## 4 Conclusive Remarks  
Our study of the Casamicicola fault system at the volcanic island of Ischia with a large-depth Ground-Penetrating Radar system aimed at constraining the source characteristics of this active and capable fault system. Our survey, performed by the Loza low-frequency (15-25 MHz) GPR system calibrated by TDEM results, highlighted variations in the electromagnetic signal due to the presence of contacts, i.e., buried and outcropping faults down to  
Figure 8: Q-factor computed on a portion of the ISH2 section acquired using a 100 MHz antenna. The Q-factor outcome is overlaid on row data (transparent) and shows a sub-vertical contact dipping towards the north.  
a depth larger than 100 m below the surface. As it is known, GPR techniques are mostly used to study the shallowest portions of fault systems.  
Our study demonstrates the optimal potential of combined TDEM and deep GPR surveys for investigating active buried faults at depths in volcanic areas.  
None of the geophysical studies carried out so far at Ischia involved such a high-resolution investigation (comparable only with what can be obtained through field surveys) of its active fault system at depths of about 100 m.  
The main results of our research highlighted the following (Figure 9): (1) a good correlation of the subsoil dielectric permittivity variations detected by LDGPR with the position of (i) the 2017 earthquake primary co-seismic ruptures [3], (ii) the synthetic and antithetic fault system bordering the Casamicicola Holocene graben inferred by geological studies [10; 11] and morphological analysis from LIDAR data [43]; (2) a northward dip displacement of the synthetic fault system inferred by the Q-factor GPR attribute.  
GPR represents an effective tool in terms of spatial resolution, portability, and costs and may be efficiently used for volcanological and palaeoseismological studies by employing a variety of GPR frequencies and attributes. Within palaeoseismology, GPR has been used for different purposes: (1) preliminary investigations; (2) identification of trenching sites; (3) parallel analysis of GPR and trench data. Our study suggests that the application of  
Figure 9: Outcome of our LDGPR survey: the white lines are the surface projections of faults found by the LDGPR survey. The blue box includes the portion of the ISH2 profile studied by Q-factor analysis (Figure 8). Active faults are from [PERSON] (1998) [10] and [PERSON] and Vezzoli (1998) [11]. The co-seismic ruptures of the 2017 earthquake are from [PERSON] et al. (2018) [3]. The 21 August 2017 mainshock (yellow star) is from [[https://terremoti.ov.ingv.it/gossip/ischia/2017/index.html](https://terremoti.ov.ingv.it/gossip/ischia/2017/index.html)]([https://terremoti.ov.ingv.it/gossip/ischia/2017/index.html](https://terremoti.ov.ingv.it/gossip/ischia/2017/index.html)), accessed on 13 July 2024 [12].  
LDGPR, possibly combined with other geophysical techniques, may offer a quick fault detection method for paleoseismological trench sites for future works.  
Conceptualization, R.N., V.P., G.D.N. and C.T.; methodology, D.D. and V.P.; software, D.D.; validation, V.P., R.N. and D.D.; formal analysis, V.P. and R.N.; writing--original draft preparation, R.N. and V.P.; writing--review and editing, V.P., R.N., G.D.N. and C.T.; supervision, R.N.; project administration, G.D.N. and C.T. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the PON OT4 CLIMA Project, (PON ARS01_00405 OR3), and by INGV Project 9999.832 \"Multidisciplinary Approach for Capable Fault system study at Ischia Island, northern sector of Mt Epomeo\" (MACFI).  
Not applicable.  
Data Availability Statement: Data are contained within the article.  
The Authors are grateful to [PERSON] for his constructive comments and discussion on GPR data applied to Casamicicola area. Moreover, we thank the reviewers for their helpful suggestion.  
The authors declare no conflicts of interest.