PSI - Issue 84

Nicola Perilli et al. / Procedia Structural Integrity 84 (2026) 805–812

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2.2 Comparison with Literature The distinction between susceptibility , slope instability , and hazard is essential for robust and interpretable landslide risk assessments. These concepts operate at different analytical levels and are not interchangeable (Tab. 4). Misinterpretation may lead to inappropriate conceptual and procedural decisions, weaken the collection of results, and compromise their interpretation. A brief, literature-based summary is provided below. • Susceptibility –The spatial predisposition of an area to landslides, determined by intrinsic geo-lithological, environmental, and anthropogenic characteristics. It is independent of time and allows the identification of potentially instable areas requiring further investigation. • Slope Instability (SI) – The measurable physical state of a slope, reflecting its actual condition along a continuum from incipient deformation to active downslope mass movement, namely landslides. SI is inferred from deformation precursors (incipient instability) or from indicators of landslide primary parameters: state of activity (P A ), movement/velocity (P V ), and magnitude (P M ). Unlike susceptibility , SI requires field observations or monitoring data. •Hazard – The probability that a landslide of a given size occurring in a defined area within a specific time interval. Its integrates spatial predisposition, temporal probability, expected magnitude, and triggering factors. Its assessment relies on monitoring, historical data, and trigger modeling, supporting quantitative risk evaluation. In compliance with LLG 2020, these definitions provide a conceptual basis for the solution proposed in Section 3. Table 4. Conceptual and operational distinction among Susceptibility , Slope instability (SI), and Hazard . Simbols ✗= exclude and +=include. Concept Diagnostic Question Dimensions Considered Key References

•Brabb (1984): likelihood of slope failure given local terrain and environmental conditions. •Reichenbach et al. (2018): degree to which terrain may be affected by future slope movements. • Guzzetti et al. (2005): predisposition to landslides based on morphological and geological traits, ignoring time and magnitude. •Crozier (1986) & Hungr et al. (2014): continuum from stable conditions to initial deformations and active landslides. States of SI: Incipient landslide : early instability signals, often detectable only through precise monitoring. Active landslide : measurable mass movement along the slope. •Guzzetti et al. (2005); Van Westen et al. (2006): probability that a landslide of given size occurs in a defined area within a specific time interval. •UNDRR (2017): combines spatial predisposition with temporal triggers (rainfall, seismic events). •Dahal et al. (2024): requires reliable time series, trigger modeling, and historical inventories.

Space only ✗ time ✗ movement ✗ velocity ✗ magnitude Space + movement + activity +velocity + magnitude ✗ time Space + movement + activity + magnitude + time + triggers

Where may landslides occur in a given area? Is the slope currently unstable and showing incipient or effective signs of movement? Where, when, might a landslide occur, and how severe could it be?

Susceptibility

Slope Instability (SI)

Hazard

3. Solutions for robust Slope Instability Class Assessment This section presents solutions to ambiguities in some key terms, along with the revision of two secondary parameters (Tabs. 5, 6). The key terms, redefined or recalibrated help reduce arbitrariness in the assessment of the Factor of Susceptibility , thereby improving consistency and reproducibility. In this context the relationships between susceptibility and slope instability , are clarified and operationalized for consistent field survey. The logic flow linking SI Degree , SI Class, and the Factor of Susceptibility is also explained, including the logical exclusion of the potential landslide and its corresponding primary parameter (P C ). Furthermore, the decision rules for the two secondary parameters - Reliability of primary parameter estimation and Status of Mitigation Works- are presented (Tabs. 5, 6). Table 5 provide a comparison of key landslide-related concepts in LLG 2020 and the refined definition proposed in this study, highlighting their operational interpretation. The refined terminology and operational definitions are below reported. • SI Degree – calculated as the summed numerical values of P A + P V + P M - replaces the term SI (Tab. 5, Fig. 1 ).