## 2 The Study Area  
Methana peninsula is located in the western Saronic Gulf south of Athens city (Figure 1) and is one of the three volcanic centres (i.e., Aegina, Poros and Methana) representing the north-western part of the active volcanic arc of the South Aegean. This Hellenic active volcanic arc also includes the islands Milos, Santorini and Nisyros in its extension [19]. Methana and the other volcanic centres in the Saronic Gulf are mostly monogenetic, and no composite volcanic structures are present [20].  
According to [24], the Saronic Gulf is the longest active area of the South Aegean with geothermal springs which are influenced by the active tectonic structure of the area as well as the existence of the magmatic chamber. Volcanic activity in Methana lasted until 1700, when a submarine eruption occurred north of Kameni Chora. Present day, active geothermal springs are reported mainly in the eastern and northern parts of the peninsula [3]. Within Methana town, in the south-east, the thermal springs are exploited at the open-air baths of the Thermal Spa and its neoclassical dwelling built in 1930.  
It is also crucial to mention the presence of the Pausanias submarine volcano, located 2 km off the north-western shore of the peninsula [25]. According to [26], this submarine volcano was discovered using seismic reflection data and volcanic activity occurred during the 3 rd century BC.  
The Saronic Gulf constitutes a neootectonic basin which was created 4-5 million years ago [27; 28]. NE-SW normal faults dominate the broader region, while N-S, E-W and NNE-SSW striking normal faults are also recognizable in the area [25] and are characterized by low seismic activity, chiefly with shallow events [29].  
Figure 1: (**a**) Simplified geological map of the Methana peninsula, Greece (modified after [21; 22; 23]), with indication of Global Navigation Satellite System (GNSS) stations and local benchmarks and (**b**) topography from a 5.5-m resolution digital surface model by the Hellenic Mapping and Cadastral Organisation.  
The geology of Methana consists of various volcanic formations with Late Pliocene-Pleistocene small andesitic to facitic flows and domes [30; 31], which overlay the older volcanic rocks, as well as the Mesozoic sedimentary basement [21; 32].  
A simplified geological map (Figure 1a) shows that the grey limestones are present in the northwest (Upper Triassic-Lower Jurassic), while limestones with dolomites are present in the south (Upper Jurassic-Cretaceous). These limestones constitute the basement of Methana and are located under the volcanoclastic formations (rhyolite, dacite and andesite) [33]. Additionally, the Methana peninsula has many tectonic faults, with a major fault crossing the town of Methana from west to east.  
The tectonic regime of the region combined with the volcanic activity determined a rough topography of the peninsula, which shows both flat basin areas with Quaternary fill and zones dominated by high slope gradients and narrow valleys, often modelled by erosion processes (such as in the south, between Vathi and Methana) [18]. The maximum elevation is 740 m above sea level (a.s.l.) and occurs at Helona Mountain (Figure 1b).