Exploring thematic activity changes in black holes and volcanoes of urban population mobility under daily rhythms

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ABSTRACT Urban functional structures and daily rhythms significantly impact population mobility. Detecting and quantifying thematic activity changes in the short-term aggregated inflow and outflow of urban population mobility (referred to as black holes and volcanoes, respectively) contribute to the economy and public services. Current research is focused on the changes in intensity of single-region population activity, but it overlooks the daily rhythms of the aggregated flows and latent thematic activity changes in urban populations. Here, we propose an Aggregated Inflow-Outflow Thematic Detection (AIOTD) method. It can detect thematic activity changes in population inflows and outflows from a spatiotemporal aggregation perspective by leveraging traffic flow theory and semantic models. Considering the stationary of flow sequences within the same time periods and the spatial continuity of flows, we designed a spatial aggregation method based on the relative ratios of population flows in spatiotemporal units. This method enables a quantitative depiction of the spatiotemporal evolution of black holes and volcanoes. Furthermore, due to the spatial proximity of land features and category imbalances, we utilized the SMOTETomek-Place2vec model to construct a spatial context information dataset at the grid level, enhancing the accuracy of capturing thematic activities. Results demonstrate that our method outperforms existing approaches in capturing the number of spatiotemporal units for black hole and volcano clusters at both the 500 m grid and 1000 m grid scales, in terms of both semantics and spatiotemporal dimensions. It reveals the spatiotemporal complementarity and thematic cross-symmetry of urban population mobility between black holes and volcanoes. By applying this method to daytime and nighttime economies and public services, we quantified changes in the fine-grained functional service vitality and the distribution of functional facilities in commuting zones. These findings offer guidance for urban services and insights into the behavioral preferences of residents.