For kMAD (asymmetrial MAD) is a root of the equation:
$$\mathop{\rm kMAD}(F,k) = \inf\{t>0\;\mid \;F(m+kt)-F(m-t)\ge 1/2 \}$$,
where F is the cumulative distribution function, m is the median of F.
References
Ruckdeschel, P., Horbenko, N. (2010): Robustness Properties for Generalized Pareto Distributions. ITWM Report 182.