scales.

Strain initiation at head scarp (sequence 1)
After about 8 h of rainfall, the onset of strain is observed at the northeast corner of the cable at the scarp (Fig. 4b, f). Over the subsequent eight-hour period, we observed a strain rate of ~–4 nmm−1s−1 near the main scarp (corresponding to an average inferred velocity of –1 mm per day). The observed pattern at the scarp over this time corresponds to a low amplitude slip surface pattern (Fig. 2e). The strain change correlates with a steady increase in soil moisture content from 43 to 47%.

Rupture zone triggering (sequence 2)
On January 14 at ~3:00 UTC, a slip surface pattern develops southwest of the main scarp (Fig. 2c). Based on the greater magnitude of the observed strain changes at this location during this time compared to other cable locations, we interpret this as the triggering of a rupture zone. The strain rate decreases to a minimum value of –35 nm/m·s (inferred velocity of –12 mm/day) at 12:00 on January 14 (Fig. 4c). Over this period, the soil moisture content increases from 47% to a maximum of 53%.

Retrogression towards scarp (sequence 3)
From the triggering of the rupture zone (January 14 at ~3:00) to ~12 h later, the strain-rate front propagates from the rupture zone upslope towards the scarp (over a ~20 m distance; Fig. 2b). We interpret this as a retrogressive deformation of the landslide. This coincides with accelerating strain near the main scarp (Fig. 4d, 2c, upper solid line).

Flow lobe surge (sequence 4)
Further downslope, propagation of the strain-rate front at the flow lobe occurs across ~10 m in the downslope direction over a 4-h period (average velocity of the strain-rate front propagation is ~2.5 m/h; Fig. 2d and Fig. S4). This is interpreted as a surge of superficial material flowing over the cable (not representative of the motion of the rupture zone assessed above, where the material is assumed to be well coupled with the cable, see Discussion).

Stabilization of rupture zone (sequence 5)
The final sequence coincides with a stabilization of the rupture zone, where the strain rate approaches near-zero values (Fig. 2d, 4e). The strain continues to increase at more gradual levels at the main scarp up ~30 m until the end of the DAS acquisition period (Fig. 4b). The soil moisture content decreases to 43%.

Further to the above, historical aerial photography shows the westward propagation of the landslide and the development of the main scarp, occurring following major movement in 201638. With data acquisition over only a 3-day period, our results show that the landslide is continuing to propagate westward, considering the maximum strain changes in the rupture zone to the southwest of the existing scarp (Fig. 5).

DAS-derived displacements with geotechnical instrumentation
We rely on collocated geotechnical instrumentation to evaluate the inferred displacements from the DAS strain. Our comparison provides evidence for the reliability of the relative magnitudes of the DAS strain-rate, strain and inferred displacements. Furthermore, it illustrates the low noise levels at a single DAS channel in comparison with the ShapeArray, permitting unprecedented resolution of changes in movement to be detected.

We compare relative displacement data from two vertical ShapeArrays installed near the fiber optic cable to a depth of 2.5 m, at the west flow lobe and east flow lobe in 2013 (Fig. 6). The near-surface (~0.3-m depth) ShapeArray data were processed to obtain relative displacement from a common baseline for comparison with the inferred DAS absolute displacements (Methods). Generally, higher magnitudes of both ShapeArray and DAS data are observed at the west lobe. The total displacement at the east-lobe and west-lobe ShapeArrays are 0.6 and 0.8 mm, respectively. The DAS-derived displacements at nearby locations vary from 0.3 mm (between east and west lobes) to 0.4 mm (adjacent to west-lobe ShapeArray). The observed discrepancies are likely a result of (1) the accuracy and precision of the ShapeArray data at very small (<1 mm) displacements; (2) the strain transfer occurring inside the fiber-optic cable and the coupling of the cable to the surrounding ground; (3) the different depths of measurement and sensor locations; and (4) the different nature of the measurements, where the ShapeArray data provide a discrete measurement at a sensor and the DAS data provide a distributed measurement over a 4-m length of optical fiber (i.e., the gauge length; LG). The observed variability between subsequent ShapeArray samples (±~0.1 mm) illustrates higher noise in comparison with the DAS estimates (Fig. 6). As the strain is enclosed within a fiber-optic cable comprising multiple layers with imperfect coupling to the surrounding ground, this results in a lower strain in the core versus the strain in the surrounding ground39,40. The temporal changes in displacement are well correlated, with the main increase in strain observed to occur on January 14 at noon in both the ShapeArray and DAS data.

Methods
Technology background
DAS relies on Rayleigh light backscattering, where small-scale variations in the refractive index of the fiber, due to its crystalline structure, allow for Rayleigh backscattering to occur. The phase of the backscattered light is altered where there are localized disturbances to the fiber. By measuring the difference in phase between two sections of the fiber, a signal is obtained that can be converted to a strain or strain rate for output, using the speed of light and known properties of the fiber22,56. As such, DAS does not measure absolute strain, but instead measures relative change in strain dependent on the optical phase change attributed to a disturbance in the fiber21. The DAS measurement corresponds to a discrete point (i.e., channel) along the fiber, which is averaged over a length of optical fiber (the gauge length LG)51,57 (Fig. 7).

The output of DAS is dependent on the type of interrogator employed. For this experiment, the raw DAS output was provided in units of optical phase change (Δφ) and can be linearly related to the average strain (ε) along the axis of the fiber over the gauge length LG (Eq. 1)23. For this study, n corresponds to a refractive index of 1.468 for the single-mode fiber, LG corresponds to a gauge length of 4.084 m, λ corresponds to the wavelength of the coherent laser pulse in a vacuum of 1550.12 nm and ζ corresponds to a scalar multiplicative factor of 0.81, accounting for changes in the index of refraction. Separate from the gauge length, the channel spacing determines the spatial distance between each sample along the optical fiber (Fig. 7). When the channel spacing is less than the LG, the effect of the latter is the same as applying a moving average window of length LG centered at each channel.

As the output from DAS is represented as an average strain ε over the gauge length, displacement d is inferred by integrating the average strain over the gauge length LG

d = εLG (2)

Strain rate is obtained by performing a temporal derivative on the average strain ε

ε̇ = dε / dt (3)

Velocity v is inferred by integrating the average strain rate ε̇ over the gauge length LG

v = ε̇LG (4)

Temperature effects
In addition to strain changes, temperature changes ΔT also cause a linear change in the optical phase change Δφ recorded by DAS58–60. This can be represented using the DAS thermal coefficient CT and the DAS strain coefficient Cϵ58, where

Δφ = CTΔT + Cϵε (5)

We estimate the influence of temperature changes on our DAS data by using an nearby temperature sensor installed at the fiber optic cable depth (10 cm below ground surface). The temperature sensor is installed at the location of the weather station (Fig. 1a). We follow the approach described by ref. 58 to compute the equivalent strain for the general temperature change of 1.7 °C (Fig. S7) occurring over our DAS acquisition period. The DAS thermal (CT) and strain (Cϵ) coefficients using Eqs. 6 and 7.

CT = (4π / λ) * (nLG * ξT + ξϵ * α + α) (6)

Cϵ = (4π / λ) * (nLG * (1 + ξϵ)) (7)

The variables used to obtain CT and Cϵ are outlined in Table 1. Temperature and strain are represented as average values over the gauge length of 4.1 m. The DAS thermal and strain coefficients were computed as 427 radians/°C and 3.8×107 radians/strain. As such, a change in 1 °C corresponds to a change of 1.1×10−2 millistrain. Over the three-day DAS data acquisition period, the ground temperature decreased by about 1.7 °C (Fig. S7). This change can be converted into an equivalent strain of 1.84×10−2 millistrain. This value is approximately two orders of magnitude lower than the main strain changes observed over the rainfall period that were used to inform our conceptual model (Fig. S3). As such, we deem it reasonable to neglect the effects of temperature in our work. However, incorporating the effects of temperature change could be used to increase the DAS measurement precision at lower strains.

Data processing
Although most geotechnical studies involving DAS to date focus on its applications as a seismic sensor61–63, we focus on low-frequency (<1 Hz) DAS measurements to extract meaningful information on changes in static strains. This was demonstrated by ref. 29, who used low-frequency DAS to measure small changes in strain for hydraulic fracturing applications. Here, we extend the application of low-frequency DAS toward a slow-moving landslide using a tight-buffered fiber optic cable. The following data processing steps are implemented to obtain low-frequency DAS strain, displacement, strain rate, and velocity measurements.

DAS data were acquired using the OptaSense ODH-F interrogator unit in the quantitative mode. The obtained data represent the optical phase change at a sampling rate of 500 Hz. Data were acquired from OptaSense in a proprietary file format of 32 files, with individual file sizes of 10.7 gigabytes. They are decimated to 50 Hz with a low-pass antialiasing filter and exported to HDF5 file format using the OptaSense DxS software, where each file contains all DAS channels over a 1-min period.

The remainder of the data processing steps are completed using the following open-source Python modules: SciPy64, NumPy65, Pandas, H5Py, Obspy, Zarr, and Xarray66. The processed strain and strain-rate DAS datasets are saved using Xarrays containing appropriate metadata (time, channel, and coordinates) in NetCDF files corresponding to individual cable segments (six segments numbered in increasing order from west to east) with explanatory Jupyter notebooks detailing the following processing steps67:

1. The DAS data were decimated to 1 Hz following the application of a low-pass antialiasing filter.
2. Following decimation, the data arrays are reshaped, concatenating the one-minute files to incorporate the full data acquisition time period (~3 days).
3. The concatenated optical phase data are converted to strain using Eq. 1.
4. Strain measurements are obtained by subtracting subsequent samples from the initial sample at time 2021-01-12T11:38:10 for each DAS channel. No median filter is implemented on the strain data.
5. Strain-rate data were obtained by computing the temporal derivative of strain using the central differences method between consecutive samples (Eq. 3).
6. A 2D median filter is implemented on the strain-rate data, using a window size of three channels and 59 s, with zero padding on data extents.
7. Displacement and velocity data are inferred by integrating the strain and strain-rate data over the gauge length, respectively (Eqs. 2 and 4).

The locations and elevations of the fiber optic cable were collected using a Leica Viva GS14 antenna and CS15 handset at 20-m intervals. Linear interpolation between each as-built location was used to infer location data at each DAS channel.

Data gaps
Three periods of data gaps, ~45-min duration, are visible in Figs. 2, 4, 6 are due to corrupted data files, where the original data were unable to be recovered. These periods are excluded from the above data processing steps. The data were reindexed to a continuous time vector using the timestamp associated with each filename, and the data gaps were incorporated as “NAN” (i.e., not a number) values.

Geotechnical site data used to support our interpretation are described in the following paragraphs.

Weather station
A weather station, installed in 20089, records soil moisture with a sensor installed at 0.1-m depth and rainfall data recorded at 30-min intervals (Fig. 1). Data from this weather station is publicly available as part of the UK Centre for Ecology and Hydrology68.

Displacement measurements (ShapeArray)
ShapeArrays (also known as Shape Acceleration Arrays; SAAs) were installed in 2013 and have been providing near continuous measurements since that time9. ShapeArrays consist of an array of microelectromechanical system (MEMS) accelerometers. These instruments measure the acceleration relative to gravity in the x-, y-, and z-directions at rigid segments along the length of the instrument to obtain a tilt measurement at each segment. Using a fixed reference point, relative displacements can be calculated from the tilt data69. ShapeArrays were installed to depths of ~2.5 m below ground level with 0.25-m sensor spacing. Instruments were assumed to be installed with the x-direction oriented horizontally parallel to the direction of movement along the downslope axis. In comparison, the DAS represents axial strain along the direction of the cable, parallel to the slope (average slope gradient of 15°). To account for the difference in magnitude of a displacement along the horizontal axis recorded by the ShapeArray and at a slope gradient of 15° recorded by DAS, a correction factor of 0.97 (cos 15°) could be implemented. For this work, we do not implement a correction factor, as 0.97 is negligible for our purposes. However, we note that this should be considered for future experiments with greater slope gradients comparing DAS and ShapeArray data. Over the DAS acquisition period, the ShapeArrays acquired data with a varying sample interval between 30-min to 1 h. The west lobe ShapeArray was not recording data from January 14 at 11:00 to January 15 at 13:00 for reasons unknown. To compare the ShapeArray data with the DAS data, we selected a common baseline reference to review relative displacement. We selected the closest available time (January 12 at 12:00) based on the ShapeArray sampling interval to the beginning of the DAS recording (January 12 at 11:38). The x-direction displacement at the shallowest sensor (at ~0.3-m depth) was selected for comparison with DAS. A full-depth profile of the west lobe and east lobe ShapeArrays is included in Fig. S8, illustrating changes over the three-day period. Based on the lateral deformation along the ShapeArray installation depth, we observe an initiation of movement at 1.5 m depth at the west lobe ShapeArray, and at 1.0 m depth at the east lobe ShapeArray. Deformation is observed to be distributed towards the surface. We select two DAS channels at adjacent cable sections closely aligned with the ShapeArrays for comparison.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The paper provides a comprehensive analysis of a rainfall-induced slow-moving landslide event using a DAS strain data at low frequency. The results are of great interest and offer extremely valuable insights into the dynamics of slow-moving landslides. In addition, as highlighted numerous time in the paper it brings promising results for the development of early warning systems. However, the paper might need some improvements before being consider for publication. Notably, to enhance the overall flow of the paper, it is advisable to eliminate repetitions and maintain consistency in terminology and units.\nHere are specific suggestions to help improving the manuscript:\nIntroduction:\n- L54: Consider rephrasing the term \"survey\".\n- L92: Consider using consistent units for clarity and comparability.\nBackground:\n- L142: Strain change is described but the you only refer to strain and strain rate. It would be\nnice to include the units and maintain consistency throughout the text.\n- Fig 1: Consider enhancing clarity by using a different method to highlight profile b without\narrows (maybe circle it in black) and improving the readability of the LiDAR background.\n- Consider removing reference to CIF formation in the map since it’s not visible. In addition,\nin caption “Cleveland Ironstone Formation 130 (CIF) and Staithes Sandstone Formation (SSF)\ngeological boundaries are per Merritt et al, 2014.” There is no reference to the CIF\nformation in Merritt et al, 2014.\n- Consider clarifying the meaning of arrow length in the map caption if there’s a specific\nmeaning to these.\n- L181: “corresponds”: would alternative terms like \"correlates\" or \"is associated\" be more\nappropriate?\nYou wrote: “We estimate an average retrogression velocity of ~1.7 m/hour, as the strain-\nrate front propagates over a 20-m distance in a 12-hour period.” I assume the retrogression\nvelocity is computed as 20 divided by 12. Is the 20 meters indicative of continuous\ndisplacement over 12 hours, a deformation propagating along a 20-meter transect, or a 20-\nmeter transect with movement during 12 hours, possibly peaking at 12mm/day?\nIn the latter cases, inferring a constant retrogression velocity of 1.7m/hr may not accurately\nrepresent the observed dynamics at Hollin Hill. Figure 2 suggests a combination of the two\nlast scenarios, involving both transect propagation and intermittent movement within the\n20-meter distance. May be interesting to clarify and be careful with the terminology you use\nto clarify the message.\nL194: “2.5 m/hour; Fig. S1, Sequence 3B” consider adding “Fig2” as well.\nI guess that the velocity of 2.5m/hr corresponds to the propagation of the displacement\nfront and not the landslide velocity?\nL195-197: Would it more accurate to the propagation velocity rather than the velocity itself\nin this context?\nL201: You use microstrain (1000) and have figures with millistrain and microstrain.\nFig3B, You changed the scale of millistrain to microstrain, using -0.2 to 0.2 millistrain would\nhave been fine.\nTo improve the clarity consider using the same unit everywhere.\nL235-237: The reference provided, 'Whiteley, J. S., et al. 2019,' does not pertain to the use\nof satellite imagery.\nThe order of figure citation is not continuous, Fig 6 cited in the Fig 3 caption and appearing\nbefore Fig 5.\nTo enhance readability, it might be beneficial to draw the scarps on Figure 4, preventing the\nneed to cite Fig 6 in L239 and maintaining a more logical order.\nRegarding Fig 5, if the positions of the lobes shown by LiDAR are consistent, it's worth\nreconsidering the relevance of comparing the east SAA and DAS at the green dot. The green\ndot appears to be situated between two lobes, potentially indicating minimal displacement.\nThe east SAA, on the other hand, is positioned at the border or within a lobe itself. While\nthis comparison may hold value for assessing temporal changes, it can be interesting to\ninclude other lines in the comparison.\nDiscussion:\n- Consider clarifying the meaning of the term \"retrogression velocity\" and its calculation.\nMaybe by reevaluating the interpretation of rapid movement in terms of “displacement\nfront propagation velocity” (or something shorter) rather than actual velocity.\n- L275: Consider using terms like \"small variations\" instead of \"slow variances.\"\n- Consider clarifying the relevance of citing Hungr and Varnes in the context of landslide\nhazard assessments.\n- Consider limiting repetitions (The 2 first paragraphs end with the same conclusion).\n- Consider include a conceptual drawing to replace fig6, illustrating the interpreted sliding\nprocesses more clearly. Also, Fig 6 is not cited in the discussion.\nL358-361: this assumption seems a bit risky with only 2 SAA to compare with at places\nrelatively close.\nIn overall, the discussion could be streamlined to eliminate repetition and directly address\nkey points. Additionally, it would be beneficial to emphasize the limitation of only\nmonitoring surface displacement and its impact on understanding the landslide's dynamics.\nCan the method effectively capture or infer deformation processes at depth? While the\napproach shows promise for depicting pattern of surface movements and providing early\nwarnings, the accuracy of quantifying deformation is debatable, especially considering\nspatial heterogeneity and temporal variations during events in coupling. A constructive\ndiscussion on potential method improvements for a more comprehensive understanding\nwould be valuable.\nMaterial and Methods:\n- Consider providing additional details about discrete points in L394-396, emphasizing their\nposition relative to the gauge.\n- Consider clarifying the definition of velocity, differentiating between L188 and L191.\n- L440: consider adding a paragraph for “The remainder…”\n- Consider clarifying the distinction between phase 3 and 4 and ensure consistency with\nterms like \"relative strain.\" That you never mentioned before.\n- L456: Are you suggesting that the fiber optic's location is determined through interpolation\nof positions at 'nb' points, measured along the fiber at 20-meter intervals using an RTK GPS\nsystem (LEICA, ...)?\n- Consider removing “complementary …” L460-461.\n- L523: unit missing\nL524-526: you already mentioned it L504-505.\nI noticed that all sections were named except the first one.\nPerhaps it would be beneficial to move the 'Temperature Effect' section to the beginning\nand potentially streamline or reduce its content since it has been neglected.\nSupplementary:\n- Consider organizing supplementary materials in the order of their citation in the main\npaper.", "reviewer_2": "Dear authors,\nThank you for your very interesting contribution. Please find my comments and suggestions\nin the attached document.\nSincerely\n[Editorial note: Please find reviewer #2’s attached document beginning on the next page.]\nReview of: “Previously hidden landslide processes using distributed acoustic sensing with\nnanostrain-rate sensitivity”, by Ouellet et al\nThis manuscript describes the use of low-frequency Distributed Acoustic Sensing (DAS) for monitoring the\nspatio-temporal evolution of landslide movements at the well-characterized Hollin Hill Landslide Observatory in\nthe UK. The study develops a processing flow to extract reliable low-frequency (< 1 Hz) strain and strain-rate\ninformation from the DAS data which shows the spatial and temporal evolution of deformation within the\nlandslide during a period of movement reactivation due to rainfall. Strain-rate and strain data are evaluated to\ndetermine 4 different deformation sequences affecting different zones of the landslide, which describe the\nlandslide movement from reactivation of the scarp, to development of a rupture zone in the central area of the\nlandslide and retrogression to the scarp and final stabilization. The low-frequency DAS measurements are also\nused to estimate displacements that are compared with independent deformation measurements from a\nShapeArray, with good agreement. The paper clearly demonstrates that low-frequency DAS has potential as a\nmonitoring tool in shallow landslides.\nFirst, I would like to start by thanking the authors for this very interesting study. I find results exciting and\nvery encouraging, and I think that this paper is an important contribution for the DAS and landslide\nmonitoring community. The paper is in general well written and illustrations are of good quality, clear and\nillustrative of the work carried out. I also appreciate the authors considering effects such as temperature\nchanges on the strain data, as this is one of the significant issues when interpreting low-frequency DAS\nmeasurements.\nI think that this paper is valuable and should be published. However, I also think that some changes and\nadditions could be made, which would improve the clarity and impact of the paper. Below, I have provided\nsome general and some more detailed suggestions that I hope the authors find useful to improve this\ninteresting contribution.\nMajor general comment\nThe Hollin Hill landslide has been monitored for a long time. A lot of work has been done and publications exist\ndescribing the inferred landslide structure and dynamics and discussing conceptual models for the landslide.\nHowever, not much of this work is mentioned in the paper. I encourage you to take advantage of this\ninformation available, and use it to do some more comparisons between what is learned and interpreted here\nfrom the DAS data, and what is already known about the landslide. The paper mentions that this investigation\nilluminates landslide processes that could not be resolved before, but it never really discusses what is already\nknown and what exactly new information is being provided by the strain data. Some more background\ninformation and discussion on this respect is needed to clarify what are the new insights gained from this\napproach.\nDetailed comments on Results\nIn general, I find this section a little bit difficult to follow. The observations are very complex (which is great, it\nmeans that you can see a lot), but then you need to be very careful and clear when you are describing the\nresults. My general suggestion is that you always start by describing what you see in the data, and then\ncontinue to say what you interpret that observation to be. Also, I suggest that you be specific about whether\nyou are referring to strain-rate or strain (which sometimes can show different trends depending on where you\nare in the temporal evolution curve). Another suggestion is to make specific references to Figures and the\nlocation of the features that you are describing (specially when you refer to Figure 2). For the reader who is not\nfamiliar with these types of measurements and cannot remember the morphology of the landslide just by\nlooking at Figure 1 once, it will be difficult to know exactly where to look in each of the panels if you don’t\nmention it.\n- I think it would be most helpful to already indicate in Figure 1 the location of the chosen channels described in\nthis section and plotted in Figure 3. I think it would also be helpful to mark the position of these channels in\nFigure 2; that would make the comparison of Figures 2 and 3 easier.\n- Line 174: “Sequences 1 through 4” - Please specify here that you are referring to the numbered squares in\nFigure 2.\n- I think you don’t discuss anything in the text, but there seems to be a relatively prominent anomaly\ndeveloping in the lower half of all profiles (within the flow dominated area of the landslide) at the time of\nmaximum soil moisture content (e.g. seen between channels 1140 and 1160 on L1). This anomaly seems to be\nfurther north as we move eastwards. Do you have any interpretation about what this feature could be based on\nwhat you know about the landslide?\n- Minor comment: I suggest to add a mention to Figure S2 here (somewhere in/after lines 163/166), so that the\nreader can compare the strain-rate images in Fig. 2 with the strain panels in Fig. S2.\nStrain initiation at head scarp\n- I am a bit confused about your observed “onset of strain” in Sequence 1. In the text (lines 177-182) you\ndescribe a “steady increase in strain near the main scarp, with an averaged inferred velocity of 1 mm per day”.\nAlthough slightly positive strain is observed in the upper few channels in Figure 2 for L6, the black line in Figure\n3B shows decreasing and not increasing strain. The gray line seems to indicate positive strain-rate just before\nthe start of Sequence 1 (just before the yellow band), but it is immediately followed by a decreasing strain-rate.\nAlthough this onset strain signal might be there, I don’t think it is clearly observable in these Figures.\n- Line 187: “… develops increasing strain centered around cable section L4” - A reference to the corresponding\npanel in Figure 2 would be helpful here.\nTriggering of rupture zone (Sequence 2), retrogression towards scarp (Sequence 3A) and flow lobe surge\n(Sequence 3B)\n- I also found the description of Sequences 2 and 3 difficult to follow. Two things that make this description\nconfusing for me:\n1) You describe strain evolution in terms of inferred velocities, but your Figures are all displayed in units of\nstrain or strain-rate. It would make it easier to link the text to the Figures if you talk about strain/strain-rate\nvalues first, and then mention what this means for the landslide processes in terms of inferred velocities.\n2)You talk about “strain” without mentioning if it is positive strain or negative strain. For example, in the\ndescription of Sequence 2 (lines 187-188), you mention “The strain accelerates to a maximum inferred\nvelocity of 12 mm/day at 12:00 on January 14 (Fig. 3A, sequence 2)”. When I look at the corresponding panel\nin Figure 3A, I see decreasing strain and strain-rate, and a large negative anomaly in L4 in Figure 2. So, in this\ncase, you are referring to a negative strain/strain-rate anomaly, which indicates compression. It is important\nthat you mention that during your description, otherwise it is difficult for the reader to follow, since they\nhave to look at two figures and read at the same time. Describing the observations in terms of “extension”\nand “compression” will make it clearer to understand what the strain/strain-rate data shows.\n- When you describe Sequence 3A, you start by saying “Here, a general retrogression of the rupture zone\ntowards the scarp occurs”. This is an interpretation, not an observation. Thus, I suggest that you start by saying\nthat you observe a relatively sharp increase in strain/strain-rate, which you interpret as a retrogression of the\nrupture zone.\n- Lines 192-193: “ This coincides with accelerating strain near the main scarp” - Please add a reference to the\npanel in Figure 2 where you can see this, and mention channel ranges so that the reader can easily locate the\narea and feature you are referring to.\n- Line 193: “ Further downslope...” - As above, where can I see this? Please refer to the Figures and the channel\nranges I should be looking at.\nStabilization of rupture zone with gradual deceleration in strain at scarp (Sequence 4)\n- Lines 201-205: “A maximum strain of ~1000 microstrain occurs at the rupture zone…” and “the strain\ncontinues to increase at more gradual levels at the main scarp...” - Please indicate where you can see these\nfeatures in Figures 2 and 3.\nDiscussion\n- Lines 297-308: I think this is a nice summary of the dynamics that you can see in the strain-rate data. Are\nthese interpretations solely based on your data, or are they also informed by previous work done on this\nlandslide? I think that your results agree nicely with the observations and conceptual models developed in\nUhlemann et al (2016)*, Figures 1 and 10. Specially, Figure 10 of this paper shows their interpreted\ncompression and tension zones in the shallow subsurface of the landslide, which I think agree reasonably well\nwith the profile that you show in Figure 1b (which is very nice to see!). Although this could have changed since\nthen due to the rapid movement of the landslide, I think it would be interesting to look into it and compare\nyour findings with theirs. You can even consider reproducing the figure (with permission) and show it along side\nyour profile if it makes sense.\n*Uhlemann, S., Hagedorn, S., Dashwood, B., Maurer, H., Gunn, D., Dijkstra, T., & Chambers, J. (2016). Landslide\ncharacterization using P- and S-wave seismic refraction tomography — The importance of elastic moduli. Journal of Applied\nGeophysics, 134, 64–76. https://doi.org/10.1016/j.jappgeo.2016.08.014\n- Lines 315-316: “Our observations are consistent with earlier studies that demonstrate the influence of\nseasonal precipitation on slope movement”. There is a lot of monitoring work done on this landslide using\ndifferent types of geophysical techniques (e.g. ERT and seismic monitoring). However, here you only compare it\nwith the ShapeArray data. This section should include a bit more (general) discussion on what these other\ngeophysical techniques show that is relevant to your study, and how your results provide more/complementary\ninformation about the landslide processes. I don’t suggest a detailed comparison as what you have done with\nthe ShapeArray data, but a description of what other works show and how they compare to your observations\nwould be granted here.\n- Lines 338-339: “Future research should investigate the effect of cable geometry on the detectable strain\nthresholds.”. I agree. Along the same lines, how about the effect of cable construction on the\nmeasured/detectable strain magnitudes? I know that you only had one type of cable, but can you (very\nbriefly) comment on how using loose-tube, gel-filled cable, for example, would affect strain measurements? A brief\nmention is sufficient since it is beyond the scope of your paper to compare cables, but I think it is an important\nconsideration when planning deployments for low-frequency DAS strain monitoring and it is worth mentioning.\nYou could also discuss this after the discussion on strain transfer in comparison with ShapeArray (ending in line\n361).\n- Throughout the discussion, I think it is worth making clear that this is a shallow landslide and movements are\nin the near-surface, where the cable is deployed. The methods could be less effective for deep-seated\nlandslides where the movements happen at depth.\nMaterials and Methods\n- How long are the data files recorded by the IU? Do you concatenate the data for the entire period of interest\nbefore you conduct your decimation, or do you apply your decimation file by file? Are steps 6 and 7 applied to\nindividual files or a long file encompassing the entire recording period? Please clarify.\n- Line 458-460: “ Short period of data gaps...” - DAS measures relative strain change, and stopping and\nrestarting the measurements causes loosing your reference strain state, sometimes. Can you comment on how\nlarge these data gaps are and how you deal with them in your integration? How do you make sure that you are not\nseeing a change in strain at each data gap?\n- Line 485: “)” needs to be removed.\n- I appreciate the effort to calculate the effect that temperature changes affecting the cable can have on strain\nmeasurements. Did you observe any effects associated with temperature changes affecting the IU itself? They\nshould manifest as “common-mode noise” along the cable. Did you have any temperature control in the barn?\n- Another way of monitoring temperature changes along the fiber would be to deploy DTS along the same cable\n(different fiber). I suggest mentioning this in the section describing the temperature effect evaluation, or even\nin the Discussion section of the paper, as an additional consideration for future experiments of this type.\nGeneral Minor suggestions\n- I suggest mentioning the name of the study site already in the Abstract. It is mentioned for the first time in the\nIntroduction with not much context (lines 78-79), and then directly introduced in the Background section.\n- Why is the photo of fiber installation the very last one in the Supplementary (Fig. S6) if it is the first one to be\nmentioned in the text? I would suggest moving it to the be the first one.\n- Line 146: “.. n corresponds to a refractive index” - I would be a bit more specific and say “refractive index of\nthe fiber”\n- Line 409-410 : “ Selecting a channel spacing less than the LG is akin to applying a moving average filter to the\ndataset” - I think this sentence is a bit confusing and could be rephrased. The gauge length is what acts as a\nmoving average filter. So, I would rather say that when channel spacing is smaller than the gauge length, the\neffect of the gauge length is the same as applying a moving average to the data, moving one channel at a time\n(or something along these lines).\n- Figure 3: the time labels seem to be displaced with respect to the tick marks, which makes it a bit confusing.\n- Figure 5: is there a data gap In the ShapeArray data on the West lobe? I think it is not mentioned in the text.\n- Figure 5: it might be useful to plot precipitation and soil moisture on top of the displacements, for\ncompleteness and aid reference to the previous figures and observations."}, "author_response": "Reply to peer reviewer comments\nComments from Reviewer 1:\nThe paper provides a comprehensive analysis of a rainfall-induced slow-moving landslide event using a\nDAS strain data at low frequency. The results are of great interest and offer extremely valuable insights\ninto the dynamics of slow-moving landslides. In addition, as highlighted numerous times in the paper it\nbrings promising results for the development of early warning systems. However, the paper might need\nsome improvements before being consider for publication. Notably, to enhance the overall flow of the\npaper, it is advisable to eliminate repetitions and maintain consistency in terminology and units.\nResponse: Thank you for providing your valuable input and constructive feedback on our manuscript. We\nhave addressed your feedback to improve the manuscript, including eliminating any repetitive statements\nand ensuring consistency with terminology and units. Our point-by-point responses are below.\nHere are specific suggestions to help improving the manuscript:\nIntroduction:\n- L54: Consider rephrasing the term \"survey\".\nResponse: Thank you for your suggestion. We have rephrased this term for clarity.\nApplied changes:\n- L53: Rephrased \"survey\" as \"geodetic surveying\"\n- L92: Consider using consistent units for clarity and comparability.\nResponse: We have revised out statement to use consistent units.\nApplied changes:\n- L90-92: \"For example, monitoring methods such as ground-based interferometry and automated\ninclinometers (i.e., ShapeArrays) can resolve up to sub-millimeter displacements over minute\ntimescales, but here we resolve changes in the order of nanometers with 1 Hz sampling frequency\"\nBackground:\n- L142: Strain change is described but then you only refer to strain and strain rate. It would be nice to\ninclude the units and maintain consistency throughout the text.\nResponse: Thank you for pointing out these important inconsistencies in our terminology and units. We\nhave incorporated several sentences to clarify the units. revised our manuscript to confirm consistent usage\nof 'strain change' and have added in a clarification note below.\nApplied changes:\n- L164-168: \"The optical phase change and corresponding strain measurement represent a change from\na baseline measurement occurring at the start of data acquisition. For clarity, we underscore that all\nreferences to strain 𝜀 and strain-rate 𝜀̇ herein represent a relative measurement from our baseline\nmeasurement on January 12, 2021 at 11:38 UTC.\"\n1\n- Fig 1: Consider enhancing clarity by using a different method to highlight profile b without\narrows (maybe circle it in black) and improving the readability of the LiDAR background.\nResponse: Thank you for your suggestion. We have updated Figure 1 accordingly.\nApplied changes:\n- Figure 1: Included a black outline to highlight Profile B and remove annotation. Updated figure caption\nto highlight Profile B captured in black outline.\n- Consider removing reference to CIF formation in the map since it’s not visible. In addition, in caption\n“Cleveland Ironstone Formation 130 (CIF) and Staithes Sandstone Formation (SSF) geological\nboundaries are per Merritt et al, 2014.” There is no reference to the CIF formation in Merritt et al, 2014.\nResponse: Thank you for pointing this out. We have removed the reference to the CIF formation on the\nmap and from the caption.\nApplied changes:\n- Figure 1: Removed reference to CIF from map and caption.\n- Consider clarifying the meaning of arrow length in the map caption if there’s a specific\nmeaning to these.\nResponse: Thank you for your suggestion. The length of the arrows are qualitative. We've revised our\ncaption to clarify this more appropriately.\nApplied changes:\n- Figure 1. Revised caption as follows: \"Strain vector lengths are qualitative and do not represent actual\nstrain magnitudes.\"\n- L181: “corresponds”: would alternative terms like \"correlates\" or \"is associated\" be more\nappropriate?\nResponse: We agree with your suggestion and feel that \"correlates\" would be more appropriate. We've\nincorporated this change into our revision.\nApplied changes:\n- L243-244: \"The change in strain correlates with a steady increase in soil moisture content from 43 to\n47%.\"\nYou wrote: “We estimate an average retrogression velocity of ~1.7 m/hour, as the strain-rate front\npropagates over a 20-m distance in a 12-hour period.” I assume the retrogression velocity is computed as\n20 divided by 12. Is the 20 meters indicative of continuous displacement over 12 hours, a deformation\npropagating along a 20-meter transect, or a 20-meter transect with movement during 12 hours, possibly\npeaking at 12mm/day? In the latter cases, inferring a constant retrogression velocity of 1.7m/hr may not\naccurately represent the observed dynamics at Hollin Hill.\nResponse: Correct, we computed the retrogression velocity as 20 divided by 12. The 20 meters is indicative\nof a deformation propagating along a 20-meter transect. In consideration of your comments, we have chosen\nto remove a reference to this retrogression velocity to improve the clarity of this section. We have reworded\nthis section as per our changes below.\nApplied changes:\n- L254-258: We modified this statement and removed the reference to the average retrogression velocity\nto improve clarity. The revised statement is as follows: \"From the triggering of the rupture zone\n(January 14 at ~3:00) to ~12 hours later, the strain-rate front propagates from the rupture zone upslope\ntowards the scarp (over a ~20 m distance; Fig. 2B). We interpret this as a retrogressive deformation\nof the landslide (Sequence 3A). This coincides with accelerating strain near the main scarp (Fig. 4D).\"\nintermittent movement within the 20-meter distance. May be interesting to clarify and be careful with the\nterminology you use to clarify the message.\nResponse: Thank you for your comments. We have made significant changes to this section which we feel\nhelp the improve clarity of our interpretation.\nApplied changes:\n- Created new figure to support interpretation (Figure 3. Conceptual framework to support\ninterpretation of strain-rate patterns).\n- L202-217: Added additional explanation of the strain-rate observations and their interpretation.\nL194: “2.5 m/hour; Fig. S1, Sequence 3B” consider adding “Fig2” as well. I guess that the velocity of\n2.5m/hr corresponds to the propagation of the displacement front and not the landslide velocity?\nResponse: Correct, the velocity of 2.5 m/hr corresponds to the propagation of the strain (displacement)\nfront. We describe this in our manuscript as \"propagation of the strain front at the flow lobe\". Thank you\nfor your suggestion, we agree that it is appropriate to add Figure 2 here as well. We have also revised Figure\n2 to improve the clarity and aid referencing the corresponding event sequences.\nApplied changes:\n- Figure 2. Revised for clarity (only including select cable section referenced within the text, highlighting\nthe corresponding event sequences appropriately within the figure).\n- L262: Updated figure references: \"~2.5 m/hour; Fig. 2D, Fig. S4\"\nL195-197: Would it more accurate to the propagation velocity rather than the velocity itself in this\ncontext?\nResponse: Per above. We have revised our wording of the velocity for clarity.\nApplied changes:\n- L260-265: \"Further downslope, propagation of the strain-rate front at the flow lobe occurs across ~10\nm in the downslope direction over a four-hour period (average velocity of the strain-rate front\npropagation is ~2.5 m/hour; Fig. 2D, Fig. S4). This is interpreted as a surge of superficial material\nflowing over the cable (not representative of the motion of the rupture zone assessed above, where the\nmaterial is assumed to be well coupled with the cable, see Discussion).\"\nL201: You use microstrain (1000) and have figures with millistrain and microstrain.\nFig3B, You changed the scale of millistrain to microstrain, using -0.2 to 0.2 millistrain would have been\nfine. To improve the clarity consider using the same unit everywhere.\nResponse: Thank you for your suggestion. We have chosen to only use units of millistrain when describing\nand displaying strain measurements for clarity and consistency.\nApplied changes:\n- L271: Upon revision, we have removed this statement (\"A maximum strain of ... \") as we feel it is not\nnecessary for the explanation of the event sequence and decreases clarity of the text.\n- L544: Revised from microstrain to millistrain \"As such, a change in 1°C corresponds to change of\n1.1·10-2 millistrain\"\n- Figure 4F (former Figure 3B) Revised units to millistrain.\nL235-237: The reference provided, 'Whiteley, J. S., et al. 2019,' does not pertain to the use of satellite\nimagery.\nResponse: Thank you for pointing out this error. We have updated this with the appropriate reference\n'Whiteley, J.S. et al. 2020. Landslide monitoring using seismic refraction tomography - The importance of\nincorporating topographic variations'. We also revised our sentence for accuracy.\nApplied changes:\n3\n- L303: Replaced \"satellite imagery\" with \"aerial photography\" and updated the corresponding reference.\nThe order of figure citation is not continuous, Fig 6 cited in the Fig 3 caption and appearing before Fig 5.\nResponse: Thank you for your feedback. We have revised all figure citations to be in continuous order of\ncitation.\nTo enhance readability, it might be beneficial to draw the scarps on Figure 4, preventing the need to cite\nFig 6 in L239 and maintaining a more logical order.\nResponse: Thank you for your suggestion. We agree with this approach and have updated Figure 4 (now\nFigure 5) accordingly.\nApplied changes:\n- Updated Figure 4 (now Figure 5) to include scarps. We have also removed the former Figure 6 as we\nfelt it decreased overall clarity of the manuscript.\nRegarding Fig 5, if the positions of the lobes shown by LiDAR are consistent, it's worth\nreconsidering the relevance of comparing the east SAA and DAS at the green dot. The green dot appears\nto be situated between two lobes, potentially indicating minimal displacement. The east SAA, on the other\nhand, is positioned at the border or within a lobe itself. While this comparison may hold value for assessing\ntemporal changes, it can be interesting to include other lines in the comparison.\nResponse: Thank you for your suggestions. We have revised our figure comparing the SAA and DAS\nchannels to include two additional DAS channels in the comparison.\nApplied changes:\n- Revised Figure 5 (now Figure 6) to include DAS channels from L3 and L4. We have also added in the\nprecipitation and moisture content to this figure, per a separate reviewer suggestion.\nDiscussion:\n- Consider clarifying the meaning of the term \"retrogression velocity\" and its calculation. Maybe by\nreevaluating the interpretation of rapid movement in terms of “displacement front propagation velocity”\n(or something shorter) rather than actual velocity.\nResponse: Thank you for your comments. We have made significant changes to this section to clarify.\nNotably, we have created a new figure to support our interpretation of strain-rate patterns, and revised our\nlanguage for clarity.\nApplied changes:\n- Created new figure (Figure 3) to support interpretation of results and clarify the meaning of terms such\nas retrogression velocity.\n- Revised language to describe observations in Results.\n- L354-357: 'Our study provides observations of strain-rate front propagation in the flow-dominant zone\nto characterize the spatial extents of an interpreted flow surge event occurring over an ~8-hour period,\nalong with its associated strain-rate front propagation velocity of 2.5 m/hour.'\n- L275: Consider using terms like \"small variations\" instead of \"slow variances.\"\nResponse: Thank you for your suggestion. Upon further review, we have revised this statement for\nimproved clarity and conciseness.\nApplied changes:\n- L357: Reworded former statement of, \"Our method can infer small variations ... \" as follows: \"Our\nmethod can be used to distinguish kinematic zones ... \"\n4\n- Consider clarifying the relevance of citing Hungr and Varnes in the context of landslide hazard\nassessments.\nResponse: Thank you for your feedback. We have revised this section to improve clarity of our citation by\nadding a separate statement with the Hungr et al. (2014) citation.\nApplied changes:\n- L357: Removed Hungr et al. (2014) citation from the following sentence: \" Our method can be used to\ndistinguish kinematic zones which vary over time to support landslide hazard assessment ...\"\n- L361-363: Added the following statement with the Hungr et al. (2014) citation: \" ... improves our ability\nto characterize landslide events and behaviours using current classification systems.\"\n- Consider limiting repetitions (The 2 first paragraphs end with the same conclusion).\nResponse: Thank you for your helpful input. We have revised this section accordingly.\nApplied changes:\n- L365-367: Modified the conclusion of the initial paragraph by merging with information from\nfollowing paragraph, as follows: \"Therefore, our DAS-based method can provide crucial information\nfor landslide early-warning applications by enabling monitoring over broad temporal and spatial\nscales.\"\n- L367: Deleted the following sentence from the end of the second paragraph to avoid repetition: \"For\nlandslide early-warning applications, DAS strain rates (and the inferred velocities) could become an\nimportant tool to enable monitoring over broad temporal and spatial scales.\"\n- Consider include a conceptual drawing to replace fig6, illustrating the interpreted sliding processes\nmore clearly. Also, Fig 6 is not cited in the discussion.\nResponse: Thank you for your suggestion. We have included a separate drawing to support our interpreted\nsliding processes (new Figure 3) and have removed the former Figure 6 for clarity.\nApplied changes:\n- Figure 3 (created new figure, titled: Conceptual framework to support interpretation of strain-rate\npatterns.).\nL358-361: this assumption seems a bit risky with only 2 SAA to compare with at places relatively close.\nResponse: Thank you for your comment. We have added an additional statement here to address this.\nApplied changes:\n- L440-443: \"However, with only two ShapeArrays sensors for comparison, and one of them at a location\noffset from the flow lobe, further studies on the inferred quantitative DAS-derived displacements are\nneeded to provide a more robust estimate of these assumptions.\"\nIn overall, the discussion could be streamlined to eliminate repetition and directly address key points.\nAdditionally, it would be beneficial to emphasize the limitation of only monitoring surface displacement\nand its impact on understanding the landslide's dynamics. Can the method effectively capture or infer\ndeformation processes at depth? While the approach shows promise for depicting pattern of surface\nmovements and providing early warnings, the accuracy of quantifying deformation is debatable, especially\nconsidering spatial heterogeneity and temporal variations during events in coupling. A constructive\ndiscussion on potential method improvements for a more comprehensive understanding\nwould be valuable.\nResponse: Thank you for your suggestions. We have made several changes to our discussion to address\nyour comments.\nApplied changes:\n5\n- L414-418: \"Future research should investigate the effect of cable geometry on the detectable strain\nthresholds and on the accuracy of quantifying deformation. For example, acquiring DAS data from a\ndownhole fiber optic cable paired with an inclinometer or ShapeArray, combined with a trench\ninstallation would provide useful information on the capabilities of DAS to capture deformations\noccurring at depth.\"\n- L440-443: \"However, with only two ShapeArrays sensors for comparison, and one of them at a location\noffset from the flow lobe, further studies on the inferred quantitative DAS-derived displacements are\nneeded to provide a more robust estimate of these assumptions.\"\nMaterial and Methods:\n- Consider providing additional details about discrete points in L394-396, emphasizing their\nposition relative to the gauge.\nResponse: Thank you for your helpful input and constructive feedback on our manuscript. We have\nincluded additional details accordingly.\nApplied changes:\n- L486-489: \"The DAS measurement corresponds to a discrete point (i.e. channel) along the fiber,\nrepresenting an average measurement which is centered over a length of fiber referred to as the gauge\nlength (L ) 51,57 (Fig. 7).\"\nG\n- Consider clarifying the definition of velocity, differentiating between L188 and L191.\nResponse: Thank you for your suggestion. We have made substantial changes to this section as per above,\nincluding the creation of a new figure that we feel helps the reader to follow our interpretation of the results.\nWe have made changes to this section as follows.\nApplied changes:\n- L204-207: \"Due to the spatiotemporal characteristics of the DAS dataset, velocity features of the\nlandslide are obtained per the following approaches: ( (1) the velocity occurring at a single location\n(i.e. DAS channel), (2) the velocity of a strain or strain-rate front propagating over multiple DAS\nchannels with time.\"\n- L246-252: \"On January 14 at ~3:00, a slip surface pattern develops ~30 m southwest of the main scarp\nat cable sections L3 and L4 (Fig. 2). Based on the greater magnitude of the observed strain changes at\nthis location and over this time period, in comparison with other cable locations, we interpret this as a\ntriggering of the rupture zone. The strain rate decreases to a minimum value of -35 nm/m•s (inferred\nvelocity of -12 mm/day) at 12:00 on January 14 at cable section L4 (Fig. 3A and Fig. 2, Sequence 2).\nOver this period, the soil moisture content increases from 47% to a maximum of 53%.\"\n- L440: consider adding a paragraph for “The remainder…”\nResponse: Thank you for your suggestion. We agree that this will help clarify this section and have revised\naccordingly.\nApplied changes:\n- L569: Removed numbered bullet and moved into standalone paragraph.\n- L569-574: Added a paragraph to clearly separate processing steps using open-source Python modules\nand initial data acquisition using proprietary software.\n- Consider clarifying the distinction between phase 3 and 4 and ensure consistency with terms like\n\"relative strain.\" That you never mentioned before.\ndefinitions of the changes in strain representing a relative measurement (L169-171)\nApplied changes:\n- L511: Removed \"relative\" from sentence for consistency with manuscript nomenclature and earlier\ndefinition.\n- L166-168: \"All references to strain 𝜀 and strain-rate 𝜀 ̇ herein represent a relative measurement from\nthe baseline measurement on January 12, 2021 at 11:38 UTC.\"\n- L456: Are you suggesting that the fiber optic's location is determined through interpolation of positions\nat 'nb' points, measured along the fiber at 20-meter intervals using an RTK GPS system (LEICA, ...)?\nResponse: Correct.\nApplied changes: N/A\n- Consider removing “complementary …” L460-461.\nResponse: Thank you for your suggestion. We have removed \"complementary\" from this sentence.\nApplied changes:\n- L601: \"Complementary Geotechnical site data used to support our interpretation ...\"\n- L523: unit missing\nResponse: Thank you for pointing this out. We have added in the unit.\nApplied changes:\n- L544: \"As such, a change in 1°C corresponds to change of 1.1·10-2 millistrain.\"\nL524-526: you already mentioned it L504-505.\nResponse: Thank you for pointing this out. We have removed this sentence.\nApplied changes:\n- L548: Deleted the following repetitive sentence: \"We used a temperature sensor (installed at the\nweather station (Fig. 1A) at 10 cm depth) to estimate the relative contribution of temperature changes\non the optical phase data.\"\nI noticed that all sections were named except the first one.\nPerhaps it would be beneficial to move the 'Temperature Effect' section to the beginning\nand potentially streamline or reduce its content since it has been neglected.\nResponse: Thank you for suggestions. We added names to the initial sections and moved the \"Temperature\neffects\" section after the \"Technology background\", near the beginning of \"Materials and Methods\". We\nchoose to maintain the content in the temperature section considering the Reviewer 2 comments.\nApplied changes:\n- L479: Added section name, \"Technology background\"\n- L519: Moved \"Temperature effects\" to section following \"Technology background\"\n- L553: Added section name, \"Data processing\"\nSupplementary:\n- Consider organizing supplementary materials in the order of their citation in the main paper.\nResponse: Thank you for your suggestion. We have revised the organization of the supplementary materials\nto follow the order of citation in the main paper.\nThis manuscript describes the use of low-frequency Distributed Acoustic Sensing (DAS) for monitoring\nthe spatio-temporal evolution of landslide movements at the well-characterized Hollin Hill Landslide\nObservatory in the UK. The study develops a processing flow to extract reliable low-frequency (< 1 Hz)\nstrain and strain-rate information from the DAS data which shows the spatial and temporal evolution of\ndeformation within the landslide during a period of movement reactivation due to rainfall. Strain-rate and\nstrain data are evaluated to determine 4 different deformation sequences affecting different zones of the\nlandslide, which describe the landslide movement from reactivation of the scarp to development of a rupture\nzone in the central area of the landslide and retrogression to the scarp and final stabilization. The low-\nfrequency DAS measurements are also used to estimate displacements that are compared with independent\ndeformation measurements from a ShapeArray, with good agreement. The paper clearly demonstrates that\nlow-frequency DAS has potential as a monitoring tool in shallow landslides.\nFirst, I would like to start by thanking the authors for this very interesting study. I find results exciting and\nvery encouraging, and I think that this paper is an important contribution for the DAS and landslide\nmonitoring community. The paper is in general well written and illustrations are of good quality, clear and\nillustrative of the work carried out. I also appreciate the authors considering effects such as temperature\nchanges on the strain data, as this is one of the significant issues when interpreting low-frequency DAS\nmeasurements.\nI think that this paper is valuable and should be published. However, I also think that some changes and\nadditions could be made, which would improve the clarity and impact of the paper. Below, I have provided\nsome general and some more detailed suggestions that I hope the authors find useful to improve this\ninteresting contribution.\nResponse: Thank you for your helpful and constructive feedback. We have addressed your comments with\ndetails of our applied changes below.\nMajor general comment\nThe Hollin Hill landslide has been monitored for a long time. A lot of work has been done and publications\nexist describing the inferred landslide structure and dynamics and discussing conceptual models for the\nlandslide. However, not much of this work is mentioned in the paper. I encourage you to take advantage of\nthis information available, and use it to do some more comparisons between what is learned and interpreted\nhere from the DAS data, and what is already known about the landslide. The paper mentions that this\ninvestigation illuminates landslide processes that could not be resolved before, but it never really discusses\nwhat is already known and what exactly new information is being provided by the strain data. Some more\nbackground information and discussion on this respect is needed to clarify what are the new insights gained\nfrom this approach.\nResponse: Thank you for your suggestion. We have incorporated additional information into our text to\nmore clearly describe what is already known about the landslide and how our findings reveal new processes.\nApplied changes:\nBackground:\n- L107-118:\"Landslide processes at Hollin Hill have been studied using a variety of characterization\nand monitoring techniques, including seismic refraction tomography, geoelectrical resistivity, self\npotential, inclinometers, piezometers, lidar change detection, interferometric synthetic aperture radar\n(inSAR) and cone penetration testing9,32,34–38. Earlier studies have integrated multiple techniques to\nreveal different factors impacting landslide movement. For example, 3-D time lapse imaging of inferred\nslope moisture content (from inverted resistivity models) demonstrate moisture accumulation in the\n8\nupper slope over wintertime correlating with landslide reactivation and drainage from the mudstone\nto sandstone formation in the summertime34. Seismic refraction tomography was then carried out along\nearlier geoelectrical resistivity profiles. The resulting ground model, incorporating both seismic and\nearlier resistivity modelling, describes zones of materials with different relative densities, leading to\naccumulating strains downslope. Drainage, occurring near the base of the flow lobes, results in\nstabilization of material near the toe39.\"\nDiscussion:\n- L345-363: \"We observe landslide processes with previously unresolved spatiotemporal resolution and\ndemonstrate how these processes mimic those occurring over longer (i.e., seasonal) time scales. Our\nresults are supported by findings from earlier studies describing similar movement patterns and that\ndemonstrate the influence of seasonal precipitation and soil moisture content on slope movement\n34,35,38,39. This agreement with earlier studies supports the validity of our findings. Our study reveals\nnew insights into landslide dynamics, as it demonstrates the scale invariance of the earlier observed\nlandslide processes because of the extreme sensitivity of our DAS-based method. For example, a past\nstudy hypothesized that deforming materials of different relative densities may contribute to a\ndeformation wave progressing through the slope 39 (i.e., a flow surge event). Our study provides\nobservations of strain-rate front propagation in the flow-dominant zone to characterize the spatial\nextents of an interpreted flow surge event occurring over an ~8-hour period, along with its associated\nstrain-rate front propagation velocity of 2.5 m/hour (Fig. S4). Our method can be used to distinguish\nkinematic zones which vary over time to support landslide hazard assessments (e.g., discerning the\nmotion characteristics of slower soil-creep events from more rapid flow-surge events). This new\ncapability enables landslide monitoring on spatiotemporal scales that are currently poorly understood\nand improves our ability to characterize landslide events and behaviours using current classification\nsystems43.\"\nDetailed comments on Results\nIn general, I find this section a little bit difficult to follow. The observations are very complex (which is\ngreat, it means that you can see a lot), but then you need to be very careful and clear when you are describing\nthe results. My general suggestion is that you always start by describing what you see in the data, and then\ncontinue to say what you interpret that observation to be. Also, I suggest that you be specific about whether\nyou are referring to strain-rate or strain (which sometimes can show different trends depending on where\nyou are in the temporal evolution curve). Another suggestion is to make specific references to Figures and\nthe location of the features that you are describing (specially when you refer to Figure 2). For the reader\nwho is not familiar with these types of measurements and cannot remember the morphology of the landslide\njust by looking at Figure 1 once, it will be difficult to know exactly where to look in each of the panels if\nyou don’t mention it.\nResponse: We appreciate your constructive comments. Upon further review, we agree that the observations\nare very complex, and the results may have been difficult to follow. To improve this, we have made\nsignificant changes to this section. Notably, we have added a conceptual framework (and corresponding\nfigure) that outline how we obtain our interpretations from the strain-rate spatiotemporal patterns. Further\ndetails of our applied changes are below.\n- I think it would be most helpful to already indicate in Figure 1 the location of the chosen channels described in\nthis section and plotted in Figure 3. I think it would also be helpful to mark the position of these channels in\nFigure 2; that would make the comparison of Figures 2 and 3 easier.\nResponse: Thank you for your suggestion. We have updated Figures 1 and 2 to mark the position of these\nchannels for clarity.\nApplied changes:\n9\n- Figure 1: Added channel location labels on Figure 1A for comparison with Figure 3.\n- Figure 2 (now 3): Added channel locations on Figure 2 for comparison with Figure 3.\n- Line 174: “Sequences 1 through 4” - Please specify here that you are referring to the numbered squares\nin Figure 2.\nResponse: Thank you for your suggestion. We have modified this statement accordingly.\nApplied changes:\n- L229-234: \"Following our conceptual framework of strain-rate patterns (Fig. 3), we interpret the five\nevent sequences which are ordered chronologically: (1) initiation of strain at the head scarp, (2)\nsubsequent triggering of a rupture zone, (3) retrogressive strain towards the scarp, (4) a flow-lobe\nsurge near the toe, and (5) stabilization of the rupture zone with a gradual increase in strain at the\nscarp. Sequences 1 through 5 are numbered and highlighted by dashed rectangles in Figs. 2b-e.\"\n- I think you don’t discuss anything in the text, but there seems to be a relatively prominent anomaly\ndeveloping in the lower half of all profiles (within the flow dominated area of the landslide) at the time of\nmaximum soil moisture content (e.g. seen between channels 1140 and 1160 on L1). This anomaly seems\nto be further north as we move eastwards. Do you have any interpretation about what this feature could be\nbased on what you know about the landslide?\nResponse: Thank you for highlighting this anomaly. The anomaly appears to be related to topographical\neffects of the landslide, as it occurs at a change of slope. We have revised our Results to include additional\ndetail on general interpretation and include further details of this anomaly in our results and discussion. We\nhave also included an additional figure in our Supplementary materials (Fig. S4) to help demonstrate the\nimpact of topography on the resulting strain and strain-rate observations.\nApplied changes:\n- L216-217: A sequence of paired extensional and compressional strain-rate observations are attributed\nto topographic and material strength variations (Fig. 3d).\n- L387-390: \"Additional observed strain-rate patterns are likely impacted by a combination of factors,\nincluding slope topography, strain redistribution from the shallow landslide movements, and variations\nin material parameters.\"\n- Supplementary Figure S4\n- Minor comment: I suggest to add a mention to Figure S2 here (somewhere in/after lines 163/166), so\nthat the reader can compare the strain-rate images in Fig. 2 with the strain panels in Fig. S2.\nResponse: Thank you for your suggestion. We have modified this statement accordingly. We have also.\nmodified our Figure 2 for clarity by only including strain-rate spatiotemporal images which are referenced\nin the main text, with figures of all six cable sections moved to the Supplementary (new Fig. S2).\nApplied changes:\n- L190-191: \"Strain spatiotemporal images are also available for comparison (Fig. S3).\"\nStrain initiation at head scarp\n- I am a bit confused about your observed “onset of strain” in Sequence 1. In the text (lines 177-182) you\ndescribe a “steady increase in strain near the main scarp, with an averaged inferred velocity of 1 mm per\n10\nday”. Although slightly positive strain is observed in the upper few channels in Figure 2 for L6, the black\nline in Figure 3B shows decreasing and not increasing strain. The gray line seems to indicate positive\nstrain-rate just before the start of Sequence 1 (just before the yellow band), but it is immediately followed\nby a decreasing strain-rate. Although this onset strain signal might be there, I don’t think it is clearly\nobservable in these Figures.\nResponse: Thank you for your comments. We have made significant revisions to this section to the clarity\nof our results and interpretations. We hope these changes increase the clarity of this section.\nApplied changes:\n- L202-217: Added an additional paragraph to illustrate our conceptual framework used to guide our\ninterpretation.\n- Figure 3. Created a new figure to support our interpretation.\n- L238-244. Reworded this section for clarity, as follows: \"After about eight hours of rainfall, the onset\nof strain is observed at the northeast corner of the cable at the scarp (Fig. 4B, Fig. 4F). Over the\nsubsequent eight-hour period, we observe a strain rate of ~ –4 nm m-1 s-1 near the main scarp\n(corresponding to an average inferred velocity of –1 mm per day). The observed pattern at the scarp\nover this time corresponds to a low amplitude slip surface pattern (Fig 2E). The strain change\ncorrelates with a steady increase in soil moisture content from 43 to 47%.\"\n- Line 187: “… develops increasing strain centered around cable section L4” - A reference to the\ncorresponding panel in Figure 2 would be helpful here.\nResponse: Thank you for your suggestion. We have made the following changes to increase clarity.\nNotably, we have revised our Figure 2 (strain-rate spatiotemporal images) to remove references to cable\nsections, instead adding lettering for clarity and only including segments referenced in the text.\nApplied changes:\n- L229-234: \"Following our conceptual framework of strain-rate patterns (Fig. 3), we interpret the five\nevent sequences which are ordered chronologically: (1) initiation of strain at the head scarp, (2)\nsubsequent triggering of a rupture zone, (3) retrogressive strain towards the scarp, (4) a flow-lobe\nsurge near the toe, and (5) stabilization of the rupture zone with a gradual increase in strain at the\nscarp. Sequences 1 through 5 are numbered and highlighted by dashed rectangles in Figs. 2b-e.\nL246-247: \"On January 14 at ~3:00 UTC, a slip surface pattern develops ~30 m southwest of the\nmain scarp (Fig. 2c).\nTriggering of rupture zone (Sequence 2), retrogression towards scarp (Sequence 3A) and flow lobe surge\n(Sequence 3B)\n- I also found the description of Sequences 2 and 3 difficult to follow. Two things that make this\ndescription confusing for me:\n1) You describe strain evolution in terms of inferred velocities, but your Figures are all displayed in\nunits of strain or strain-rate. It would make it easier to link the text to the Figures if you talk about\nstrain/strain-rate values first, and then mention what this means for the landslide processes in\nterms of inferred velocities.\nResponse: Thank you for your suggestion. We believe we have addressed this comment as per our response\nto the following comment below.\nApplied changes:\n- See below.\n11\n2) You talk about “strain” without mentioning if it is positive strain or negative strain. For example,\nin the description of Sequence 2 (lines 187-188), you mention “The strain accelerates to a\nmaximum inferred velocity of 12 mm/day at 12:00 on January 14 (Fig. 3A, sequence 2)”. When I\nlook at the corresponding panel in Figure 3A, I see decreasing strain and strain-rate, and a large\nnegative anomaly in L4 in Figure 2. So, in this case, you are referring to a negative strain/strain-\nrate anomaly, which indicates compression. It is important that you mention that during your\ndescription, otherwise it is difficult for the reader to follow, since they have to look at two figures\nand read at the same time. Describing the observations in terms of “extension” and\n“compression” will make it clearer to understand what the strain/strain-rate data shows.\nResponse: Thank you for your suggestion. We have made significant changes to this section to increase\nclarity. Notably, we have created a new Figure (Figure 3) that provides the primary strain-rate patterns\nwhich support our interpretation of rotational slip surface, retrogression, flow-lobe surge, topographic\neffects. We feel that this figure makes a significant improvement to aid the reader's understanding and\navoids the requirement of adding additional text to describe the strain-rate observations.\nApplied changes:\n- Created new figure (Figure 3) to support interpretation of results, and revise Figure 2 for clarity.\n- We have revised our text to use extension / compression descriptors and describe our interpretation as\nfollows:\n- L203-217: \"Positive and negative strain and strain-rate observations correspond to cable extension\nand compression, respectively, and we use the extensional or compressional descriptors herein. Due to\nthe spatiotemporal characteristics of the DAS data, velocity features of the landslide are obtained per\nthe following approaches: (1) the velocity occurring at a single location (i.e. DAS channel), (2) the\nvelocity of a strain or strain-rate front propagating over multiple DAS channels with time. We guide\nour analysis using a conceptual framework to interpret key strain and strain-rate patterns, as follows:\n(1) Extensional strain-rate observations that are upslope of compressional strain-rate occurring over\nthe same time period are interpreted as a shallow slip surface intersecting with the cable (Fig. 3a). (2)\nPropagation of the extensional strain-rate processes upslope with time are interpreted as slope\nretrogression. We characterize the retrogressive behaviour by analyzing the slope of the strain and\nstrain-rate fronts with time (Fig. 3b). (3) A paired extensional and compressional strain-rate\nobservation propagating downslope is interpreted as a surge of saturated materials (i.e., a flow surge)\npropagating over the cable (Fig. 3c). (4) A sequence of paired extensional and compressional strain-\nrate observations are attributed to topographic and material strength variations (Fig. 3d).\"\n- Revised language to describe observations in Results.\n- When you describe Sequence 3A, you start by saying “Here, a general retrogression of the rupture zone\ntowards the scarp occurs”. This is an interpretation, not an observation. Thus, I suggest that you start by\nsaying that you observe a relatively sharp increase in strain/strain-rate, which you interpret as a\nretrogression of the rupture zone.\nResponse: Thank you for your suggestion. See our above comments for changes. We have also revised our\nSequence 3A description as per below.\nApplied changes:\n- L254-258: Retrogression towards scarp (sequence 3): From the triggering of the rupture zone\n(January 14 at ~3:00) to ~12 hours later, the strain-rate front propagates from the rupture zone upslope\ntowards the scarp (over a ~20 m distance; Fig. 2B). We interpret this as a retrogressive deformation\nof the landslide. This coincides with accelerating strain near the main scarp (Fig 4D).\n12\n- Lines 192-193: “ This coincides with accelerating strain near the main scarp” - Please add a reference to\nthe panel in Figure 2 where you can see this, and mention channel ranges so that the reader can easily\nlocate the area and feature you are referring to\nResponse: Thank you for your suggestion. We have modified this statement accordingly.\nApplied changes:\n- L257-258: \"This coincides with accelerating strain near the main scarp (Fig 4D).\"\n- Line 193: “ Further downslope...” - As above, where can I see this? Please refer to the Figures and the\nchannel ranges I should be looking at.\nResponse: We have modified this statement and revised Figure 2 to improve clarity.\nApplied changes:\n- L233-238: \"Following our conceptual framework to interpret strain-rate patterns, we identify the\nfollowing event sequences, demonstrating: (1) initiation of strain at the head scarp, (2) subsequent\ntriggering of a rupture zone, (3) retrogressive strain towards the scarp and flow lobe surge, and (4)\nstabilization of the rupture zone with a gradual increase in strain at the scarp (Sequences 1 through 4,\nhighlighted as numbered squares and circles in Fig. 2A and Fig. 2B, respectively).\"\n- L260-262: \"Further downslope, propagation of the strain-rate front at the flow lobe occurs across ~10\nm in the downslope direction over a four-hour period (average velocity of the strain-rate front\npropagation is ~2.5 m/hour; Fig. 2D, Fig. S4).\"\nStabilization of rupture zone with gradual deceleration in strain at scarp (Sequence 4)\n- Lines 201-205: “A maximum strain of ~1000 microstrain occurs at the rupture zone…” and “the strain\ncontinues to increase at more gradual levels at the main scarp...” - Please indicate where you can see these\nfeatures in Figures 2 and 3.\nResponse: We have revised this section for clarity.\nApplied changes:\n- L268-271: \"The final sequence coincides with a stabilization of the rupture zone, where the strain rate\napproaches near-zero values (Fig. 2D and Fig. 4E). The strain continues to increase at more gradual\nlevels at the main scarp up until the end of the DAS acquisition period (Fig. 4A).\"\nDiscussion\n- Lines 297-308: I think this is a nice summary of the dynamics that you can see in the strain-rate data.\nAre these interpretations solely based on your data, or are they also informed by previous work done on\nthis landslide?\nResponse: Thank you for your feedback. These interpretations are based solely on our data. We have also\nincorporated an additional statement to this section as per the below.\nApplied changes:\n- L387-390: \"Additional observed strain-rate patterns are likely impacted by a combination of factors,\nincluding slope topography, strain redistribution from the shallow landslide movements, and\nvariations in material parameters (Fig. 3D, Fig. SS)\".\nUhlemann et al (2016)*, Figures 1 and 10. Specially, Figure 10 of this paper shows their interpreted\ncompression and tension zones in the shallow subsurface of the landslide, which I think agree reasonably well\nwith the profile that you show in Figure 1b (which is very nice to see!). Although this could have changed since\nthen due to the rapid movement of the landslide, I think it would be interesting to look into it and compare\nyour findings with theirs. You can even consider reproducing the figure (with permission) and show it\nalong side your profile if it makes sense.\n*Uhlemann, S., Hagedorn, S., Dashwood, B., Maurer, H., Gunn, D., Dijkstra, T., & Chambers, J. (2016).\nLandslide characterization using P- and S-wave seismic refraction tomography — The importance of\nelastic moduli. Journal of Applied Geophysics, 134, 64–76. https://doi.org/10.1016/j.jappgeo.2016.08.014\nResponse: Thank you for your suggestion. We have included an additional paragraph incorporating this.\nApplied changes:\n- L350-358: \"Our results are supported by findings from earlier studies describing similar movement\npatterns and that demonstrate the influence of seasonal precipitation and soil moisture content on\nslope movement 34,35,38,39. This agreement with earlier studies supports the validity of our findings.\nOur study reveals new insights into landslide dynamics, as it demonstrates the scale invariance of the\nearlier observed landslide processes because of the extreme sensitivity of our DAS-based method. For\nexample, a past study hypothesized that deforming materials of different relative densities may\ncontribute to a deformation wave progressing through the slope 39 (i.e., a flow surge event). Our study\nprovides observations of strain-rate front propagation in the flow-dominant zone to characterize the\nspatial extents of an interpreted flow surge event occurring over an ~8-hour period, along with its\nassociated strain-rate front propagation velocity of 2.5 m/hour (Fig. S4).\"\n- Lines 315-316: “Our observations are consistent with earlier studies that demonstrate the influence of\nseasonal precipitation on slope movement”. There is a lot of monitoring work done on this landslide using\ndifferent types of geophysical techniques (e.g. ERT and seismic monitoring). However, here you only\ncompare it with the ShapeArray data. This section should include a bit more (general) discussion on what\nthese other geophysical techniques show that is relevant to your study, and how your results provide\nmore/complementary information about the landslide processes. I don’t suggest a detailed comparison as\nwhat you have done with the ShapeArray data, but a description of what other works show and how they\ncompare to your observations would be granted here.\nResponse: Thank you for your suggestion. We have revised this section to incorporate further details of\nother studies.\nApplied changes:\n- As per above.\n- Lines 338-339: “Future research should investigate the effect of cable geometry on the detectable strain\nthresholds.”. I agree. Along the same lines, how about the effect of cable construction on the\nmeasured/detectable strain magnitudes? I know that you only had one type of cable, but can you (very\nbriefly) comment on how using loose-tube, gel-filled cable, for example, would affect strain measurements?\nA brief mention is sufficient since it is beyond the scope of your paper to compare cables, but I think it is\nan important consideration when planning deployments for low-frequency DAS strain monitoring and it is\nworth mentioning. You could also discuss this after the discussion on strain transfer in comparison with\nShapeArray ending in line 361).\ntube cable installed. However, analysis and comparison of the two cable types was outside of the scope of\nthis paper. We have added details on this in the introduction.\nApplied changes:\n- L123-127: \"Most of the fiber-optic cable installed is of tight-buffered construction, but 140-m of gel-\nbuffered cable was placed alongside a similar length of the tight-buffered cable for comparison. Our\nanalysis focuses exclusively on data acquired from the tight-buffered cable due to its improved strain\ntransfer properties. The British Geological Survey acquired data along the entire cable length using ...\"\n- L443-453: \"Further to the above, different types of cable construction will result in different strain\ntransfer coefficients. Our study uses DAS results acquired from a tight-buffered fiber optic cable,\nproviding improved strain transfer from the surrounding formation to the cable. In contrast, a loose-\ntube (gel-filled) cable type is designed to minimize the strain transferred to the optical fiber from the\nsurrounding formation. As such, this cable type is deemed less effective for monitoring changes in\nstrain47. However, this same attribute increases the effectiveness of loose-tube cables for temperature\nmonitoring. Although our study focuses on strain changes in the near-surface for landslide\ncharacterization using DAS, complementary DFOS technologies such as distributed temperature\nsensing could be considered to provide measurements of temperature changes along the fiber and\nsupport interpretation of DAS data.\"\n- Throughout the discussion, I think it is worth making clear that this is a shallow landslide and\nmovements are in the near-surface, where the cable is deployed. The methods could be less effective for\ndeep-seated landslides where the movements happen at depth.\nResponse: Thank you for your suggestion. We have made revisions throughout the discussion to clarify\nthis is a shallow landslide and the cable is deployed in the near surface. We also include our original\nstatements below that are included in our discussion for completeness.\nApplied changes:\n- L369-370: (original): \"This is likely a result of the main scarp providing a direct rainfall-infiltration\npathway to the near-surface cable\"\n- L380-382 (original): This is likely a result of our interpreted slip-surface geometry, where the greatest\nobserved tensile strains at the near-surface cable are expected to occur where the slip surface intersects\nwith the cable.\n- L387-390: \"Additional observed strain-rate patterns are likely impacted by a combination of factors,\nincluding slope topography, strain redistribution from the shallow landslide movements, and variations\nin material parameters\"\n- L415-419: \"For example, acquiring DAS data from a downhole fiber optic cable paired with an\ninclinometer or ShapeArray, combined with a trench installation would provide useful information on\nthe capabilities of DAS to capture deformations occurring at depth. Regardless, our experiment\ndemonstrates the valuable insights gleaned from a near-surface trench installation.\"\nMaterials and Methods\n- How long are the data files recorded by the IU? Do you concatenate the data for the entire period of\ninterest before you conduct your decimation, or do you apply your decimation file by file? Are steps 6\nand 7 applied to individual files or a long file encompassing the entire recording period? Please clarify.\nResponse: Thank you for your questions. Answers are as follows:\n• The data files recorded by the interrogator are of unknown time length. The data was shared by\nOptaSense using a proprietary format (.cbt) containing 32 files of 10.7 GB individual file sizes). The\ndata was processed in OptaSense DxS software to decimate from 500 Hz to 50 Hz and were exported\nto the HDF5 format for individual cable segments with one-minute duration file lengths.\n• The HDF5 one-minute files were decimated from 50 Hz to 1 Hz using SciPy (Python)\n15\n• The resulting data arrays are then concatenated over the entire period of interest prior to converting the\noptical phase data to strain data via Equation 1. As such, the final steps of implementing the median\nfilter and integration are applied to a long file encompassing the entire recording period.\nWe agree these are important points to include in our description of data processing steps and have updated\nthis section accordingly.\nApplied changes:\n- L562-580: DAS data are acquired using the OptaSense ODH-F interrogator unit in the quantitative\nmode. The obtained data represent the optical phase change at a sampling rate of 500 Hz. Data are\nacquired from OptaSense in a proprietary file format of 32 files, with individual file sizes of 10.7\ngigabytes. They are decimated to 50 Hz with a low-pass anti-aliasing filter and exported to HDF5 file\nformat using the OptaSense DxS software, where each file contains the channels corresponding\nto a cable segment (labelled L1 to L6 from west to east) of one-minute duration.\n- The remainder of the data processing steps are completed using the following open-source Python\nmodules: SciPy64, NumPy65, Pandas, H5Py, Obspy, Zarr and Xarray66. The processed strain and strain-\nrate DAS datasets are saved using Xarrays containing appropriate metadata (time, channel and\ncoordinates) in NetCDF files corresponding to individual cable segments (six segments numbered L1\nto L6 from west to east) with explanatory Jupyter notebooks detailing the following processing\nsteps67:\n1. The DAS data are decimated to 1 Hz following the application of a low-pass antialiasing filter.\n2. Following decimation, the data arrays are reshaped, concatenating the one-minute files to\nincorporate the full data acquisition time period (approximately three days).\n3. The concatenated optical phase data are converted to strain using Equation 1.\n- Line 458-460: “Short periods of data gaps...” - DAS measures relative strain change, and stopping and\nrestarting the measurements causes losing your reference strain state, sometimes. Can you comment on\nhow large these data gaps are and how you deal with them in your integration? How do you make sure\nthat you are not seeing a change in strain at each data gap?\nResponse: Thank you for pointing out this important point for consideration. We have modified this section\nfor clarity by creating a section header and adding appropriate details. Following review, we confirm that\nthese data gaps are not attributed to power-related issues of the DAS interrogator. Indeed, as you point out,\nstopping and restarting the measurements will cause losing the reference strain state. We have added an\nadditional statement in the Background to reflect this.\nApplied changes:\n- L164-165: \"The optical phase changes and corresponding strain measurements represent a change\nfrom a baseline measurement occurring at the start of data acquisition.\"\n- L595-599: \"Data gaps: Three periods of data gaps, approximately 45-minutes duration, are visible in\nFigures 2, 3 and 5 are due to corrupted data files, where the original data was unable to be recovered.\nThese periods are excluded from the above data processing steps. The data is reindexed to a continuous\ntime vector using the timestamp associated with each filename, and the data gaps are incorporated as\n'NAN' (i.e., not a number) values.\"\n- Line 485: “)” needs to be removed.\nResponse: We have updated this to remove the \")\".\nApplied changes:\n- L497: Revised to \"(Equation 1)\"\n16\n- I appreciate the effort to calculate the effect that temperature changes affecting the cable can have on\nstrain measurements. Did you observe any effects associated with temperature changes affecting the IU\nitself? They should manifest as “common-mode noise” along the cable. Did you have any temperature\ncontrol in the barn?\nResponse: No, we did not observe any effects associated with temperature changes affecting the IU. There\nwas no temperature control in the barn.\nApplied changes:\n- N/A\n- Another way of monitoring temperature changes along the fiber would be to deploy DTS along the same\ncable (different fiber). I suggest mentioning this in the section describing the temperature effect\nevaluation, or even in the Discussion section of the paper, as an additional consideration for future\nexperiments of this type.\nResponse: We agree with your suggestion and have added a paragraph to our Discussion on the deployment\nof DTS, as per the below.\nApplied changes:\n- L450-453: \"Although our study focuses on strain changes in the near-surface for landslide\ncharacterization using DAS, complementary distributed fiber optic sensing technologies such as\ndistributed temperature sensing could be considered to provide measurements of temperature changes\nalong the fiber and support interpretation of the DAS dataset.\"\nGeneral Minor suggestions\n- I suggest mentioning the name of the study site already in the Abstract. It is mentioned for the first time\nin the Introduction with not much context (lines 78-79), and then directly introduced in the Background\nsection.\nResponse: Thank you for your suggestion. We have updated our abstract to mention the name of the study\nsite.\nApplied changes:\n- L27 (Abstract): Updated statement as follows, \"We employ distributed acoustic sensing (DAS) strain\ndata below 1 Hertz frequency over a three-day period of rainfall at the Hollin Hill landslide, and\nquantify strain-rate changes at meter and sub-minute scales.\"\n- Why is the photo of fiber installation the very last one in the Supplementary (Fig. S6) if it is the first one\nto be mentioned in the text? I would suggest moving it to the be the first one.\nResponse: Thank you for your suggestion. We have reorganized the Supplementary figures such that their\norder follows their mention in the text.\nApplied changes:\n- Supplementary Figures: Reorganized all figures such that they follow the order they are mentioned in\ntext.\n- Line 146: “.. n corresponds to a refractive index” - I would be a bit more specific and say “refractive\nindex of the fiber”\nResponse: Thank you for your suggestion. We have modified this statement accordingly.\n- L162: \"...where n corresponds to a refractive index of the fiber ,\"\n- Line 409-410 : “ Selecting a channel spacing less than the LG is akin to applying a moving average filter\nto the dataset” - I think this sentence is a bit confusing and could be rephrased. The gauge length is what\nacts as a moving average filter. So, I would rather say that when channel spacing is smaller than the gauge\nlength, the effect of the gauge length is the same as applying a moving average to the data, moving one\nchannel at a time (or something along these lines).\nResponse: Thank you for your suggestion. We have modified this statement to improve clarity.\nApplied changes:\n- L502-504: Updated sentence as follows: \"When the channel spacing is less than the L , the effect of\nG\nthe latter is the same as applying a moving average window of length L at each channel.\"\nG\n- Figure 3: the time labels seem to be displaced with respect to the tick marks, which makes it a bit\nconfusing.\nResponse: We have revised the time labels on this figure for clarity.\nApplied changes:\n- Figure 3 (now Figure 4). Adjusted time labels to align with tick marks.\n- Figure 5: is there a data gap In the ShapeArray data on the West lobe? I think it is not mentioned in the\ntext.\nResponse: Although we mention the data gap in our Materials and Methods, we appreciate your comment\nand have included an additional mention in the Figure 5 caption for clarity.\nApplied changes:\n- Former Figure 5 (now Figure 6). We added the following statement to the caption: \"The west lobe\nShapeArray did not record data from January 14 at 11:00 to January 15 at 13:00 for reasons\nunknown.\"\n- Figure 5: it might be useful to plot precipitation and soil moisture on top of the displacements, for\ncompleteness and aid reference to the previous figures and observations.\nResponse: Thank you for your suggestion. We have revised this figure to include precipitation and moisture\ndata.\nApplied changes:\n- Revised Figure 5 (now Figure 6) to include precipitation and moisture data.\nReviewer #2 (Remarks to the Author):\nDear authors,\nThank you for your work on the manuscript review. I think that the article is now much\nclearer and highlights the novelty and contributions of this study to the understanding of\nprocesses in the Hollin Hill landslide, and also to the use of low-frequency DAS for\ndeformation monitoring.\nMost of my comments have been properly addressed, and the authors have done a great\njob improving the clarity of the results and the discussion. I appreciate the addition of Figure\n3, explaining the conceptual framework followed by the authors to interpret the strain and\nstrain-rate measurements in terms of landslide processes.\nI think the paper is of good quality and deserves publication after conducting some minor\nclarifications and corrections that I have detailed in the attached file.\nOnce again, thank you for this interesting contribution and for your hard work on the\nreview.\n[Editorial note: Please find reviewer #2’s attached document beginning on the next page.]\nMinor comments to “Previously hidden landslide processes revealed using distributed acoustic\nsensing with nanostrain-rate sensitivity”, by S. Oullet et al. (R1)\n• Lines 75-76: “...demonstrate the capability of low-frequency DAS to measure changes in strain at\nthe Hollin Hill Landslide Observatory.” - I still think that this is a bit of a “sudden” mention of the\nsite. I think you could remove the mention to Hollin Hill here and say “ … demonstrate the\ncapability of low-frequency DAS to measure changes in strain associated to the movement of slow-\nmoving shallow landslides”. Then, later in line 80, you can add: “… the kinematics of the slow-\nmoving landslide of the Hollin Hill Landslide Observatory in England.”\n• Line 31: “… are like previously observed...” - I would change it to “agree with previously\nobserved...” or “correlate with previously observed...”\n• Line 94: Do you mention somewhere the geographical location of Hollin Hill? This information\nshould be added here.\n• Lines 115-116: “describes zones of materials with different relative densities, leading to\naccumulating strains downslope” - I am not sure what you mean by this sentence. Are you talking\nabout the strain observed in DAS? If so, I would suggest that you don’t mention strain here, as you\nhaven’t talked about the DAS results here yet. I also don’t really understand how “material with\ndifferent relative densities” lead to “accumulating strain downslope”. Can you clarify this sentence?\n[This sentence is much clearer in the Discussion… I suggest you rephrase it here, or you remove it\nsince it reads as a comparison with your data, which you haven’t shown yet.]\n• Line 121: “...gel-buffered cable...” - I think it should be “loose-tube, gel-filled”.\n• Line 144 (Figure 1 caption): I think this “2” should not be there.\n• Line 154: “… over the gauge length of the measurement...” - Please add “(L )” after “gauge\nG\nlength”, as this symbol appears in Equation 1 but I think it is not defined.\n• Line 196-197 (Figure 2 caption): “DAS channel locations featured in Fig.4C (upper solid line) and\nFig.4D (lower solid line)” - I think it should be the other way around. The channel in Figure 4C\nshows a decrease in strain (lower solid line) and the channel in Figure 4D shows an increase in\nstrain near the scarp (upper solid line).\n• Line 199 (Figure 2 caption)- Figure 4A should be Figure 4B.\n• Line 202: “Due to the spatiotemporal characteristics...” - Do you mean “Thanks to...”? It would\nwork better, I think.\n• Line 246: “Figure 4c” - I guess it should be “4C” to be consistent with your capitalization of the\nletters as in all other figure references.\n• Line 256-257: “This coincides with accelerating strain near the main scarp (Fig 4D).” - You could\nalso add a reference to Figure 2C here, upper solid line, since that is where the channel shown in\n4D is taken from.\n• Line 268-270: Fig. 4A should be “Fig 4B”.\n• Lines 305-306: “maximum strain changes in the rupture zone to the west of the existing scarp” -\nBased on Figure 5, should this be “to the southwest of the existing scarp”?\n• Line 315: “Figure 5” should be “Figure 6”.\n• Line 350 - “...as it demonstrates the scale invariance of the earlier observed landslide processes\nbecause of the extreme sensitivity of our DAS-based method” - This sentence could be a bit clearer.\nWhat do you exactly mean by “scale invariance”? Are you referring to improved spatial and\ntemporal resolution? Or also to sensitivity of small movements (since you mention the “sensitivity”\nhere as well)? I would make this sentence a bit more explicit and say that your work reveals high\nvariability of landslides processes both in space and time which could not be resolved by previous\nmeasurements as they had limited spatial and temporal resolution and/or limited sensitivity to small\nmovements (or something along these lines).\n• Line 362-363: “this addresses existing limitations of state-of-the-art remote sensing technologies\nsuch as ground-based interferometry and Doppler radar” - I would suggest to add “...addresses\nexisting limitations of current deformation monitoring techniques based on state-of-the-art remote\nsensing...”, to emphasize that you are talking about the tools that are currently used to monitor\nmovement on landslides.\n• Lines 379-380: “...This is likely a result of our interpreted slip-surface geometry at this location...” -\nA reference to Figure 3A might be useful here.\n• Lines 436-439: The difference in magnitude can also be due to the fact that the ShapeArray is a\npoint measurements and DAS measures strain-rate over the gauge length, which acts as an\naveraging filter. You could add a sentence on this effect too, as an additional consideration.\n• Lines 448-449: “However, this same attribute increases the effectiveness of loose-tube cables for\ntemperature monitoring” - I thought that tight-buffered cables are also more sensitive to\ntemperature changes than loose-tube, gel-filled cables?\n• Lines 503-504: “...average window of length L at each channel.”” - I would maybe say “… of\nG\nlength L centered at each channel.”\nG\n• Line 626: The bracket should be removed.\nReply to peer reviewer comments\nReviewer #2 (Remarks to the Author):\nDear authors,\nThank you for your work on the manuscript review. I think that the article is now much clearer and\nhighlights the novelty and contributions of this study to the understanding of processes in the Hollin Hill\nlandslide, and also to the use of low-frequency DAS for deformation monitoring.\nMost of my comments have been properly addressed, and the authors have done a great job improving the\nclarity of the results and the discussion. I appreciate the addition of Figure 3, explaining the conceptual\nframework followed by the authors to interpret the strain and strain-rate measurements in terms of\nlandslide processes.\nI think the paper is of good quality and deserves publication after conducting some minor clarifications\nand corrections that I have detailed in the attached file.\nOnce again, thank you for this interesting contribution and for your hard work on the review.\nResponse: Thank you for sharing your feedback and for your encouraging review of our submission. We\nare pleased to hear that we were able to adequately address your comments. We appreciate your thorough\nreview and details of our responses are below, in point-by-point format.\nFurther to this, we have revised all our figures and labels to meet the editorial requirements of the journal.\nDuring the updates to the figures, we realized that there had been an error in the strain axis labels of Figure\n4f. Previously, the strain axis labels varied from +1 to -2 millistrain, when they should have been +0.1 to -\n0.2 millistrain. The strain-rate axis labels are correct and have not been changed. We have revised the strain\naxis labels in Figure 4f accordingly and have confirmed these did not affect any of our results in the main\nmanuscript text.\nL75-76: “...demonstrate the capability of low-frequency DAS to measure changes in strain at the Hollin\nHill Landslide Observatory.” - I still think that this is a bit of a “sudden” mention of the site. I think you\ncould remove the mention to Hollin Hill here and say “ … demonstrate the capability of low-frequency\nDAS to measure changes in strain associated to the movement of slow-moving shallow landslides”. Then,\nlater in line 80, you can add: “… the kinematics of the slow-moving landslide of the Hollin Hill Landslide\nObservatory in England.”\nResponse: Thank you for your suggestion. We agree with your proposed changes and have reworded this\nsection to improve the introduction of the site. We have also modified the paragraph in L77-80 to improve\nreadability.\nApplied changes:\n• L72-75: Other studies, referring to a DC-coupled DAS as a distributed Rayleigh sensing system,\ndemonstrate the capability of low-frequency DAS to measure changes in strain associated with the\nmovement of slow-moving shallow landslides.\n1\n• L77-80: We reveal the kinematics of a slow-moving landslide at the Hollin Hill Landslide\nObservatory in England by employing low frequency (<1 Hz) DAS data over a three-day period. The\ndata are acquired with nanostrain-rate sensitivity, 1 Hz temporal sampling and a 4-meter spatial\nresolution over 925 m of optical fiber.\nL31: “… are like previously observed...” - I would change it to “agree with previously\nobserved...” or “correlate with previously observed...”\nResponse: We agree with your suggested rewording and have updated this sentence per the below (\"agree\nwith ...\").\nApplied changes:\n• L31: \"However, the millimeter processes over three days agree with previously observed seasonal\nlandslide patterns.\"\nLine 94: Do you mention somewhere the geographical location of Hollin Hill? This information should be\nadded here.\nResponse: We did not include the geographical location of Hollin Hill. We agree with your suggestion and\nhave added a sentence to include this information.\nApplied changes:\n• L92: \"The Hollin Hill Landslide Observatory, located in North Yorkshire, UK32, is one of the most\nstudied slow-moving landslides in the world and has been monitored by the British Geological Survey\nsince 2008 3,33.\"\nLines 115-116: “describes zones of materials with different relative densities, leading to\naccumulating strains downslope” - I am not sure what you mean by this sentence. Are you talking\nabout the strain observed in DAS? If so, I would suggest that you don’t mention strain here, as you\nhaven’t talked about the DAS results here yet. I also don’t really understand how “material with\ndifferent relative densities” lead to “accumulating strain downslope”. Can you clarify this sentence?\n[This sentence is much clearer in the Discussion… I suggest you rephrase it here, or you remove it\nsince it reads as a comparison with your data, which you haven’t shown yet.]\nResponse: Thank you for your comments on this section. This section describes an earlier study and is\nintended to provide additional background to our work. Considering your feedback and that we have also\ncommented on the relevant findings from this study in our later discussion, we agree with removing this\nsentence.\nApplied changes:\n• L112-114: Removed the following sentences from the introduction: \"A Hollin Hill landslide model,\nincorporating seismic refraction tomography and geoelectrical resistivity modelling, describes zones of\nmaterials with different relative densities, leading to accumulating strains downslope. Drainage near\nthe base of the flow lobes results in stabilizing material near the toe.\"\nLine 121: “...gel-buffered cable...” - I think it should be “loose-tube, gel-filled”.\nResponse: Thank you for pointing this out, that is correct. We have revised this accordingly.\nApplied changes:\n• L116: Revised sentence as follows: \"Most of the fiber-optic cable installed is of tight-buffered\nconstruction, but 140-m of loose-tube, gel-filled cable was placed alongside a similar length of the\ntight-buffered cable for comparison.\"\nLine 144 (Figure 1 caption): I think this “2” should not be there.\nApplied changes:\n• Figure 1 Caption: Removed standalone \"2\".\nLine 154: “… over the gauge length of the measurement...” - Please add “(L )” after “gauge\nG\nlength”, as this symbol appears in Equation 1 but I think it is not defined.\nResponse: Thank you for your suggestion. That is correct, we had defined it later on, but it was not defined\nbeforehand. We have added the gauge length acronym to this sentence.\nApplied changes:\n• L135: \"...Rayleigh-backscattered light over the gauge length (L ) of the measurement \"\nG\nLine 196-197 (Figure 2 caption): “DAS channel locations featured in Fig.4C (upper solid line)\nand Fig.4D (lower solid line)” - I think it should be the other way around. The channel in Figure\n4C shows a decrease in strain (lower solid line) and the channel in Figure 4D shows an increase\nin strain near the scarp (upper solid line).\nResponse: Thank you for pointing this out. We have revised this accordingly.\nApplied changes:\n• Figure 2 caption: \"c Strain-rate for cable section 4, highlighting Sequence 2 and DAS channel locations\nfeatured in Fig.4c (lower solid line) and Fig.4d (upper solid line).\"\nLine 199 (Figure 2 caption)- Figure 4A should be Figure 4B.\nResponse: Thank you for pointing this out. You are correct, we have revised this accordingly.\nApplied changes:\n• Figure 2 caption: \"e Strain-rate for cable section 6, highlighting Sequence 1 and DAS channel location\nfeatured in Fig. 4b.\"\nLine 202: “Due to the spatiotemporal characteristics...” - Do you mean “Thanks to...”? It would\nwork better, I think.\nResponse: We agree with your suggestion and have updated this accordingly.\nApplied changes:\nL173: \"Thanks to the spatiotemporal characteristics of the DAS data, velocity features of the landslide \"\nLine 246: “Figure 4c” - I guess it should be “4C” to be consistent with your capitalization of the\nletters as in all other figure references.\nResponse: Thank you - we have updated all figure labels to conform with the editorial standards and for\nconsistency (lower-case for all).\nApplied changes:\nUpdated all figure labels to conform with editorial standards.\nLine 256-257: “This coincides with accelerating strain near the main scarp (Fig 4D).” - You could\nalso add a reference to Figure 2C here, upper solid line, since that is where the channel shown in\n4D is taken from.\nResponse: Thank you - we agree with your suggestion and have updated this accordingly.\nApplied changes:\nL218: \"This coincides with accelerating strain near the main scarp (Fig 4d, Fig 2c, upper solid line).\"\nLine 268-270: Fig. 4A should be “Fig 4B”.\nResponse: Thank you for pointing this out - we have updated this accordingly.\nL231: \"The strain continues to increase at more gradual levels at the main scarp up until the end of the DAS\nacquisition period (Fig. 4b).\"\nLines 305-306: “maximum strain changes in the rupture zone to the west of the existing scarp” -\nBased on Figure 5, should this be “to the southwest of the existing scarp”?\nResponse: That is correct - southwest is more precise. We have updated this accordingly.\nApplied changes:\nL238: \" ... considering the maximum strain changes in the rupture zone to the southwest of the existing\nscarp \"\nLine 315: “Figure 5” should be “Figure 6”.\nResponse: Thank you for pointing this out - we have updated this accordingly.\nApplied changes:\nL246: \"We compare relative displacement data from two vertical ShapeArrays installed near the fiber optic\ncable to a depth of 2.5 m, at the west flow lobe and east flow lobe in 2013 (Fig. 6).\"\nLine 350 - “...as it demonstrates the scale invariance of the earlier observed landslide processes\nbecause of the extreme sensitivity of our DAS-based method” - This sentence could be a bit clearer. What\ndo you exactly mean by “scale invariance”? Are you referring to improved spatial and temporal\nresolution? Or also to sensitivity of small movements (since you mention the “sensitivity” here as well)? I\nwould make this sentence a bit more explicit and say that your work reveals high variability of landslides\nprocesses both in space and time which could not be resolved by previous measurements as they had\nlimited spatial and temporal resolution and/or limited sensitivity to small movements (or something along\nthese lines).\nResponse: Thank you for your feedback on this section. We have revised this statement to improve clarity.\nApplied changes:\nL273-275: \"This agreement with earlier studies supports the validity of our findings and suggests that the\nprocesses governing landslide behavior are scale invariant (i.e., similar regardless of the scale at which they\nare observed).\"\nLine 362-363: “this addresses existing limitations of state-of-the-art remote sensing technologies such as\nground-based interferometry and Doppler radar” - I would suggest to add “...addresses existing limitations\nof current deformation monitoring techniques based on state-of-the-art remote sensing...”, to emphasize\nthat you are talking about the tools that are currently used to monitor movement on landslides.\nResponse: Thank you for your feedback - we agree with your suggestion and have updated accordingly.\nApplied changes:\nL286-287: \"Importantly, this addresses existing limitations of current deformation monitoring techniques\nbased on state-of-the-art remote sensing technologies such as ground-based interferometry and Doppler\nradar15 which lack temporal resolution combined with sensitivity to small displacements.\"\nLines 379-380: “...This is likely a result of our interpreted slip-surface geometry at this location...” -\nA reference to Figure 3A might be useful here.\nResponse: Thank you for your suggestion. We agree it would be useful, and have included a reference to\nFigure 3A.\nApplied changes:\nL304-307: \" This is likely a result of our interpreted slip-surface geometry at this location, where the greatest\nobserved tensile strains at the near-surface cable are expected to occur where the slip surface intersects with\nthe cable (Fig. 3a).\npoint measurements and DAS measures strain-rate over the gauge length, which acts as an\naveraging filter. You could add a sentence on this effect too, as an additional consideration.\nResponse: That's a great suggestion. We have added a statement in the Results section on \"DAS-derived\ndisplacements with geotechnical instrumentation\" to account for the difference in magnitudes , and then\nrevised this statement to include a consideration for the different nature of the measurements.\nApplied changes:\nL257-260: \"... and (4) the different nature of the measurements, where the ShapeArray data provide a\ndiscrete measurement at a sensor and the DAS data provide a distributed measurement over a 4-m length\nof optical fiber (i.e., the gauge length; L ).\"\nG\nL362-366: \"This relationship between ground displacement from ShapeArray instrumentation and DAS\ninferred displacement can be extended to estimate a strain transfer of 0.5, under the assumption of similar\ncoupling throughout the length of installed fiber optic cable and with consideration to the different nature\nof the measurement (i.e., point-based versus distributed; see Results).\nLines 448-449: “However, this same attribute increases the effectiveness of loose-tube cables for\ntemperature monitoring” - I thought that tight-buffered cables are also more sensitive to\ntemperature changes than loose-tube, gel-filled cables?\nResponse: Thank you for your comment. We were unable to find any studies providing a direct comparison\nof tight-buffered and loose-tube cables with DAS for temperature change monitoring. Sidenko et al. (2022)\nperform two separate experiments, where (1) a gel-filled loose-tube cable is used with DAS/DTS and (2)\nthe response of a tight-buffered cable and a bare fiber are compared. However, they do not compare the\nloose-tube with the tight-buffered in the same experiment. We have removed this sentence as we do not\nhave references to support this statement and it is not necessary for the discussion.\nApplied changes:\nL374-376: Removed the following sentence: \"However, this same attribute increases the effectiveness of\nloose-tube cables for temperature monitoring.\"\nLines 503-504: “...average window of length LG at each channel.”” - I would maybe say “… of length LG\ncentered at each channel.”\nResponse: Thank you for your suggestion. We have updated this sentence accordingly.\nApplied changes:\nL425-427: \"When the channel spacing is less than the L , the effect of the latter is the same as applying a\nG\nmoving average window of length L centered at each channel.\nG\nLine 626: The bracket should be removed.\nResponse: Thank you for pointing this out. We have corrected this.\nApplied changes:\nL549-551: \"Over the DAS acquisition period, the ShapeArrays acquired data with a varying sample interval\nbetween 30-minutes to one hour.\"\n5"}]

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