1. Introduction - Context and Related Work

1.3. Satellite-Remote Sensing Based Landslide Detection

Detecting landslides using satellite-RS based solutions provides crucial information and evidence for landslide- and earthquake related research. Landslide detection time, location and spatial extent of identified landslides, together with information about changes in land surface materials, are not only important for landslide risk/susceptibility modelling, and for disaster impact assessment [12], but also for early disaster response and monitoring. Captured satellite imagery available as near real-time (NRT) products with latency below three hours [20,21], together with effective cloud-based imagery processing and distribution of detected landslide locations to emergency response systems, are key to early response and can save lives and reduce damage.

Multiple relevant satellite imagery sensors for earthquake applications have been identified [7]. RS-based landslide detection approaches utilize both, optical sensors (e.g., Landsat-8 (L8), Sentinel-2 (S2)), and radar sensors (e.g., Sentinel-1 (S1), PALSAR-2 (P2)), as well as elevation and slope raster datasets, usually derived from SAR (Synthetic Aperture Radar) acquisitions by Interferometric Synthetic Aperture Radar (InSAR) [22]. Most SAR-based landslide detection methods utilize InSAR to quantify ground surface deformation through measured changes of radar phase between two (pre- and post-landslide event) acquisitions [23,24]. Alternatively, SAR radar backscatter intensity- and coherence can also be used to detect changes in ground surface properties (e.g., reflectance, roughness, dielectric properties). While optical sensors provide useful spectral information, SAR bands can inform about local surface deformation.

Processing and analysis of relevant satellite imagery for investigating earth surface properties is generally done by utilizing spectral indices or image classification [25]. For change detection, a time series of imagery data, including pre- and post-landslide event, is required [26]. When processing optical satellite imagery, cloud pixels can be avoided or reduced by mosaicking of quality-filtered multi-temporal images [27].

Several reviews have discussed and compared satellite RS-based methods for landslide detection [28,29,30,31,32]. In summary, most existing techniques apply a spectral index threshold (e.g., NDVI, rdNDVI (Relative Different NDVI), BSI (Bare Soil Index)), supervised image classification, or radar phase and backscatter change to an imagery obtained from a single satellite sensor. Most methods include pixel-based slope data in their algorithms and several approaches mask pixels below a defined slope threshold, excluding them from potential landslide pixels. In [33] for instance, a slope threshold of 15 degrees was applied for masking.

Some methods also consider Principal component analysis (PCA) that transforms multiple spectral bands acquired at different times into distinct linear components which results in noise reduction and removal of redundant information in the data, ultimately enhancing the distinction between spectral bands [34,35]. An overview of existing studies is provided in Table 1.

The Sudden Landslide Identification Product (SLIP) [36] algorithm is widely used for landslide detection. It is an multi-sensor approach that explores changes using four thresholds: 1) Increases in red wavelength band to signify the exposure of bare earth; 2) Variations in the Shortwave Infrared (SWIR) bands to indicate changes in soil moisture; 3) Steep slopes, determined from DEM, to restrict the detection process to areas with pronounced topographic inclines; and 4) A land-cover mask to minimize errors of commission specifically within recognized agricultural regions. SLIP had been developed for MODIS and L8 imagery, adapted by integrating the inverse NDVI to assess the soil bareness (aSLIP) [37] and improved to utilize S2 instead of L8 imagery (iSLIP) [38]. In addition, Zhang, et al. [39] discussed the potential presence of “old landslides” – areas of previously triggered landslides that did not (fully) recover at the time of the new incident. Such “old landslides” could be falsely detected as a new landslide but can be masked to avoid detecting them as False Positives (FP), if spatial data about the boundaries of such former landslides are available. Previous studies detecting landslides based on S2, L8, or similar multispectral optical sensors utilized in most cases Level-2A surface reflectance (SR) products, and in a few cases Level-1C Top of Atmosphere (TOA) products [40,41].

Validating the accuracy of landslide detection approaches is usually based on available ground truthing, in the form of either point samples of confirmed landslide locations or polygons representing their spatial boundaries. The latter is either captured in the field or created via digitization from Very High-Resolution (VHR) aerial or satellite RGB imagery on which the boundaries of landslides are clearly visible. Ground truthing-based accuracies assessment utilizes confusion matrix derived metrics including Overall Accuracy (OA), Balanced Accuracy (BA), Balanced Error (BE), Specificity (Spec), Recall, Precision (Prec), F1-Score (F1S), Negative Predictive Values (NPV), Positive Predictive Values (PPV), Omission Error (OE), Commission Error (CE), True negative rate (TNR), False positive rate (FPR), False negative rate (FNR), Quality percentage (QP), or Kappa Coefficient [14,42,43].

The accuracies of published landslide detection methods (Table 1) are challenging to compare because they strongly depend on detection and validation method, used imagery pixel resolution, reliability of ground truthing data, and on the study area size. For a reliable comparison, the methods would need to be implemented and applied to the same landslide events while using a comprehensive and accurate ground-truthing dataset for validation. With regard to reported accuracies, existing landslide detection methods (Table 1) produced good, but not excellent, detection rates with most accuracies being between 55% and 75%. Combining different optical and SAR imagery while applying advanced Machine Learning-based models could further improve landslide detection accuracies [32].

Table 1. Comparison of existing literature addressing Satellite Imagery based landslide detection, including information about sensor source, detection method, and study location. ‘Change detection’ indicates whether images pre- and post- (‘yes’), or only post- (‘no’) landslide event were utilized. ‘GEE’ indicates whether a method was implemented using Google-Earth-Engine. ‘Coseismic’ indicates whether case studies used in each publications include coseismic landslides or not.

Table 1. Comparison of existing literature addressing Satellite Imagery based landslide detection, including information about sensor source, detection method, and study location. ‘Change detection’ indicates whether images pre- and post- (‘yes’), or only post- (‘no’) landslide event were utilized. ‘GEE’ indicates whether a method was implemented using Google-Earth-Engine. ‘Coseismic’ indicates whether case studies used in each publications include coseismic landslides or not.

No.	Publication	Satellite Sensors	Detection method	Change detection	GEE	Study area	Co -seismic
M1	[44]	L8, SRTM DEM	∆NDVI, Supervised classification	yes	yes	Nepal	yes
M2	[45]	S2, SRTM DEM	∆NDVI or rdNDVI	yes	yes	Sulawesi	yes
M3	[46]	S2, L8	rdNDVI	yes	yes	Papua New Guinea, Kenya	yes1
M4	[43]	S2	∆BSI	yes	no	Central America	yes
M5	[33]	S2, DTM (5m)	∆NDVI, slope	yes	yes	Italy	no1
M6	[35]	S2, ALOS GDEM	Unsupervised classification (NDVIpost, slope, S2post bands)	no	no	India, China, Taiwan	yes1
M7	[42]	S2, ALOS GDEM	Supervised OBIA (NDVIpost, slope)	no	no	India, China, Taiwan	no1
M8	[47]	L8	Supervised classification (NDWIpost, NDVIpost, DEM, slope)	no	yes	India	no1
M9	[48]	S1 or S2	∆NDVI, SAR backscatter (VV-VH)	yes	yes	Norway	no1
M10	[23,49,50]	S1	∆SAR backscatter (VH), heatmap for visual landslide interpretation	yes	yes	Haiti; Vietnam;Japan: Hokkaido, Hiroshima;	yes1
M11	[51]	S1	∆SAR backscatter (VV-VH)	yes	no	Mexico	yes
M12	[14,52]	P2	∆SAR backscatter (HH)	yes	no	Japan: Hokkaido	yes
M13	[36]	L8	SLIP (%RedChange, ∆mNDMI)	yes	no	Nepal, Cameron	yes1
M14	[37]	L8	aSLIP (mRedChange, ∆iNDVIn, ∆mNDMI)	yes	no	Nepal, Cameron	no1
M15	[38]	S2	iSLIP (mRedChange, ∆mNDMI)	yes	no	Japan: Hokkaido	yes
M16	[40]	S1, S2	∆SAR backscatter (VV) or SLIP (%RedChange, ∆mNDMI)	yes	no	India	no
M17	[53,54]	GE RGB imagery	ML: RetinaNet, YOLO v3, Mask R-CNN, YOLOX	no	no	China	yes

¹ Case studies include landslides triggered by floods or other weather events.

2. Materials and Methods

2.1. Case Studies

Below, four co-seismic earthquake case studies used in this paper are described. Table 2 provides an overview of each event, including details of the sourced landslide inventory data. In addition, Figure 1 maps these inventory data as well as the used study area boundaries.

2.1.1. Japan, 2018 Mw 6.6 Hokkaido Earthquake

On September 6, 2018, a moment magnitude 6.6 earthquake struck the Iburi Subprefecture in southern Hokkaido, Japan, following Typhoon Jebi’s passage. The 2018 Hokkaido Eastern Iburi earthquake is hereafter referred to as “JPN case study”. Aftershocks indicated compression along the ENE-SWS direction in the northern, southern, and Shallow Iburi faults [66]. The earthquake caused extensive damage, triggering thousands of landslides. A comprehensive inventory map identified nearly 6,000 coseismic landslides, mostly small to medium translational ones with high mobility and long run-out distances ranging from 25 m² to up to 57,000 m². This dataset, representing ground-truth polygons of all landslides triggered during the 2018 Hokkaido earthquake, is hereafter referred to as “landslide inventory”. The spatial extent of the study area was defined by applying a convex hull around the landslide inventory polygons, resulting in an extend of 22 km (East-West) by 24 km (North-South) geodetic distance, while the convex hull-shaped study area spans a geodetic area of 359 km². Note that for computational reasons a few distant/isolated landslide inventory polygons had been excluded to reduce the total study area. Despite the area’s low population density (less than 10 persons per km²), the landslides resulted in 41 fatalities and 691 people injured. The landslide concentration formed an elliptical area parallel to the region’s (active) faults, with the most affected aspect being southerly, perpendicular to the NNW/SSE striking faults. These coseismic landslides occurred in regions with a seismic intensity ranging from 7.0 to 8.0 on the Modified Mercalli intensity scale and peak ground acceleration between 0.4 and 0.7 g. They were densely distributed in hilly regions at elevations between 100 and 250 m, with slope angles ranging from 15° to 35°. Most of the landslides were shallow, several meters deep, and categorized as planar and spoon types [67]. The area predominantly consists of Neogene sedimentary rocks overlaid by pumice layers from Tarumai volcano to the east. Surface soil layers in the low to middle mountain ranges contain interbedded pumice and ash, with a thickness of approximately 4-5 m [14]. The landslide-prone area primarily consists of Miocene sedimentary rock. Slope failures occurred in stratified pyroclastic fall deposits due to a combination of strong seismic ground motion and intense antecedent precipitation [39].

2.1.2. Haiti, 2021 Tiburon Peninsula Mw 7.2 Earthquake

On August 14, 2021, a seismic event of moment magnitude 7.2 struck the region of Nippes, Haiti, specifically on the Tiburon Peninsula, marking a significant geological event. This seismic disturbance was attributed to activity along the Enriquillo–Plantain Garden Fault, a seismogenic fault renowned for its tectonic significance in the area. The aftermath of this earthquake revealed a notable impact on the landscape, triggering over 8,444 landslides across an expanse of approximately 2,700 km². Among these, an area totaling 45.6 km² experienced direct landslide activity, with particular concentration observed in the western region of the Tiburon Peninsula. Notably, the Pic Macaya National Park emerged as a focal point, with approximately 6,100 landslides occurring within or near its confines, constituting 72.2% of the total landslide occurrences, hereafter referred to as “HTI case study”. Further analysis revealed that 89.4% of the landslides were predominantly situated in the hanging wall area and regions characterized by high relief, featuring slopes ranging between 35° and 55° [68]. The consequences of these landslides were multifaceted, directly resulting in fatalities and extensive damage to infrastructure. Moreover, the obstruction of roads and other vital pathways hindered response efforts, exacerbating the challenges faced in the wake of this seismic event [69].

The spatial extent of the study area was defined by applying an envelope around the landslide inventory polygons, resulting in an extend of 16 km (East-West) by 11 km (North-South) geodetic distance, while the convex hull-shaped study area spans a geodetic area of 170 km². Note that for computational reasons a few distant/isolated landslide inventory polygons had been excluded to reduce the total study area.

2.1.3. Papua New Guinea, 2018 Mw 7.5 Earthquake

In the Highlands of central Papua New Guinea (PNG), close to Komo in the Hela Province, a seismic event of moment magnitude 7.5 took place on February 25, 2018. This event, hereafter referred to as “PNG case study”, stands as the most remarkable earthquake documented within this geographical expanse over the preceding century. Subsequently, the affected area experienced four substantial aftershocks (each with a moment magnitude equal to or exceeding 6.0) within a span of 9 days following the mainshock. The affected area is highly susceptible to landslide due to its climatic, geologic, and tectonic influences. Furthermore, local environmental conditions serve to amplify the seismic shaking, exacerbating the propensity for landslides. The aftermath of the seismic event was characterized by the huge amount of landslides throughout the region [70].

The initial earthquake and its ensuing aftershocks triggered widespread landslides, with an estimated total exceeding 11,600, of which > 10,000 were triggered by the principal earthquake. These landslides collectively generated a cumulative planimetric failure area encompassing approximately 145 km². Steep hillslopes caused the activation of large landslides reaching dimensions of up to approximately 5 km² for single landslides. Analysis of the seismic event delineated a predominant reverse fault motion, with discernible displacement reaching up to approximately 0.7 meters along faults extending to depths surpassing 25 km [71].

The spatial extent of the study area was defined by applying an envelope around the landslide inventory polygons, resulting in an extend of 148 km (East-West) by 84 km (North-South) geodetic distance, while the convex hull-shaped study area spans a geodetic area of 5163 km². Note that for computational reasons a few distant/isolated landslide inventory polygons were excluded to reduce the total study area.

2.1.4. New Zealand, 2016 M_w 6.7 Kaikōura Earthquake

On November 14, 2016, the northeastern South Island of New Zealand experienced a seismic event of Mw 7.8 known as the Kaikōura earthquake. This earthquake, hereafter referred to as “NZL case study”, exhibited a remarkably intricate rupturing mechanism, unprecedented in complexity within recorded seismic activity. The seismic instrumentation in place within the region provided comprehensive data on the event. A total of 14,233 landslides were documented, covering an extensive area of approximately 14,000 km². The earthquake’s rupture sequence impacted a series of active faults, extending offshore and significantly affecting coastal and inland areas across the northeastern part of the South Island.

Analysis of global moment tensor solutions revealed the intricate nature of this multi-fault rupture earthquake. The rupture process was observed to propagate from south to north, establishing connections between the Hikurangi subduction system of the North Island and the oblique collisional regime of the South Island, notably the Alpine Fault. The impacts of the Kaikōura earthquake on both infrastructure and the environment were severe and widespread. Reconstruction efforts were estimated to cost between 3 and 8 billion NZD, underscoring the scale of the challenges posed by the event’s aftermath [72].

The spatial extent of the study area was defined by applying an envelope around the landslide inventory polygons, resulting in an extend of 48 km (East-West) by 69 km (North-South) geodetic distance, while the convex hull-shaped study area spans a geodetic area of 1370 km². Note that for computational reasons a few distant/isolated landslide inventory polygons had been excluded to reduce the total study area.

Table 2. Inventory data of coseismic landslides case studies used in this paper.

Table 2. Inventory data of coseismic landslides case studies used in this paper.

Earthquake date	Epicentre location	Epicentre Lat/Lon	Focal depth(km)	Mw, death, injured	Inventory method	Ref.	No. inventory landslides	Used landslides	Study area (km2)
2018 Sept 6	Japan, Hokkaido, Iburi	42.662°N 142.011°E	37	6.6, 41, 691	VHR UAV imagery, PlanetScope	[73]	5,625	5,208 (93%)	359
2021 Aug 14	Haiti, Tiburon Peninsula, Pic Macaya NP	18.434°N 73.482°W	10	7.2, 2,200, 12,200	GE imagery, PlanetScope	[68]	6,100	approx. 80%	170
2018 Feb 25	PNG, Hela Province, Komo	6.070°S 142.754°E	15-30	7.5, 160, 500	GE imagery, PlanetScope, Rapid Eye	[71]	11,607	8,912 (77%)	5,163
2016 Nov 14	New Zealand, South Island, Kaikōura	42.737°S 173.054°E	15	6.7, 2, 618	GE imagery, S2	[72]	14,233	2,521 (18%)	1,370

2.2. Methodology

Our ML-LaDeCORsat approach involves two principal steps: a) preprocessing and b) training and evaluation of a ML algorithm used to classify a combined S2 optical S1 SAR imagery dataset into landslide and non-landslide pixels.

2.2.1. Data Preparation

Considering previous approaches (see Table 1), several satellite sensors and imagery bands that are available in GEE are relevant for landslide detection as proposed in this work (Table 3).

To prepare multispectral S2 data for landslide detection for each case study, two cloud-free sets of imagery covering the entire Area of interest (AOI) are necessary, representing the situations before and after the landslide events.

As indicated in Table 2 and in Figure 1, the inventories of landslides used in this work do not include the entire available inventory source dataset. For GEE performance reasons, a convex hull was applied covering the majority of all inventory data while excluding a small amount of rather isolated inventory landslides. In addition, for the NZL case study, available Sentinel-1 post event imagery only covered the South-East part of the available inventory data.

Except for the NZL case study, a single S2 imagery scene available in GEE did cover the respective AOI. In case of NZL, three adjacent cloud-free S2 imagery had to be joined using the GEE ImageCollection.mosaic() function.

Due to the likeliness of cloud coverage, it is suggested to compute multi-temporal image composites (“mosaics”) [48]. In this work, S2 multi-temporal image mosaics were created for the pre- and post-earthquake scenario over the PNG case study due to significant cloud coverage in all S2 imagery in 2017 and 2018. In total 88 selected S2 imagery during 2017 were used to create a cloud-free S2 pre-earthquake mosaic and 67 selected S2 imagery between March and July 2018 to create a cloud-free S2 post-earthquake mosaic. Due to PNG’s geographic location being close to the equator causing its hot, humid climate with all months above 18°C, seasonal reflectance dynamics have only marginally impact on these mosaics if at all. For the other three case studies, nearly or completely cloud-free S2 imagery were available within a few months after each earthquake event. A cloud-free pre-earthquake S2 imagery was then extracted approximately one year earlier to avoid seasonal reflectance dynamics.

Since a S2-L1C (TOA) image product can be available (on GEE) up to a few hours earlier than S2-L2A (SR), it was decided to use the S2-L1C product, and to apply, dependent on its impact to landslide detection accuracies, the Sensor Invariant Atmospheric Correction (SIAC) method [74] to convert TOA radiance into estimated SR values in GEE. Masking was applied to the few cloudy pixels in the pre-earthquake S2 image using the S2 QA60 (cloud mask) band.

Sentinel-1 imagery data accessible via GEE consists of radiometric calibrated, ortho-rectified SAR C-band backscatter images available in four polarization modes: 1) Single co-polarization: vertical transmitting, vertical receiving (VV), 2) Single co-polarization: horizontal transmitting, horizontal receiving (HH), 3) dual-band cross-polarization: vertical transmitting, vertical and horizontal receiving (VV and VH), and 4) dual-band cross-polarization: horizontal transmitting, horizontal and vertical receiving (HH and HV). As argued by [23], HH and HV polarizations are less useful for landslide detection, thus only VV and VH polarization were considered due to their sensitivity to forest biomass structure [75].

Due to the longer wavelength, the emitted L-band radar of ALOS-2/P2 can better penetrate through denser vegetation compared to S1’s C-band. The GEE-provided ortho-rectified and radiometrically calibrated and terrain-corrected normalized backscatter data of P2 used in this work, are available as HH and HV polarization bands.

The available S1 SAR imagery data included in this study comprised the period four months before and after the earthquake in order to identify sufficient ascending and descending pre-earthquake respectively post-earthquake imagery. Following the approach of [23], the temporal median of the pre-event and post-event SAR data was computed, and ascending and descending data were combined by calculating their mean values.

Due to the differences between S1 and P2 in terms of sensor characteristics, polarimetry, orbit parameters, imaging strategies, and data availabilities in GEE, a much larger time window for P2 had to be selected. Even including all available P2 imagery since 2014 and until present, sufficient ascending and descending pre- and post-event P2 imagery were only available for the JPN case study.

Besides using elevation data sourced from ASTER GDEM to extract landslide conditioning factors, slope values derived from the GDEM were used to mask out irrelevant pixels below a certain slope threshold (e.g., 10 degrees).

Another potentially relevant conditioning factor is pixel-based solar radiation. However, the only globally available raster dataset in GEE is the Global Solar Atlas, generated using Shuttle Radar Topography Mission (SRTM) derived elevation data combined with MTSAT and Himawari-8 reflectance data as reported in [76]. Due to its coarse spatial resolution of 250 m, the solar atlas was not incorporated into this work.

2.2.2. Landslide Conditioning Factors

As shown in Figure 2, several bands and derived bands (“pseudobands”) were identified from literature as relevant landslide conditioning factors – and used in the ML-LaDeCORsat approach. For S2, these include the following bands: a) all spectral bands of the prepared S2 post-earthquake image except B1 (aerosol), B9 (water vapor), and B10 (cirrus), b) the differences of each spectral band by subtracting pre-earthquake reflectance values from post-earthquake ones, and c) the below-listed computed spectral indices1:

• Soil index: BSIpost, ∆BSI [43]
• Vegetation index: NDVIpost, ∆NDVI, rdNDVI, ∆iNDVIn [35,42,43,44,45,46,47,48]
• Water index: NDWIpost, ∆NDWI [47]
• Drought index: ∆mNMDI [36,38]
• Sudden Landslide Identification Product/index: SLIP [36], aSLIP [37], iSLIP [38]

Figure 2. Conceptual overview of the ML-LaDeCORsat approach, involving Pre-processing (top box) and Model training and evaluation (bottom box).

Figure 2. Conceptual overview of the ML-LaDeCORsat approach, involving Pre-processing (top box) and Model training and evaluation (bottom box).

Specific index formulas and thresholds can be found in the cited references. All these indices had been used in previous landslide detection methods, except BSI_post which was added to explore its importance for detection performance.

Utilizing S1 polarization bands, the same methodology as suggested in [23] was followed to determine log ratio (and percentiles of it) for pre- and post-event S1 SAR intensity using different polarizations. It was then decided to include the following three bands which had been identified via empirical investigation as the best performing bands for landslide detection:

• S1_log_VH: log ratio for pre- and post-event S1 SAR (VH) intensity
• S1_90p_VH: 90th percentile of S1 log ratio (VH)
• S1_90p_VH_VV: 90th percentile of S1 log ratio (VH and VV)

Utilizing P2 polarization bands, the same process as for S1 was applied and the following band was identified to be included in the ML classifier used in this study: P2_log_HV, the log ratio for pre- and post-event P2 SAR (HV) backscatter.

Derived from GDEM, the following topographic factors were included: slope, aspect, curvature, and elevation [33,35,42]. These topographic data are derived from imagery taken prior to the earthquake event. Post-earthquake elevation data were not available in this work, but data including detailed changes in elevation might further improve detection results.

Further relevant topographic indices are TWI and STI, computed using slope and flow accumulation as inputs. While slope can be easily created in GEE from a given DEM, GEE does not provide a function to compute flow accumulation out of the box due to a significant amount of required iterative operations. TWI and STI could be, however, computed from a DEM using GIS software and uploaded into GEE. In GEE, available rainfall datasets do not provide the required spatial resolution to be considered for this work. The Global Precipitation Measurement (GPM) dataset [77], for instance, supplies global precipitation estimates updated every 30 minutes, but only at a spatial resolution of 0.1 degrees (approximately 11 km). As the focus in this work is to produce a replicable and ready to use GEE landslide detection product, the use of TWI and STI was not further investigated.

In summary, these identified indices, allow the proposed ML-LaDeCORsat detection method to address changes in surface vegetation, vegetation moisture, bare soil, soil moisture, and bare-earth exposures derived from S2 imagery; to consider alterations in ground surface properties (e.g., roughness, dielectric properties) through S1 and P2 SAR intensity and coherence changes; and to incorporate topography features of the study area (e.g., slope, aspect) using the GDEM.

In preparation for the image classification, pre-processed SAR and topographic bands were spatially reprojected and resampled to match pixel dimension (10 m) and position of S2 pixels. Next, all 40 landslide conditioning bands were combined into a single image using the addBands GEE function. This multi-band stacked image was then fed into the ML feature matrix.

Investigations into PCA bands derived from S2 post and S2 change bands did not lead to improved landslide detection accuracies, for none of the four case studies, hence PCA bands were not included in this work.

2.2.3. ML Sampling Strategy

A sampling strategy specifically designed for ML-based classification of landslides was developed: This strategy consists of several geoprocessing steps, illustrated in the preprocessing part of the provided flowchart in Figure 2. First, three sampling areas were defined. Utilizing the above-mentioned landslide inventories to define a ”Positive” sampling area (P_area), the GIS Erase tool was applied to generate an initial non-landslide “Negative” sampling area (N_area). To allow a stratified random sampling method to create sufficient sampling points near inventoried landslide polygons, a 100 m ring-buffer was created around the P_area, forming a second “Negative” sampling area: the N-B_area. This ring-buffer area was also erased from the initial N_area. Sampling points inside N-B_area allow the model to put emphasize on “border“ areas around landslides.

Next, the spatial extent of sampling areas and the multi-band stacked input image were reduced removing pixels within areas of low slopes, less than 10 degrees. Such slope-filtering is being applied in many of the existing RS-based landslide detection approaches, including [33,36,37,38]. This, however, can cause False Positives (FP) for scenarios in which the landslide is triggered at steeper slopes but its flow area reaches into areas of lower slopes, as illustrated in Figure 3.

To address this issue, a 100 m buffer was added to the slope filtering (using >10 degrees) before clipping the ML-input image with the final sampling areas. The buffer size of 100 m was determined empirically. Not applying such buffer has a strong impact on causing FP as can be seen in Figure 4: Pixels with slopes >20 degrees are mapped in grey on top of red-color coded landslide inventory areas. The slope threshold of >10 degrees applied to the JPN case study did only overlap 24% of the landslide inventory polygons (P_area). Only when adding the 100 m buffer, more than 99% of the inventory data were covered, as shown in Figure 4b. Table 4 lists landslide inventory areas not covered by a >10 degree slope threshold for all four case studies.

2.2.4. ML Classifier

For ML model training and evaluation, a randomly stratified point sampling approach was implemented using in total 6,000 sampling points that consists of 3,000 “landslide” points inside P_area, 2,000 “non-landslide” points inside N_area, and another 1,000 “non-landslide” points inside N-B_area. Figure 5 Each of these three parts were then split into 80% training points and 20% validation points as exemplarily shown in Figure 5.

Next, all available ML classifiers in GEE were implemented, including Classification and Regression Trees (CART), Naive Bayes (NB), Random Forest (RF), Support Vector Machine (SVM), and Gradient Tree Boost (GTB). For a consistent comparison, a fixed randomization seed with a value of ‘0’ was applied to ensure using the exact same set of randomly distributed sampling points for ML model training and validation. Aiming to compare the performance of all ML classifiers for landslide detection, the optimal settings for the classifiers’ parameter were empirically investigated and finally defined as listed in Table 5. Details about GEE classifier and their parameters are described in [78,79]. To further improve detection accuracy, a Gaussian kernel filter (radius: 60 m, sigma: 30) was applied as suggested in [80] to the classified image to remove wrongly detected isolated single landslide pixels. Older landslides triggered by previous events were excluded from training and predication. If not considered the prediction will likely suffer from high number of FP.

2.2.5. Evaluation Metrics

A range of different evaluation metrics were implemented to allow comparison of landslide detection accuracy achieved in this work with existing methods (listed in Table 1). Evaluation metrics used in this study included Kappa, OA, BA, BE, Spec, Recall, Prec, F1S, NPV, PPV, OE, CE, TNR, FPR, and FNR. Furthermore, pixel counts of TP, FP, FN, TN were computed and added to the results to allow others to determine any further evaluation metrics, such as omission or commission error. For comparing different classification settings within this work (classifier parameters, sampling input, selected bands, and pseudobands), it was decided to use the BA as one of the main comparative validation metric due to its advantages for measuring the prediction quality of all target classes as described by [81], along with OA, Recall, Prec, F1S, and Kappa. Moreover, Training Accuracy (TA) and Validation Accuracy (VA) were computed to investigate and address potential model overfit (in case of larger differences between TA and VA). A detailed description of each metric can be found in [81]. For each case study, the evaluation metrics were computed using approximately 2,000 randomly distributed validation points, containing at least 600 points inside landslide inventory polygons. Computing the same evaluation metrics was then repeated considering all pixels of the entire study area. Although a heavier computational task in GEE, the ‘all-pixel’ evaluation approach provides the best possible evaluation.

2.2.6. Band Importance Investigations

When using a set of potentially relevant features or variables (in this case spectral bands or derived indices) for a ML algorithm, it is crucial to investigate the importance of each band to achieve a better insight into what feature contributes towards performance improvement and to decide which features to exclude in order to reduce the classification computation time and memory. To investigate the importance of each band, we utilized the GEE inbuilt function: classifier.explain() which returns a dictionary that describes the results of a trained classifier and includes band importance factors.

2.2.7. Transfer Learning Investigations

Transfer learning or domain adaptation has been widely used in image classification and other fields [82,83]. It is crucial to note that models predicting landslides may be applied in geographical areas distinct from those where the model’s training data was gathered.

To verify the performance of ML-LaDeCORsat applied to landslide events, we designed and conducted across domain transfer learning for each case study with training the ML-LaDeCORsat model using different subsets of target site samples together with the complete sampling point dataset from other case studies. The model was trained on data from one up to three sites and with or without a portion of training data from site four and is applied to the test data of site four for evaluation. Comparing and validation of across domain trained ML-LaDeCORsat outputs followed the same approach as described earlier.

3. Results

4. Discussion and Conclusions

Abbreviations

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