def _get_SRF_sigma(self, imt_per):
        """
        Table 8 and equation 19 of 2013 report. NB change in notation,
        2013 report calls this term 'sigma_t' but it is referred to
        here as sigma. Note that Table 8 is identical to Table 7 in
        the 2013 report.
        """
        if imt_per < 0.6:
            srf = 0.8
        elif 0.6 <= imt_per < 1:
            srf = self._interp_function(0.7, 0.8, 1, 0.6, imt_per)
        elif 1 <= imt_per <= 10:
            srf = self._interp_function(0.6, 0.7, 10, 1, imt_per)
        else:
            srf = 1

        return srf
Table 8 and equation 19 of 2013 report. NB change in notation,
        2013 report calls this term 'sigma_t' but it is referred to
        here as sigma. Note that Table 8 is identical to Table 7 in
        the 2013 report.