## 2 Landslide Early Warning System for Busan: SEM-LEWS  
### Principles of Landslide Early Warning System: Sequential Evaluation Method  
In this study, we designed a landslide early warning system for Busan to evaluate the hazard of rainfall-induced landslides and determine appropriate warning levels based on the principles of the sequential evaluation method proposed by [PERSON] et al. (2018). As shown in Figure 1, the proposed early warning system is characterized by five distinct warning levels (Null, Attention, Watch, Alert, and Emergency) that are driven by a decision-making algorithm. The algorithm sequentially uses three different evaluation stages to determine appropriate warning levels. The first evaluation phase is a statistical evaluation, initiated at the \"Null\" warning level. This evaluation uses two different, statistically defined rainfall thresholds in combination with a statistically derived geo-spatial threshold to determine whether the warning level should be raised to \"Attention.\" The two rainfall thresholds are the _rainfall intensity-duration threshold_ (_I-D curve_) and the _extreme rainfall-induced landslide index_ (\(I_{ERL}\)), while the geo-spatial threshold is represented by a _landslide susceptibility index_ (\(I_{LS}\)). If the outcome of the first evaluation is to raise the warning level to \"Attention,\" the second evaluation phase is initiated. The second stage, a physically based evaluation, applies _critical continuous rainfall_ (\(CR_{Critical}\)) to determine whether to increase the warning level to \"Watch\" or further to \"Alert.\" Here, if the observed or predicted rainfall reaches 70 or 100% of a physically derived critical rainfall amount, the warning level will be increased to \"Watch\" or \"Alert,\" respectively. If the warning level is determined as \"Alert,\" the decision-making algorithm enters the third and final evaluation stage. This third evaluation phase is a geomorphological evaluation, where a _debris flow mobilization index_ (\(I_{DF}\)) is used to evaluate several geomorphological factors and thereby determine whether the warning level should be increased to the highest level of \"Emergency.\"  
Using a series of thresholds grouped in terms of theoretical backgrounds, each of the five warning levels offers specific theoretical interpretations of landslide hazards. Firstly, by using lower limits of the three statistically defined thresholds, \"Attention\" represents a conservative and preliminary warning that advises for preemptive preparations. Secondly, both \"Watch\" and \"Alert\" warnings focus on slope instability from a physically based perspective. Specifically, \"Watch\" functions as a precautionary warning to extend response time prior to a slope becoming unstable, while \"Alert\" indicates that a slope has become completely unstable and that a slope failure may occur at any point. Thirdly, \"Emergency\" indicates that a slope is completely unstable and, in addition, the potential failed mass of the slope is highly likely to transition into channelized debris flows, which can have severe physical and social implications for surrounding communities.  
Further detailed information regarding the sequential evaluation method including the relevant decision-making algorithm, theories of constituent thresholds, required parameter datasets, and processes to generate early warning level maps are described in [PERSON] et al. (2018).