Nisyros Volcano Hydrothermal System Response to Temporal Stress Changes Inferred from Ambient Noise Interferometry

ABSTRACT Nisyros Island, located in the eastern Hellenic arc of the Aegean Sea, is an active volcanic island. Although seismic stations have sparsely sampled the area, ambient noise interferometry measurements revealed a significant coseismic velocity drop during the 2017 Kos earthquake, 44 km away. The velocity change was located on the edifice and persisted for at least three months after the mainshock without recovering, whereas no velocity changes were recorded farther away from the volcano. Extending the analysis to the period from 2011 onward revealed similar velocity drops during other nearby earthquakes at all available stations on the volcano. The velocity-drop dependence on the correlation functions’ frequency band and coda lag time indicates that the affected area is located beneath the volcano. It appears to be associated with two permeable layers within the upper 2 km, though a deeper extension is less well constrained. Horizontal-to-vertical spectral ratios show a quick-recovering peak frequency drop after earthquakes, indicating that the surface layers are only coseismically affected. Similar to Lassen Peak, White Island, and Izu-Oshima volcanoes, the dynamic stress change induced by seismic waves suggests the presence of high-pressure hydrothermal fluids in the upper crust that reduce the effective stress and make it more susceptible to damage through increased fracturing. In addition to velocity changes following nearby earthquakes, annual seasonal fluctuations have also been observed. These fluctuations are anticorrelated with precipitation-predicted water levels, suggesting that the two hydrothermal aquifers may be unconfined. The results of this study demonstrate that Nisyros volcano should now be monitored with dense seismic networks both on land and offshore.