def _compute_focal_depth_term(self, C, hypo_depth):
        """
        Compute fourth term in equation 1, p. 901.
        """
        # p. 901. "(i.e, depth is capped at 125 km)".
        focal_depth = hypo_depth
        if focal_depth > 125.0:
            focal_depth = 125.0

        # p. 902. "We used the value of 15 km for the
        # depth coefficient hc ...".
        hc = 15.0

        # p. 901. "When h is larger than hc, the depth terms takes
        # effect ...". The next sentence specifies h>=hc.
        return float(focal_depth >= hc) * C['e'] * (focal_depth - hc)
Compute fourth term in equation 1, p. 901.