Anthropogenic-natural driven coastline expansion in China's mainland: Spatiotemporal dynamics, morphological efficiency and coastal vulnerability assessment.

The coastal zone serves as a critical interface of intensive anthropogenic activity. Utilizing Landsat imagery from 1987 to 2025, we characterized the spatiotemporal evolution of China's mainland coastline and assessed coastal vulnerability. Over 38 years, the coastline expanded by 3997.85 km and the land area increased by 9654.03 km2. Regionally, Tianjin recorded the highest seaward advancement (ASAD: 2910.77 m/km) and Zhejiang the highest morphological efficiency (MEI: 8.16 km2/km), reflecting contrasting engineering modes of extensive mudflat enclosure versus intensive bay-enclosing seawall construction. Anthropogenic activities accounted for 77.02% of expansion, with aquaculture (47.29%) and impervious surfaces (21.20%) as primary contributors. Municipal-scale attribution revealed a 'rate-type decoupling' - comparable expansion rates driven by different land-cover transformations within the same province. An Interrupted Time Series Analysis decomposed the post-2015 deceleration into a pre-existing trend and a policy-induced level drop of -3.92 m/yr following the 2018 reclamation moratorium. The Coastal Vulnerability Index (CVI) indicates moderate vulnerability nationally, with coastal slope as the primary driver. Bivariate spatial coupling analysis revealed significant spatial decoupling (Global Moran's I = -0.177, p = 0.001): only 0.4% of points were co-amplification hotspots, while 17.3% were 'sleeping risk' zones with high natural vulnerability but low anthropogenic pressure. The Coupling Coordination Degree (CCD) (0.588) places the national system in a transitional state. These findings demonstrate that reclamation control effectively suppresses anthropogenic risk but cannot eliminate climate-driven vulnerability, underscoring the need for differentiated adaptive governance.