  The increasing integration of world economies, which organize in complex
multilayer networks of interactions, is one of the critical factors for the
global propagation of economic crises. We adopt the network science approach to
quantify shock propagation on the global trade-investment multiplex network. To
this aim, we propose a model that couples a Susceptible-Infected-Recovered
epidemic spreading dynamics, describing how economic distress propagates
between connected countries, with an internal contagion mechanism, describing
the spreading of such economic distress within a given country. At the local
level, we find that the interplay between trade and financial interactions
influences the vulnerabilities of countries to shocks. At the large scale, we
find a simple linear relation between the relative magnitude of a shock in a
country and its global impact on the whole economic system, albeit the strength
of internal contagion is country-dependent and the intercountry propagation
dynamics is non-linear. Interestingly, this systemic impact can be predicted on
the basis of intra-layer and inter-layer scale factors that we name network
multipliers, that are independent of the magnitude of the initial shock. Our
model sets-up a quantitative framework to stress-test the robustness of
individual countries and of the world economy to propagating crashes.
