Building-Level Multi-Scenario Seismic Damage Assessment of 1.49 Million Buildings in Ningxia: Building-Specific PGA Scaling and Spatial Association Analysis

To characterize regional building-stock damage arising from the combined effects of spatially varying ground motions, building vulnerability, and exposure, this study develops a building-level, multi-scenario, and multi-record seismic damage assessment framework for 1,491,510 buildings across Ningxia, China. Frequent, design-basis, and rare earthquake scenarios were defined in accordance with China’s multi-level seismic design framework. Three sets of recorded ground motions, each comprising two orthogonal horizontal components, were individually scaled to the target peak ground acceleration (PGA) of each building and analyzed through nonlinear time-history analysis using the physics-based models implemented in YouSimulator. A total of 13,423,590 building–scenario–record results were obtained. Damage was characterized using building-count-averaged and floor-area-weighted damage indices (DIs), five damage grades, record-to-record sensitivity, and spatial autocorrelation at two grid resolutions. The building-count-averaged regional DIs were 0.063, 0.222, and 0.576 under the frequent, design-basis, and rare earthquake scenarios, respectively. The dominant damage state shifted from Grade 1 (essentially intact) to Grade 2 (slight damage) and then to Grade 4 (severe damage), indicating nonlinear transitions across damage thresholds. Under the rare earthquake scenario, 74.23% of the buildings reached Grades 4–5, and the three-record mean number of Grade 5 buildings was approximately 92,082. The floor-area-weighted DIs under the frequent and design-basis earthquakes were 1.68 and 1.31 times their building-count-averaged counterparts, respectively, indicating that floor-area weighting gives greater emphasis to buildings that contribute more exposed area. Approximately 844,627 rural detached houses reached Grades 4–5 and constituted the majority of buildings with severe damage. At the 0.025° grid scale, Moran’s I values were 0.429, 0.435, and 0.586 for the three scenarios, respectively (all p = 0.002), confirming significant spatial clustering. Although the regional mean DI under the rare earthquake scenario varied only from 0.563 to 0.598 among the three records, the proportion of buildings in Grades 4–5 ranged from 60.4% to 83.6%. Moreover, the Jaccard index between the top 10% of grids with high DI and those with high record sensitivity was only 0.039. These findings demonstrate that regional mean damage alone cannot adequately represent damage-grade composition, exposure, and record-to-record sensitivity within the selected record set. Damage severity, exposure scale, and record sensitivity should therefore be treated as complementary screening dimensions for regional retrofit prioritization, emergency resource allocation, and targeted verification. These screening dimensions and all statistics reported below are conditional on the present set of three-record pairs and on the specified scenario intensities.