## 1 Introduction
(2018) [15], among others, further developed and confirmed this understanding of the geological structure of the area. After the 2017 mainshock in Casamicicola, various models of the seismogenic sources of the earthquake have been proposed based on different data sets, leading to an ongoing scientific debate. Seismological studies by [PERSON] et al. (2018) [16], GPS data from [PERSON] et al. (2018) [17] and [PERSON] et al. (2018) [18], InSAR measurements by [PERSON] et al. (2018) [19], [PERSON] et al. (2018) [20], [PERSON] et al. (2019) [21], and [PERSON] et al. (2022) [22], and geological-macroseismic data sets analyzed by [PERSON] et al. (2018, 2020, 2021) [3; 4; 5] have contributed to these interpretations. These diverse analyses, utilizing different data sets, sometimes conflicting, have fueled the ongoing scientific discussion regarding the nature of the structure responsible for the recurrent earthquakes in Casamicicola. Considering its seismological characteristics, we infer that that the source of the 2017 Casamicicola earthquake is the same as that of all the strong historical earthquakes at the island. As regards previous applied geophysics studies of the island, they involve gravity data [23], aeromagnetic data [24] and references therein, and their combined analysis [25; 26], performed on the whole island. The central-western sector of the island was investigated by a magnetotelluric (MT) survey carried out along two main profiles that provided an electrical resistivity map to a depth of 3 km [27]. A characterization of the shallow structure of the southwestern Ischia's hydrothermal system was carried out by [PERSON] et al. (2011) [28] with a multidisciplinary survey including time-domain electromagnetic (TDEM) and resistivity tomography studies. A seismic tomography study was carried out all over the island by [PERSON] et al. (2015) [23]; more recently, [PERSON] et al. (2023) [29] investigated the elastic and electric characteristics of the shallower underground portion of the northern sector of Ischia Island using the Horizontal-to-Vertical Spectral Ratio (HVSR), seismic array techniques (f-k), polarization analysis, and TDEM data. None of the studies carried out so far at Ischia involved Ground-Penetrating Radar (GPR) data to characterize its active structures. GPR is a geophysical electromagnetic (EM) technique that uses microwaves in the frequency range of 10 MHz to 3 GHz to investigate shallow buried structures. The choice of operating frequency is a compromise between the need to investigate deeper regions using lower frequencies and the need for higher resolution using higher frequencies. The dielectric constant improves the achievable resolution, but electrical conductivity leads to soil loss and limits the depth of investigation. GPR is commonly used in engineering studies to identify shallow subsurface structures and sources. However, its usefulness in geological studies is compromised by the limited exploration depth that can be achieved. Previous studies have involved only shallow parts of fault systems through 2D and 3D surveys, e.g., [30] and references therein. Furthermore, GPR has also been successfully used to identify normal faults in volcanic environments, e.g., [31]. Here, we exploit the effectiveness of a low-frequency, large-depth GPR system (LDGPR), the Loza 2N system (Figure 2). The Loza 2N GPR operates in the tens of MHz frequency band, capable of investigating large depths and therefore effective for the study of buried faults. We constrained our LDGPR interpretations thanks to four TDEM soundings. The TDEM method involves sending a transient electromagnetic pulse into the ground and measuring the resulting electromagnetic response connected to the variations in soil electric conductivity. The method induces electric current flow within the subsurface and, after the transmitted signal is shut off, measures a voltage signal that is returned from the underground materials.