def _compute_f1_factor(self, rrup):
        """
        Compute and return factor f1 - see equation (5), 4th term, p. 2191
        """
        # f1 = min(log10(rrup),log10(R1))
        f1 = np.log10(rrup)
        logR1 = np.log10(self.COEFFS_IMT_INDEPENDENT['R1'])
        f1[f1 > logR1] = logR1

        return f1
Compute and return factor f1 - see equation (5), 4th term, p. 2191