Geochemical Indicators and Diatoms as True Markers of the Inundation Limit of the 2010 Maule Tsunami

It has long been known that tsunamis inundate further than the limit of the sand deposit they leave behind, and that relying on the extent of the sandy sedimentary evidence is likely to result in an under-estimation of event magnitude and risk. However, the question remains about how to identify the real limit of tsunami inundation, particularly several months or more after the event. Here we report on a study carried out at a site on the Pacific coast of Chile south of Constitución in August 2010, nearly 6 months after the Mw 8.8 Maule tsunami. Eight pits were dug along a 480 m long shore-perpendicular transect, incorporating the area covered by tsunami sediment, a zone covered by a discontinuous and decomposing debris scatter up to the point of maximum inundation, and an area further landward from this point. The tsunami sand deposit extended from ~160 to 260 m inland and ranged in thickness from 22 to less than 1 cm at the limit of sediment deposition. It consisted of dark, generally well-sorted, coarse to medium sand. Logs, pumice and various organic debris were found up to a limit of tsunami inundation (380 m inland - confirmed by a local eye witness). New grass growth also covered the tsunami deposit and the field further inland. While the chemical composition of the tsunami deposit differed to that of the underlying soil, indicators of saltwater inundation were low in the sandy units, reflecting downward leaching and dilution by rainfall in the porous material in the 6 months since the tsunami. Concentrations of saltwater indicators (e.g. chloride, sulphate, bromide) were however elevated in the area covered by scattered debris up to the limit of tsunami inundation, suggesting preferential retention in organic-rich material. Marine and brackish-marine diatoms were found both in the sandy units and beyond, at the surface of the grass field, and occurred in higher concentrations than in the underlying soil. Both diatom assemblages and geochemical marine proxies are indicative of tsunami inundation well beyond the sediment limit. While sea spray and associated wind cannot be totally excluded as the source of soluble salts and wind-blown diatoms, chloride concentrations do not decrease landward within the area of tsunami inundation. A decrease in chloride would have been expected if it was solely due to sea spray. While further research is required to allow us to distinguish the effects of marine inundation from sea spray, the use of geochemical proxies and diatoms provides a means to identify the limit of tsunami inundation beyond the limit of sand deposition, and as previous studies have shown, even after debris marking it have decayed. This is of importance for tsunami risk assessment and mitigation.