Seismic hazard and shifting channels: Exploring coseismic river response

Abstract

Large earthquakes can trigger cascading flood hazards that can influence societal risk and loss; however, the mechanisms driving coseismic river response (CRR) in seismogenic regions have not been fully characterized. This review synthesizes data from fifty-two global cases of CRR where surface deformation affected rivers and identifies the key physical and environmental parameters that control riverine flood hazard in earthquakes. We identify four primary CRR classes, ranging from channel-confined ponding to overbank flooding and avulsion, wherein a river shifts into an enduring new course within the floodplain. CRR susceptibility governs the likelihood, spatial extent, and longevity of CRR, and links back to the characteristics of the surface deformation, river planform, and host environmental setting. Dip-slip fault ruptures, with a large vertical separation at the surface capable of damming river channels, are the primary driver of CRR. Multi-meter dip-slip offsets are also linked to fault-block tilting, which reduce or reverse the gradients of overlying channels. These tilted fault blocks can induce coseismic flooding or avulsion away from the principal fault, with cases documented up c. 10 km upstream from the fault-river intersection. Lateral offsets can amplify or modulate CRR but are generally less effective than vertical offsets at causing them. Two-thirds of CRR cases occurred in single-thread rivers, the majority of which are meandering systems occupying intermontane or lowland basins. Liquefaction-prone substrates in these floodplains exacerbate CRR impacts through ground deformation that disrupts river channel profiles and the local water table. Overbank flooding, lake formation, and prolonged surface flooding are common outcomes near meandering channels and may occur away from the principal fault. Confined braided rivers with high width-to-depth ratios are more resistant to overbank flow and avulsion. In unconfined settings, erodible substrates, and low lateral bank stability amplify the potential for channel scour and rapid avulsion. Braided floodplains often host dense networks of paleochannels within their floodplain, and in any environment, paleochannels enhance the potential for, and geographic extent of flooding and avulsion. High discharge conditions elevate the potential for overbank flow and avulsion, and the hazard posed by CRR may be higher in regions where the climate is perennially or seasonally wet. The risk posed by CRR during earthquakes is heightened in areas where population expansion and land reclamation in flood and liquefaction-prone zones is commonplace. A method to assess CRR risk in advance is required, and a probabilistic framework incorporating uncertainties in fault behaviour, recurrence intervals, and flood-rating curves may offer a way to evaluate absolute hazard in areas where faults and rivers intersect. Site-specific hydrodynamic modelling can further quantify potential changes in flood patterns in high-hazard, high-exposure areas, paving the way for assessing societal and economic risks. We suggest the need is urgent in places such as India and Bangladesh's Ganges-Brahmaputra-Meghna delta. This densely populated region, home to ∼150 million people, is underlain by a locked plate boundary and active faults. Historic earthquakes in the region suggest that future large seismic events could trigger widespread changes in flood hazard, exacerbating the existing risk posed by flooding.