Rural-urban flows determine internal migration structure across scales

Historically, internal migration happened through a centralization process upwards the urban hierarchy, but in recent decades, a shift towards network-based hierarchies has occurred in developed economies. This shift has led to the development of intra-urban mobility networks that reveal how different connections move populations between cities. Yet, most studies concentrate only on major cities and their networks, often overlooking the impact of smaller-scale connections. In our analysis, we show that inter-settlement migration patterns are heterogeneous across spatial scales, but a strong hierarchical structure persists across them. Results for a multi-scale community detection algorithm show that most of internal migration happens in local, clique-like groups of smaller settlements, and these cliques form larger units through connections to towns with center roles. Surprisingly, towns with similar center roles on different scales can differ by population and administrative rank. By applying the radiation model of migration as an analytic null model, we identify network patterns that cannot be explained by the population of settlements and the distance between them. We find that the radiation model fails to predict population flows from local cliques to centers, a characteristic type of observed flow that defines the hierarchic network structure.