The death toll and economic impact of an earthquake can be greatly exacerbated if seismic ground shaking triggers landslides. Earthquake-triggered landslides typically occur in two different contexts: localized failure of steep slopes and resulting landslides that pose a major threat to life in areas below; and lateral spreading of nearly flat sediment plains due to shaking-induced liquefaction, which can damage large areas of critical infrastructure. Unexpected catastrophic landsliding triggered by the 28 September 2018 earthquake at Palu, Indonesia did not occur in either typical context, but produced both destructive outcomes. Here, we show that alluvial ground failure in the Palu Valley was a direct consequence of irrigation creating a new liquefaction hazard. Aqueduct-supported cultivation, primarily of wet rice, raised the water table to near ground level, saturating sandy alluvial soils that liquefied in response to strong ground shaking. Large-displacement lateral spreads occurred on slopes of 1°. Slopes steeper than 1.5° sourced long-runout landslides and debris flows that swept through villages occupying the gentler slopes below. The resulting damage and loss of life would probably not have occurred in the absence of a raised water table. Earthquake-triggered landsliding of gentle, irrigated alluvial slopes is an under-recognized, but avoidable, anthropogenic hazard.
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The data required to reproduce our conclusions are available at the NTU Data Repository (https://researchdata.ntu.edu.sg/). The data included are: horizontal displacements from object tracking and pixel correlation, pixel correlation strain maps, land use classification and mapped geological features. MicMac displacement maps can be found at https://doi.org/10.21979/N9/VTBCFL. Strain rasters can be found at https://doi.org/10.21979/N9/PNYMEQ. Object tracking displacements can be found at https://doi.org/10.21979/N9/WPDFX2. Geological feature KMZ files can be found at https://doi.org/10.21979/N9/BZNMU0. The land use classification map can be found at https://doi.org/10.21979/N9/AFWLAR.
The MATLAB function used to calculate horizontal strain components is available at the NTU Data Repository: https://doi.org/10.21979/N9/FLOXET.
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Generic Mapping Tools34 was used to produce the figures. This research was supported by the Asian School of the Environment, Nanyang Technological University and the National Research Foundation Singapore and the Singapore Ministry of Education under the Research Centres of Excellence initiative. J.H. was supported by a Singapore National Research Foundation Fellowship (award no. NRF-NRFF2013-06). Part of the research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. This work comprises Earth Observatory of Singapore contribution no. 230.
The authors declare no competing interests.
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