## 1 Introduction  
In this paper, we focus on the identification at depth of the synthetic fault system of the Casamicicola Holocene graben, which was responsible for the 2017 earthquake, by using a large-depth GPR system (hereinafter LDGPR). This was already employed successfully to study buried faults at depth in different geological frameworks [1]. Our LDGPR system was designed for high-conductivity soils at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN) for a planned space mission to Mars [2]. A characteristic feature of our LDGPR is the use of low frequencies (tens of MHz) combined with high energy potential, which allows for probing highly conductive soils such as wet clay that are inaccessible for standard GPR. The seismotectonic setting of the Ischia Island in the Bay of Naples, Southern Italy is characterized by an active resurgent caldera that has experienced several historical catastrophic earthquakes, making Ischia an area of high seismic risk. The island is densely inhabited and a major tourist destination, further increasing the importance of understanding the seismotectonics of the Ischia earthquake source (Figure 1). On 21 August 2017, a main shock occurred at 20:57 CEST (18:57 UTC), rated Md 4.0, reactivating the Casamicicola Terme capable fault system. Despite the moderate magnitude, several buildings and a church collapsed, causing two deaths and 42 injuries and leaving 2600 people homeless. This earthquake is important because it is the first well-documented, significant instrumental earthquake at Ischia, occurring 134 years after the catastrophic 28 July 1883 I\({}^{\circ}\) XI MCS event, which killed more than 2300 people. For the first time, detailed mapping of the primary co-seismic ruptures of the 2017 earthquake was carried out, showing extensional surface faulting of the synthetic fault system of the Casamicicola Holocene graben [3; 4; 5] (Figure 1). Other very strong damaging earthquakes (occurring in 1796, 1828, 1881, and 1883) hit Ischia Island during the last three centuries, with similar seismotectonic characteristics [6; 7; 8; 9]. The epicentral area of the 2017 earthquake is confined within just a few square kilometers in the Casamicicola Terme area at the base of the N flank of Mt. Epomeo (787 m a.s.l.). Here, a clear stratigraphic and morphological trace highlighting the effects of the earthquakes is represented by a graben structure, which originated as the result of Holocene extensional tectonic deformation [10; 11]. Mt. Epomeo is the main morphological structure created by the Ischia caldera resurgence, with an average uplift rate of ca. 3 cm/yr in the last 30 kyr [11]. Figure 1: Location of our geophysical survey, Ischia Island (Italy): ISH1, ISH2, ISH3, ISH4 are the GPR profiles (black lines); TDEM1-7 are the time-domain electromagnetic measurements (black squares) across the synthetic and antithetic active faults system of Holocene Casamicicola graben (red lines, marks on down-thrown side). Geology and active faults are from [PERSON] (1988) [10] and [PERSON] and [PERSON] (1998) [11]. The co-seismic ruptures of the 2017 earthquake (yellow lines) are from [PERSON] et al. (2018) [3]. The 21 August 2017 mainshock (the yellow large 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 1 July 2024) [12]; historical earthquakes (red circles) are from [PERSON] et al. (2021) [13]. The interpretation of the Casamicicola structure as a Holocene graben due to the uplift of Mt. Epomeo was initially proposed by [PERSON] in 1930 [14]. This interpretation was later substantiated and extensively documented by field geological data provided by [PERSON] (1988) [10], which included the first modern 1:10.000 scale geological map of Ischia Island. Subsequent studies by [PERSON] and [PERSON] (1998) [11] and [PERSON] et al.