Kyle Bradley, Rishav Mallick, Dedy Alfian, Harisma Andikagumi, Benazir Benazir, Gilles Brocard , Guangcai Feng, Emma M. Hill, Judith Hubbard, Jedrzej Majweski

The death toll and economic impact of an earthquake are greatly exacerbated when landslides are triggered by strong ground motion. These slides typically occur in two different contexts: localized failure of steep slopes 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. Catastrophic landsliding triggered by the September 28, 2018 earthquake at Palu, Indonesia did not occur in either context, but produced both outcomes. Here, we show that major alluvial landsliding was a direct consequence of irrigation that activated a previously nonexistent liquefaction hazard. Aqueduct-fed wet rice cultivation raised the water table to near ground level, saturating sandy alluvial soils that liquefied in response to strong ground shaking and enabled extensive and large-displacement lateral spreads on slopes less than 1.5°. On slopes steeper than 1.5°, lateral spreads sourced long-runout landslides and debris flows that swept through villages below. This damage and loss of life would not have occurred in the absence of aqueduct-fed wet rice cultivation. Landsliding of gentle, irrigated alluvial slopes is an unrecognized anthropogenic hazard, particularly in seismically active, rice-growing areas of Asia.