Sedimentary Record of the February 27, 2010, Chile Tsunami

The February 27, 2010 Chilean tsunami altered the coastal landscape along a section of coast characterized by broad sand beaches, fluvial deltas, and occasional rocky headlands. We examined tsunami geologic impacts at five relatively natural sites along a 200 km segment of coast centered on the earthquake epicenter. Data collected included measurements of topography, flow depths, flow directions and flow direction history, inundation distances, vertical erosion, and sediment deposition. The depositional record consisted mostly of sand deposits that were typically less than 25 cm thick. Exceptions were thick tsunami deposits near the mouths of Rio Huenchullami (La Trinchera) and Rio Maule (Constitucion) where sediment supply was abundant. Observations of tsunami geologic impacts included: (1) up to meter-scale changes in coastal-plain elevation as a result of tsunami-induced erosion and deposition; (2) erosion of the land surface was typically greatest near the shoreline and was the result of multiple processes such as tsunami return-flow, inundation scour around the bases of trees and other structures, and a general planation of the land surface near the shoreline; (3) tsunami sand deposits were widespread and extended to near the limit of inundation at most sites; (4) evidence of inundation by multiple waves that arrived at different times from different directions at open coast sites; (5) layers of plane-parallel stratification in some tsunami deposits and the presence of large bedforms at one site indicated that at least some of the sediment was transported as bed load and not as suspended load; (6) where available, coarse clasts such as mud boulders and rock cobbles were transported inland; (7) in forested coastal areas large debris ramparts were constructed near the limit of inundation. A significant finding was that vegetation height and density exerted a strong control on tsunami deposit thickness in the Coliumo Valley. Agricultural and ranching activities resulted in adjacent plots of land of similar microtopography but with distinctly different vegetation coverage. In general, tsunami sand deposits were thinnest where vegetation was low or sparse, and thickest where vegetation was high and dense. Also sand thicknesses increased along the valley margin where flow depths decreased.