弾性波
∂ 2 u 1 ∂ t 2 = c P 2 ∂ 2 u 1 ∂ x 1 2 , ∂ 2 u 2 ∂ t 2 = c S 2 ∂ 2 u 2 ∂ x 1 2 , ∂ 2 u 3 ∂ t 2 = c S 2 ∂ 2 u 3 ∂ x 1 2 {\displaystyle {\frac {\partial ^{2}u_{1}}{\partial t^{2}}}=c_{P}^{2}{\frac {\partial ^{2}u_{1}}{\partial x_{1}^{2}}},\qquad {\frac {\partial ^{2}u_{2}}{\partial t^{2}}}=c_{S}^{2}{\frac {\partial ^{2}u_{2}}{\partial x_{1}^{2}}},\qquad {\frac {\partial ^{2}u_{3}}{\partial t^{2}}}=c_{S}^{2}{\frac {\partial ^{2}u_{3}}{\partial x_{1}^{2}}}}