@classmethod
def calc_strain_ratio(cls, mag):
    """Compute the effective strain ratio using Idriss and Sun (1992).

        Parameters
        ----------
        mag: float
            Magnitude of the input motion.

        Returns
        -------
        strain_ratio : float
            Effective strain ratio

        References
        ----------
        .. [1] Idriss, I. M., & Sun, J. I. (1992). SHAKE91: A computer program
            for conducting equivalent linear seismic response analyses of
            horizontally layered soil deposits. Center for Geotechnical
            Modeling, Department of Civil and Environmental Engineering,
            University of California, Davis, CA.
        """
    return (mag - 1) / 10
