Irrigation of rice paddies led to many of the 4,000-plus deaths during the powerful earthquake that struck Palu, Indonesia, in September 2018. The waterlogged ground shook during the magnitude-7.5 quake and slid downhill, according to two teams that studied the aftermath of the disaster.

The landslides surprised many scientists because the slopes in the Palu Valley are so gentle. To investigate, Kyle Bradley at Nanyang Technological University in Singapore and his colleagues mapped four of the big landslides and evaluated the ground that had collapsed.

The team found that wet rice paddies had saturated the sandy soil. When the earthquake hit, that soil liquefied and flowed across the landscape. Had the ground been dry, the landslides would probably not have happened.

Separately, Ian Watkinson and Robert Hall at Royal Holloway University of London in Egham, UK, studied buildings that had been carried away by the landslides. They confirmed that the slides happened only on irrigated ground.

The studies suggest that even gentle slopes, when irrigated, could pose a serious landslide risk in other earthquake-prone parts of the world.