Seismic velocities, Poisson's ratios and potential auxetic behavior of volcanic rocks

Abstract

The present compilation of P- and S-wave velocities (Vp and Vs) for 395 dry and 664 wet basalt, 231 dry and 63 wet rhyolite samples provide a new database for evaluating the porosity (ϕ) dependence of Vp, Vs and Poisson's ratio (ν) and the potential auxetic behavior (i.e., negative Poisson's ratio) of volcanic rocks. The ν-ϕ correlation is always positive for the water-saturated rocks but can be either positive (rhyolite) or negative (basalt) for dry rocks, depending on the Poisson's ratio of the non-porous solid matrix (ν0). Auxetic behavior cannot occur in wet volcanic rocks but may appear in dry basalts with extremely high porosity (≥70% for basalt), such that a re-entrant foam structure forms. The auxeticity of highly porous volcanic rocks such as pumice and reticulite will be suppressed by infiltration of water into the rocks. Hence, volcanic rocks cannot be auxetic in nature if their vesicles are filled with water. Under the laboratory conditions, however, a transition from conventional to auxetic behaviors in pumice and reticulite is expected to occur by changing the degree of water saturation. These results provide a set of basic parameters for the modeling of ground deformation and temporal and spatial variations in Vp, Vs, Vp/Vs ratio as precursors of volcanic eruption or volcanic edifice collapse.