Seismic tomography and ground-penetrating radar (GPR) were combined to determine weakness zones and the degree of alteration in rock making up the Moai statues of Easter Island. The surveys were carried out on stratified argillaceous tuff that had been subjected to weathering for long periods. The argillaceous nature and unsaturated condition of the rock creates ambiguity in both the seismic and georadar data interpretation, but this ambiguity is removed by using a combined interpretation of the data. Interpretation of the combined data shows that the high seismic velocities of the inner zones of the statues must be related to water content and not to better rock quality, because of the high attenuation of the georadar signal in those zones. The weathered zone is characterized by a high signal-to-noise ratio of georadar data and low P-wave seismic velocities. Comparing the seismic and GPR data sets, we found that the seismic velocity and the signal attenuation of the georadar section differ for two statues in accordance with the degree of alteration of the rock. We show that the combined interpretation of seismic and georadar data allows the removal of the ambiguities in the interpretation of the seismic velocity in the massive tuff, the definition of the thickness of the weathered layer and the linking of seismic velocity to the degree of Moai preservation, which is useful for a classification of Moai health. Copyright © 2013 John Wiley & Sons, Ltd.