*2.1. Underreported Landslide Events in the WEAR*

The WEAR covers an area of ~350,000 km2 in tropical Africa (Figure 1). This highly populated region is characterized by high rainfall intensities, recent seismicity, deeply weathered substrates, and a complex rift topography [33,47,49–52]. These factors render the area highly susceptible to landsliding [45,49]. Indeed, recent studies incorporating observations from satellite images and fieldwork reveal high landslide activity, with hundreds of recent landslide events over the last 10 years accounting for more than 5000 individual landslides mapped in the area [44,49,50,53]. An event is defined as a single landslide or a group of landslides with a common trigger over the same area [29]. Landslide types vary greatly from dominant shallow slides and earth and debris flows to less frequent rock avalanches and deep-seated rotational slides [44,47,49,50]. Rainfall has been identified as the prevailing trigger of these recent landslides [29,35,47,49,54]. Despite this large number of landslide events, usable information about the day of their occurrence is rare [29]. Actually, for a period of observation similar to the one covered by the satellite rainfall estimates, [35] compiled an unprecedented landslide event inventory that comprises 145 events for which the location and day of occurrence is known. This clearly highlights the fact that landslides are severely underreported in this poorly accessible region [29].

In this research, the calibration data set ('CAL' in Figure 1) used for the threshold approach is the same as in [35]. The 145 dated landslide events are located with an individual accuracy better than 25 km and a mean accuracy of 7.2 km. The inventory covers the period from 2001 to 2018, with most landslides occurring from March to May, after the second rainy season [35] (Figure 2).

The event inventory used for validation ('VAL' in Figure 1) consists of newly acquired information on 39 additional events located with similar individual accuracy and a mean accuracy of 2.3 km. Information about these events was obtained through field observations and newly identified online media sources, explaining their 2002–2019 temporal coverage overlapping with that of the calibration inventory. Their seasonal distribution confirms the temporal pattern previously observed in [35] (Figure 2).

Both the calibration and validation data sets are strongly biased in space and time, due to the severe constraints inherent to the political, economic, and environmental context in the WEAR [29,35,55]. Considering the restricted accessibility of many parts of the study area, fast vegetation regrowth or

land rehabilitation, and areas with poor temporal (cloud-free) satellite coverage, uncertainties are too high to claim absence of landsliding in places and periods where no event has been reported. A distinction between landslide types cannot be asserted, owing to inadequate information in media reports which constitute the principal source of both landslide event inventories. Information on the sub-daily timing of the landslide occurrence is rare. Therefore, when the day of occurrence of the event is known, there is still an uncertainty mainly related to the ambiguous reporting of landslides that occurred during the night when the date sequence is not mentioned. For this reason, we consider that a reported landslide may have occurred randomly at any time over a 36-h period centered on the reported day [35].

**Figure 1.** Landslide susceptibility at 0.25◦ resolution, derived from (**a**) the continental-scale model of [45] and (**b**) the regional-scale model of [49], and distribution of dated and localized landslide events in the western branch of the East African Rift. Landslide events used for calibration (CAL) are shown in blue and those for validation (VAL) in green, with their respective number between brackets. A total of 184 landslide events are distributed over 63 different pixels. 1: Lake Albert; 2: Lake Edward; 3: Lake Kivu; 4: Lake Tanganyika. Background hillshade 3 arc-second SRTM (±90 m).

**Figure 2.** Monthly distribution of 145 (blue) and 39 (green) landslide events in the WEAR used for the threshold calibration and validation respectively, and mean monthly rainfall based on 20 years (2000–2019) of TRMM Multisatellite Precipitation Analysis 3B42 Real-Time, version 7 (TMPA-RT) daily data, downloaded from https://disc.gsfc.nasa.gov/ (last access: 14 April 2019).
