Pre-failure response spectra predict finite-amplitude fragility

Abstract: Failure theories often identify a single leading route to failure: the most unstable mode, weakest link, minimum-action escape path, or optimal perturbation. Yet finite-amplitude susceptibility depends not only on the nearest route but on how much of perturbation space lies near dangerous directions. We cast this distinction as a fragility problem: for each perturbation direction, the failure distance is the smallest amplitude that crosses a prescribed boundary, and the fragility curve is the fraction of directions that fail below a given amplitude. Measuring this curve directly requires nonlinear trials over many directions; instead, we show that it is predicted, before any failure occurs, by the tail of a single pre-failure quantity: the boundary-normalized fragility gain computed from the linearized response. The breadth of the associated response spectrum sets how many near-dangerous pathways coexist beyond the strongest direction. We demonstrate the mechanism in a high-dimensional nonlinear non-normal network with the strongest directional gain held fixed: the system with broader response-channel breadth has a larger nonlinear fragility curve, isolating breadth from the worst direction. An independent scalar test in deterministic traffic breakdown confirms the predicted sign: response breadth lowers calibrated jam thresholds once the strongest response is matched, with residual margins screening but never reversing the effect. Response-spectrum breadth thus emerges as a pre-failure coordinate for finite-amplitude fragility beyond the strongest path.