A structural model finds that adding a stone topknot narrows a moai’s stability margin, especially toward its face.
Researchers developed a closed-form model for the static stability of moai–pukao assemblies on their stone platforms, or ahu. The model extends earlier analyses of overturning monuments and rocking stone blocks by considering different directions of possible toppling.
The calculations indicate that a pukao raises the assembly’s centre of mass and reduces its stability margin. The authors say the model can be used as a first screening tool to rank restored assemblies by their resistance to frontal toppling and to assess future decisions about reinstalling pukao.
How toppling risk changes
For a moai with a pukao, the centre of mass rises from about 0.522 of the statue’s height to about 0.671 of the assembly height. The calculated critical tipping angle falls from about 17.7 to 13.9 degrees for sideways movement and from about 10.8 to 8.5 degrees toward the front of the statue—the direction associated with observed face-down toppling.
The model estimates that the threshold for frontal tipping during earthquake-like horizontal acceleration decreases from about 0.19 g for a moai alone to 0.15 g with a pukao. The minimum energy calculated for frontal toppling is about 6.7–6.9 kilojoules, around 37% of the lateral value. Together, the smaller angular margin and lower energy barrier make frontal toppling the favoured mode in the model.
A tool for conservation planning
About 50 moai have been re-erected on their ahu since 1956, and several have had their pukao reinstalled. Because the topknots raise the centre of mass and reduce calculated stability, the model provides quantitative criteria for identifying assemblies that may deserve closer structural health monitoring.
The approach could also help conservation teams prioritize preventive work and evaluate future restoration choices involving pukao. The study does not identify particular restored statues as the most vulnerable; rather, it provides a way to rank them once their relevant structural characteristics are assessed.
Model-based evidence and limits
This is a mathematical, closed-form structural model rather than a report of a new field survey or physical experiment. Its calculations are based on overturning-stability and two-block rocking approaches, extended to account for direction; the numerical results can be reproduced with the supplementary Python script.
The abstract does not report field or laboratory validation of the model, nor does it provide a site-by-site ranking of restored assemblies. The model defines a stability envelope within which different possible causes of toppling—including earthquakes, applied forces, neglect after contact, or ritual activity—would have to operate, but it does not establish which mechanism caused any particular failure.
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Built Heritage · 2026 · DOI: 10.1186/s43238-026-00298-0
Authors: LUCAS GIOVANI RIBEIRO, Marcos André Simonssini
Institutions: Centro Universitário do Leste de Minas Gerais
