201-7 Unraveling the Drivers and Coastal Flood Implications of Spatially Decoupled Land Subsidence at San Francisco International Airport

Abstract:

Land subsidence at coastal airports compounds sea-level rise (SLR) to increase flood exposure, yet the subsurface processes governing spatially variable subsidence at these sites remain poorly resolved. San Francisco International Airport (SFO), built on hydraulically placed fill overlying thick, compressible Young Bay Mud (YBM), provides a representative setting to examine these processes. We use Sentinel-1 InSAR observations from 2017–2024 to resolve spatially dense vertical land motion (VLM) across SFO's runway system at 15×15 m resolution, revealing rates locally exceeding −10.0 ± 0.1 mm/yr, with the most pronounced deformation along Runway 10R/28L (−7.8 ± 0.1 mm/yr near its 10R end).

A key process-level finding, resolved along a 3.2 km geotechnical transect, is that the location of peak subsidence does not coincide with the greatest YBM thickness, indicating that additional factors beyond compressible-layer geometry shape the deformation pattern. A Geographical Detector analysis quantifies these controls: along the transect, YBM thickness (47.8%), paleoenvironmental setting (35.9%), and construction year (8.2%) jointly explain 91.9% of VLM variance, with strong nonlinear interaction between thickness and depositional environment. At the runway scale, where thickness data are unavailable, construction year emerges as the dominant proxy (18.6%). A Gibson-Lo secondary consolidation model calibrated at eleven cone penetration test sites, reproduces observed rates closely (R² = 0.98, RMSE = 0.26 mm/yr), showing that calibrated creep-rate parameters decrease systematically with fill-induced effective stress (r = −0.97), linking present-day deformation to construction-era loading history.

Comparison with 2007–2010 InSAR rates reveals a broad deceleration in subsidence, with 82% of collocated pixels slowing (median rate ratio = 0.62), consistent with ongoing secondary consolidation approaching its asymptotic limit at most sites. Despite this deceleration, projected cumulative subsidence could lower airfield surface elevations by up to 0.60 m by 2100, exceeding the projected regional SLR of 0.19 m (SSP2-4.5), substantially amplifying effective relative sea-level rise at the airfield. These results demonstrate that mechanistic consolidation modeling combined with dense InSAR monitoring is essential for disentangling the drivers of heterogeneous coastal subsidence and anticipating its long-term compounding effect on coastal flood risk at reclaimed infrastructure worldwide.

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