Gravity changes and ground deformation on the island of Nisyros Volcano (Greece) for the period 2001-2006

Abstract

A long-term gravity and elevation changes time series (2001-2006) at the restless caldera of Nisyros in Greece is here reported. After a period of intense seismic activity (1995-1998), associated with an increase of fumarolic activity and ground deformation of Nisyros volcano which no any eruption occurred, and then a gravity network was settled in June 2001 and re-occupied annually up to 2004 and in June 2006. Discrete gravity measurements have been carried out at located benchmarks surrounding Nisyros island in proximity to hydrotermal area within the caldera floor and taking into account the effect of the vertical ground movements in order to quantify gravity changes of the volcano. A general positive observed gravity change was determined at all stations of the network inside caldera rim. In the same time, a temporal and spatial evolution of the gravity changes was found between 2001 and 2004: it was observed a steady-rise in gravity variations between 2001-2003, obtaining maximum positive gravity variations up to +60 Gal confined in the caldera floor area at the base of the presently active crater, where the most fumarolic activity is concentrated; in contrast a spatial distribution of gravity changes is recorded during 2003-2004 and the maximum positive gravity variations are confined in the caldera rim near Nikia village. In this way residual gravity changes were obtained and applications of Gausss theorem to the study of the excess mass recorded during 2001-2006 is explored. During recent unrest period magma replenishment genereted a great quantity of steam hot hydrothermal fluids which is released towards the surface underneath the southeastern part of the caldera floor. The gravity changes might be attributed to the migration of fluids probably occurring through pre-existing faults.