Effect of pore geometry on VP/VS: From equilibrium geometry to crack

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Abstract
Seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes, including oblate spheroids, tubes, cracks, and an equilibrium geometry controlled by a dihedral angle. The relative role of liquid compressibility and pore geometry on the “VP/VS” velocity ratio is clarified. The result clearly indicates that P and S velocity structures determined by seismic tomography can be used to verify whether interfacial energy‐controlled melt or fluid geometry (equilibrium geometry) is achieved. Relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio. As a function of the aspect ratio, a significant effect of pore geometry on “d ln VS/d ln VP”, the ratio of the fractional changes in VS and VP, is shown. Equilibrium geometry of the partially molten rocks, characterized by a dihedral angle of 20°–40°, corresponds to an aspect ratio of 0.1–0.15. The value of d ln VS/d ln VP expected for the texturally equilibrated partially molten rocks is shown to be 1–1.5, which is much smaller than that expected for cracks and dikes with an aspect ratio of −2–10−3. In the upper mantle low‐velocity regions the seismologically obtained value of d ln VS/d ln VP is within this range beneath the Bolivian Andes (1.1–1.4) but is as high as 2 beneath Iceland (1.7–2.3) and beneath northeastern Japan (2.0). The former region can be regarded as a region where equilibrium geometry is achieved, and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate.