Boulder deposits on Isla de Mona, Puerto Rico: Evidence of tsunami or storm?

Boulders deposited on land in coastal regions are evidence of extreme wave events and can be used to assess associated hazards. Discrimination between boulders emplaced by tsunamis and storms is a relatively new science. One criterium for differentiating tsunami and storm deposits is the size and mass of boulders. The argument is made that boulders above a certain size and mass can only be transported by tsunamis because the height of a storm wave is limited by breaking while a tsunami wave can be 10s of meters high. Although intuitive, uncertainties in flow accelerations in storm waves and bottom friction make it difficult to discriminate between boulder transport by a large storm and a small or moderately-sized tsunami. Another criterium is to use the geometry and spatial distribution of the boulder deposit. Richmond et al. (2011) in a study of coastal boulder deposits on the island of Hawaii found that storms tend to deposit boulders in linear ridges close to and parallel to the shore while tsunamis leave boulders in a dispersed pattern, at times 100s of meters from the shore, as they lose energy moving inland.

We investigated boulders on a low-lying karstic coastal platform in southwest Isla de Mona, which is located in the Mona Passage about 70 km west of Puerto Rico, to determine whether a tsunami or a storm deposited them. Fieldwork was limited to near the shoreline and a road running obliquely from the shore because of a dense network of poisonous plants on the platform. In all, about 50 boulders were measured and mapped. The largest boulders were over 6 m in length and 2-3 m in width and height. Boulders larger than 4 m in length were found up to 600 m inland. Boulders in gaps in the dense vegetation between the shore and road were also visible from a small plane flying at low altitude. The dispersed pattern of boulders and their inland distance favors transport by a tsunami. A tsunami interpretation is supported by the presence of a submarine landslide scarp near the coast and preliminary modeling of flow and boulder transport from a landslide-generated tsunami.