## 1 Introduction  
Campi Flegrei is a near-circular caldera with a diameter of about 12 km (Figure 1), which includes part of the city of Naples and several smaller towns. Since 2000, Campi Flegrei caldera has been subject to long-term unrest characterized by a resumption of seismicity [1; 2; 3; 4; 5], an uplift of its central area [6; 7; 8; 9; 10; 11; 12; 13], and an increase in hydrothermal activity [14; 15; 16; 17]. In December 2012, after a phase of acceleration of geochemical changes [18], uplift, and seismicity, the Civil Protection decrered the passage to the yellow alert level (second level on a four-level scale). Since then, further gradual increases in deformation, seismic and geochemical activity have been observed, and in some cases, they have been attributed to magma intrusion [6; 7; 8; 19; 20]. The most evident variations are concentrated in the hydrothermal area of Solfatara-Pisciarelli (Figure 1), which is characterized by a powerful fumarole and a pool of bubbling mud, due to the gas flux that passes through it. In this area most of the caldera's seismicity is concentrated, although secondary seismogenic zones are located slightly further north, and in the Gulf of Pozzuoli [2; 5]. In the hydrothermal area of Solfatara-Pisciarelli, an escalation in CO\({}_{2}\) flux has been measured for over ten years, and is still continuing [15; 16; 21]. To monitor hydrothermal activity, in addition to geochemical measurements, the seismic tremor produced by the Pisciarelli fumarole- mud pool system was recorded continuously for over ten years [22]. Currently, the Pisciarelli fumarole is the most powerful emission of the Campi Flegreicaldera and one of the most powerful in Europe. The measurement and study of Pisciarelli fumarolic tremors proved to be very useful for obtaining a proxy of the CO\({}_{2}\) flux of the fumarole-mud pool system [2; 23]. Furthermore, the amplitude of the fumarole tremor is a robust indicator of the intensity of the hydrothermal activity of the entire Solfatara-Pisciarelli area, since a good correlation between this parameter and the CO\({}_{2}\) fluxes measured at the Bocca Grande and Bocca Nuova fumaroles of the Solfatara was highlighted in [PERSON] et al. (2017) [23]. Moreover, [PERSON] et al. (2020) [24] documented a temporal evolution of the amplitude of the fumarolic tremor of Pisciarelli comparable with that of the Radon emission in the nearby Monte Olibano tunnel. A recent study of fumarolic tremors highlighted that a sudden increase in the amplitude of this signal occurred in conjunction with the enlargement of the gas and mud emission areas in Pisciarelli on 1 December 2018 [22]. During this episode, Short Duration Events (SDEs), with a maximum duration of 0.8-1 s, were recorded in the fumarolic tremor signal, attributable to an increase in the boiling of the mud pool. The episode of 1 December 2018 raised concern for the possible evolution in phreatic activity as reported in [15]. Another episode similar to that of 1 December 2018 occurred in early September 2020, when the amplitude of the fumarolic tremor reached one of the largest values recorded so far [3]. In the present work, we analyze the two episodes of anomalous increase in fumarolic tremor amplitude that occurred in 2018 and 2020 by using an artificial neural network. Neural networks are often applied to analyze seismo-volcanic data [8; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39]. Here we apply an unsupervised algorithm, the Self-Organizing Map (SOM), to cluster two main types of signal examples, namely the typical fumarolic tremor and the fumarolic tremor containing short-duration events. In particular, in this work, we developed an appropriate technique to extract the features of these two types of seismic tremors, compressing the representation of the seismic records and, at the same time, enhancing the discriminating characteristics of the two classes of signals.