Browsing by Author "Cisternas, M"

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    Geochemical indicators and diatoms as true markers of the inundation limit of the 2010 Maule Tsunami
    (American Geophysical Union, 2013-01-12) Chagué-Goff, C; Goff, JR; Wong, HKY; Cisternas, M
    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.
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    Insights from geochemistry and diatoms to characterise a tsunami's deposit and maximum inundation limit
    (Elsevier, 2015-01-01) Chagué-Goff, C; Goff, JR; Wong, HKY; Cisternas, M
    Geochemical proxies and diatom assemblages were used in combination with grain size characteristics not only to describe the deposit left behind by the 27 February 2010, Maule tsunami at Las Cañas, Maule Region, Chile, but also to trace the maximum inundation limit of the event. The sandy deposit was laid down between 160 and 260 m inland behind an eroded sand dune and a lagoon but reached only 60% of the total tsunami inundation distance of 380 m, which was marked by organic debris, pumice clasts and wooden logs. It consisted of coarse to medium sand that thinned and fined inland. At the most seaward point, the 22 cm thick deposit exhibited a fining upward unit overlain by a couplet of coarsening–fining upward units, suggesting deposition by at least two waves, while farther inland the fining upward deposit was probably left behind by only one wave. Chemical proxies (Ca/Ti vs Sr/Ba) allow us to distinguish the deposit from the surrounding soil and indicate that it was sourced from the beach and/or dune area, with diatom assemblages confirming the marine origin of the deposit. Saltwater indicators (e.g. Cl, S) provide evidence for the maximum inundation limit beyond the extent of the sandy deposit, despite dilution and dissolution by 500 mm of rainfall in the six months since the tsunami. Marine and marine/brackish diatom assemblages decreased landward but were found up to the inundation limit and immediately beyond, suggesting the effect of diatom-bearing sea spray at the wave front or redistribution of the detrital assemblage associated with tsunami inundation due to wind. While the latter might result in a slight over-estimation of the inundation distance, they can be used in combination with chemical proxies to trace the maximum inundation distance of recent and past tsunamis, thus allowing for a better estimation of the magnitude of past events. Post-depositional processes were found to have affected the thinner sandy deposits (< 5 cm), suggesting that these are unlikely to be preserved in the geological record. This highlights the need to be able to trace the tsunami inundation limit with geochemical and/or diatom proxies without having to rely on sedimentological evidence, as it is now widely recognised that conventional approaches used by tsunami researchers have led to an under-estimation of previous events.© 2014, Elsevier B.V.
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    Palaeotsunamis in the Pacific Islands
    (Elsevier, 2011-07-01) Goff, JR; Chagué-Goff, C; Dominey-Howes, D; McAdoo, B; Cronin, S; Bonté-Grapetin, M; Nichol, SL; Horrocks, M; Cisternas, M; Lamarche, G; Pelletier, B; Jaffe, BE; Dudley, W
    The recent 29 September 2009 South Pacific and 27 February 2010 Chilean events are a graphic reminder that the tsunami hazard and risk for the Pacific Ocean region should not be forgotten. Pacific Islands Countries (PICs) generally have short ( < 150 years) historic records, which means that to understand their tsunami hazard and risk researchers must study evidence for prehistoric events. However, our current state of knowledge of palaeotsunamis in PICs as opposed to their circum-Pacific counterparts is minimal at best. We briefly outline the limited extent of our current knowledge and propose an innovative methodology for future research in the Pacific. Each PLC represents a point source of information in the Pacific Ocean and this would allow their palaeotsunami records to be treated akin to palaeo-DART (R) (Deep-ocean Assessment and Reporting of Tsunamis) buoys. Contemporaneous palaeotsunamis from local, regional and distant sources could be identified by using the spatial distribution of island records throughout the Pacific Ocean in conjunction with robust event chronologies. This would be highly innovative and, more importantly, would help provide the building blocks necessary to achieve more meaningful disaster risk reduction for PICs. (C) 2010 Elsevier B.V.