def castagna_vp_to_vs(vp):
    """Calculate vp from vs using castagna's relation.

    Castagna's relation (the mudrock line) is an empirical relation for vp/vs
    for siliciclastic rocks (i.e. sandstones and shales). [Castagna et al.,
    1985]

        vp = 1.16 * vs + 1360 [m/s]

    :param vp: P-wave velocity [m/s]
    :returns: S-wave velocity [m/s]
    """
    return (vp - 1360.0) / 1.16
