The main volcanic structural elements of Nisyros and the tectonic lines as plotted through field work and photogeologic observations are laid out in the relevant drawing (Vougiouklakis, 1993).

The prevailing volcano-tectonic element is the existence of the caldera depression.

The existence of two main fault systems in the greater area, NE-SW and NW-SE is apparent. The above faults, having inclinations between 700-800 in Nisyros, develop several high and low tectonics and cause displacements, which are especially apparent on the caldera rim, in the order of 120 to 150m (NE-SW direction: Kokkinos Mylos site and NW-SE direction: Agios Vasileios site).

In addition to the two main systems mentioned above, a fault system whose direction is approximately E-W (Β600 to Β1100) is also present; this direction presents remarkable volcanic characteristics. All feeding dykes are aligned in this direction and the same applies to the lava necks, historic phreatic eruptions as well as the majority of fumaroles. Furthermore, post-caldera dome feeding dykes seem to follow the same direction. A highly active fault, in the N-S direction, forms the ‘Langadi’ valley at Mandraki, the main conurbation of Nisyros. This fault was active in 1871-73 and 1995-97 during the seismic crisis of the area; it further extends to the undersea region (Vougioukalakis et al, 1998). Consideration of morphotectonic and volcanic aspects (current formation occurrence altitude, tectonic displacement, lava currents directions, pyroclastic depositing directions and phases) leads to the conclusion that there never was a main volcanic cone in Nisyros with a single vent, but rather multiple outlet centers and a rather complex elevation tending to resemble the shape of a cone.

The fault systems observed in Nisyros are also present on the surrounding islets. Severe tectonism is recorded on Yali; the newer marine terrace (< 31 Ka) is located at a height of up to 120 meters above the contemporary sea level. During the seismic crisis of the area, in 1970 and 1995-97, the fault starting from ‘Italiki Skala’ in the W-NW / E-SE direction became active (Stiros and Vougioukalakis, 1996).