## 2 The Montecilfone Earthquake Case Study  
The Molise region is characterized by a very high frequency of landslides, which impact more than 28,000 sites over an area of about 4400 km\({}^{2}\), including a wide range of processes and landslide size [21; 22; 23; 24], with a different state of activity (IFFI database; [[https://idrogeo.ispmbienne.it/app/ififi](https://idrogeo.ispmbienne.it/app/ififi)]([https://idrogeo.ispmbienne.it/app/ififi](https://idrogeo.ispmbienne.it/app/ififi)); last access 10 March 2022) [25]. The high landslide concentration in the study area and the abundance of shallow landslides (i.e., soil slip), earth slides and solifiluction are strictly related to the lithological features, where slope deformations mainly involve quaternary cover deposits and weathered portions of the clayey and marly bedrock. These units are ascribable to a thick Cenozoic marine succession belonging to the Lagoneegrese-Molisan Basin [26; 27], whose lower sequence is mainly composed of scaly clays [28] with subordinate calciturbiditic intercalations and followed by Neogenic arenaceous-marly and calcareous-marly flysch deposits [26]. In the North-East sector of the study area Plio-Pleistocene blue marly-clays belonging to the lower horizon of the Mutignano Fm. [29] crop out, often involved in landslides. In this landslide-active context, on 16 August 2018, an Mw 5.1 earthquake occurred at 18:19:04 UTC in Montecilfone (CB), Molise region (Central Italy), anticipated by a seismic event of the 14 August 2018 with a Mw 4.6 (SISMIKO Group INGV), striking an area involved in the 2002 San Giuliano di Puglia seismic sequence [30], located about 20 km south of Montecilfone (Figure 1a). Figure 1: (a) Physiography of the Molise region with locations and magnitudes of historical earthquakes in the area according to the CPTI15 catalogue [31]; location of the study area struck by the  
2018 seismic sequence is highlighted by the red dashed line. (**b**) Distribution of ground effects surveyed and inventoried in the CEDIT catalogue [32] with respect to outcropping lithotechnical units, PGA, and cumulated rainfall in the Thiessen polygons. Outcropping lithotechnical units include: (1) landslide deposits; (2) fluvial and terraced deposits; (3) calcarenties; (4) blue clays; (4) peiltic flysch; (5) marly clays; (5b) scaly clays; (6) arenaceous fysch. Location of the Liscione earth dam is also reported. The mainshock of the 2018 seismic sequence caused a peak ground acceleration (PGA) of up to 0.12 g ([[http://shakemap.rm.ingv.it](http://shakemap.rm.ingv.it)]([http://shakemap.rm.ingv.it](http://shakemap.rm.ingv.it)); last access 10 March 2022), which corresponds to a return time of 745 years according to the Italian seismic hazard map ([[http://essel-gis.mi.ingv.it/](http://essel-gis.mi.ingv.it/)]([http://essel-gis.mi.ingv.it/](http://essel-gis.mi.ingv.it/)); last access 10 March 2022). The earthquake reflected a reduced macroseismic intensity (V-VI; EMS98 [33]) but induced an unexpected frequency and distribution of ground effects [20]. Ground crackrelated to landslides were observed within an area of tens of square kilometers, resulting in an induced scenario that suggested a possible combined action of multiple destabilizing actions controlling the landslide scenario. The seismic event was anticipated by intense rainfall that affected the NE part of Molise region, causing a cumulative rainfall of up to 160 mm [20], with an intensity of 145 mm cumulated in three days in the epicentral area, recorded at the Palata weather station (Figure 1b). Field surveys carried out in the days immediately after the earthquake (Figure 1a) allowed us to recognize and classify 88 earthquake-induced ground effects, grouped into 75 coherent slides (earth slide and earth flow mechanisms), 9 disrupted landslides [34] (Figure 1b), and 4 ground cracks mostly related to slides. Such landslides occurred in the summer, after more than 30 days of absence of intense rainfall, in a period characterized by the landslide's quiescence [35].