Forecasting Cascading and Compounding Flood Risk From Earthquake‐Induced Ground Deformation

ABSTRACT Large earthquakes can disrupt river systems and produce abrupt, unforeseen changes in flood risk. Forward modelling of earthquake‐related surface deformation and its influence on inundation extent, depth, and frequency enables pre‐emptive assessment of exposure to this compound hazard. We present a novel paired fault‐dislocation and hydrodynamic modelling framework that adapts the concept of Increased Flooding Vulnerability (IFV) to assess increases in flood depth associated with earthquake‐induced ground deformation. We apply this framework to simulate rupture scenarios for the real‐world case study of the Titri fault in New Zealand to evaluate changes in flood risk within the Taieri Basin, currently managed by the Lower Taieri Flood Protection Scheme (LTFPS). Results reveal zones of heightened exposure to coseismic and post‐seismic flooding and demonstrate how deformation could undermine flood defenses. At 2.6 m of slip and a 45° dip on the modelled Titri fault, Dunedin Airport is flooded to depths of 0.3–1.5 m during a 2%–5% annual exceedance probability flood event. Fault uplift also obstructs the basin's drainage outlet, while footwall subsidence reduces infiltration and may facilitate coseismic lake formation. Early identification of areas facing increased flooding vulnerability provides a basis for long‐term risk assessment and land‐use planning in regions exposed to both seismic and flood hazards.