## 2 Study Area  
### Geological Setting  
The Sarno plain is located at the SE corner of the Campana plain, a large peri-Tyrrhenian graben situated along the inner sector of the Southern Apennines. The Southern Apennines are a NE-verging mountain belt that developed in response to the Neogene collision between the African and Eurasian plates [22; 23; 24]. Formation of the peri-Tyrrhenian grabens have been ongoing since the Lower Pleistocene in response to extensional tectonics due to the opening of the Tyrrhenian Sea [25; 26; 27]. These caused the lowering of the Apennine carbonate units, which are the highest morphstructural unit of the chain, either thousands of metres, in the inner areas [28; 29], or hundreds of metres, in the axial zone, below the surface [4; 30].  
Geophysical data [28; 31] has shed light on the subsurface setting of the Campana plain, which consists of three main depocenters, with the Sarno plain being the most south-easterly of these (Figure 1). The Quaternary filling of the Sarno plain consists of a ca. 1.8 km thick sequence of marine, transitional, and continental deposits, with abundant volcanics, whose age falls between the Middle Pleistocene and the Holocene [29; 32]. The Quaternary filling becomes thinner towards the inner sector of the Sarno plain, where the carbonate substratum occurs at depths exceeding 250 m. Here, traverting, lacustrine, and peat layers lie on top of the 30-40 m thick Campania Ignimbrite (CI) chronostrigraphic regional marker near the Sarno village [20; 33].  
This stratigraphic setting hosts a multi-layered alluvial aquifer, which is recharged by local effective precipitation and, subordinately, by lateral groundwater flow coming from the Sarno Mountains karst aquifer. The groundwater circulation is directed towards the Sarno River and its tributaries [38; 39; 40]. A high number of uncontrolled and unauthorized private wells (up to approximately 90-100 wells/km\({}^{2}\)) use the plain groundwater for industrial, domestic, and agricultural purposes [5]. Other groundwater withdrawals are carried out at three well-fields and two gravity drainage galleries located near the Sarno Mountains karst aquifer.  
The inner sector of the Sarno plain has been deeply investigated by means of DInSAR analysis [1; 21], which suggested intense human-induced subsidence clustered around the Sarno village. In the same area, [PERSON] et al. [20] suggested the occurrence of a graben-like structure bounded by active faults, which can be inferred by geomorphological and stratigraphic analysis around the Lavorate embayment. In contrast, evidence of recent vertical motions in the central and north-western sector of the Sarno Mountains pediment  
Figure 1: (**A**) geological map of the sector of the Southern Apennines including the Sarno plain (modified from [20; 34]). White box indicates location of the study area, shown in inset map B. Active faults are derived from the ITHACA database [35] and from [PERSON] et al. [36]. Earthquake epicentres and magnitude are derived from the Italian Earthquake Catalogue CPT15 [37]. (**B**) geological map of the inner sector of Sarno plain (modified from [20]).  
alluvial plain junction area were not discussed in the previous work by [PERSON] et al. [20] but are discussed in this paper.  
Finally, the recent activity of the fault strands bounding the Sarno plain were not associated with diffuse and moderate-to-high magnitude seismicity (Figure 1A and Supplementary Figure S1). The strongest historical seismic events affecting the Sarno plain, and its surroundings, occurred in 1499 and in 1561, both having moment magnitudes of 5.5, and whose epicentres were located ~15 km to the NW and to the SE of the Sarno village urban area, respectively [36] (Figure 1).