def _get_stddevs(self, stddev_types, rrup):
        """
        Return standard deviations as defined in equation 3.5.5-2 page 151
        """
        assert all(stddev_type in self.DEFINED_FOR_STANDARD_DEVIATION_TYPES
                   for stddev_type in stddev_types)
        std = np.zeros_like(rrup)
        std[rrup <= 20] = 0.23
        idx = (rrup > 20) & (rrup <= 30)
        std[idx] = 0.23 - 0.03 * np.log10(rrup[idx] / 20) / np.log10(30. / 20.)
        std[rrup > 30] = 0.20
        # convert from log10 to ln
        std = np.log(10 ** std)
        return [std for stddev_type in stddev_types]
Return standard deviations as defined in equation 3.5.5-2 page 151