Ratio of P-wave ( α {\displaystyle \alpha } ) to S-wave ( β {\displaystyle \beta } ) velocities. Poisson's ratio
![{\displaystyle \sigma ={\left[\left({\frac {\displaystyle \alpha }{\displaystyle \beta }}\right)^{2}-2\right] \over \left[2\left({\frac {\displaystyle \alpha }{\displaystyle \beta }}\right)^{2}-2\right]}\quad }](https://en.wikipedia.org/api/rest_v1/media/math/render/svg/72d5c5d9f4814b8d54e34fee6534a4c7f9f71548)
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