Urban Vitality Within the Daily Activity System: Multidimensional Spatial Patterns and Nonlinear Relationships with the Built Environment in Nanjing, China

Urban vitality is central to sustainable urban development, yet conventional research is rarely organized around the complete range of everyday activities: it seldom distinguishes activity types, so a place may be rated vital on one activity alone, and the behavioral reasons behind reported nonlinearities remain little discussed. Oriented to everyday activities, this study differentiates residential, work, and visiting vitality, integrates them through CRITIC weighting into a comprehensive vitality, and—alongside that overall level—examines subsystem coupling and dominant combinations to form three layers of spatial patterns; it then uses an expanded “5D” built-environment system and XGBoost–SHAP with spatial cross-validation to examine the nonlinear relationships between comprehensive vitality and the built environment. In Nanjing, a large Chinese metropolis, comprehensive vitality shows a sharp core–periphery polarization with a single core and multiple nodes, declining steeply from the innermost rings to the fringe. The three activities are strongly coupled across the city, and low coupling-coordination values mainly reflect low overall activity intensity rather than structural mismatch. Work–visiting combinations dominate, followed by residential–visiting combinations, whereas grids dominated by a single activity type are few. Employment, shopping, and leisure are the leading correlates, with threshold, interaction, and spatially varying associations; the model attribution also varies geographically, clustering in the core. These findings support differentiated, people-centered strategies and 15-min living-circle planning in place of uniform redevelopment.