A P-wave parameter for a medium in which the elastic properties exhibit vertical transverse isotropy. Epsilon (ε) is the P-wave anisotropy parameter and equal to half the ratio of the difference between the horizontal and vertical P-wave velocities squared divided by the vertical P-wave velocity squared.

ε ≡ ½ [(C_{11} − C_{33}) / C_{33}] = ½ [(V_{P⊥}^{2} − V_{P∥}^{2}) / V_{P∥}^{2}]

P-wave parameter (ε) for a medium in which the elastic properties exhibit vertical transverse isotropy, where C_{11} is the horizontal P-wave modulus (perpendicular to the symmetry axis), C_{33} is the vertical P-wave modulus (parallel to the symmetry axis), V_{P⊥} is the horizontal P-wave velocity and V_{P∥} is the vertical P-wave velocity.