def _get_distance_term(self, C, rrup, backarc):
        """
        Returns the distance scaling term, which varies depending on whether
        the site is in the forearc or the backarc
        """
        # Geometric attenuation function
        distance_scale = -np.log10(np.sqrt(rrup ** 2 + 3600.0))
        # Anelastic attenuation in the backarc
        distance_scale[backarc] += (C["c2"] * rrup[backarc])
        # Anelastic Attenuation in the forearc
        idx = np.logical_not(backarc)
        distance_scale[idx] += (C["c1"] * rrup[idx])
        return distance_scale
Returns the distance scaling term, which varies depending on whether
        the site is in the forearc or the backarc