1. n. [Geophysics]

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 [(C₁₁ − C₃₃) / C₃₃] = 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 C₁₁ is the horizontal P-wave modulus (perpendicular to the symmetry axis), C₃₃ 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.

Reference: Thomsen L: “Weak Elastic Anisotropy,” Geophysics 51, no. 10 (October 1986): 1954–1966.

See: delta,  gamma (γ),  eta (η)