Relating seismic velocities, thermal cracking and permeability in Mt. Etna and Iceland basalts

At low pressure (0-10 MPa), the P-wave velocity of the columnar Seljadur basalt was 5.4 km/s, while for the Etnean lava flow basalt it was only 3.0-3.5 km/s. On increasing the pressure to 80 MPa, the velocity of Etnean basalt increased by 45%-60%, whereas that of Seljadur basalt increased by less than 2%. Furthermore, the velocity of Seljadur basalt thermally stressed to 900 °C fell by about 2.0 km/s, whereas the decrease for Etnean basalt was negligible. A similar pattern was observed in the permeability data. Permeability of Etnean basalt fell from about 7.5×10−16 m2 to about 1.5×10−16 m2 over the pressure range 5-80 MPa, while that for Seljadur basalt varied little from its initial low value of 9×10−21 m2. Again, thermal stressing significantly increased the permeability of Seljadur basalt, whilst having a negligible effect on the Etnean basalt. These results clearly indicate that the Etnean basalt contains a much higher level of crack damage than the Seljadur basalt, and hence can explain the low velocities (3-4 km/s) generally inferred from seismic tomography for the Mt. Etna volcanic edifice.