Sedimentary and diatom evidence of the 2015 tsunami on the north-central coast of Chile in the absence of significant coastal deformation
Abstract
On September 16, 2015 a Mw 8.3 earthquake struck offshore north-central Chile. The Illapel earthquake triggered a tsunami that impacted the coastline from Huasco (28.5°S) to San Antonio (33.5°S), registering as much as 4.5 m on the tide gauge at Coquimbo (29.95°S) with runup heights >10 m at a few exposed locations between Limari and Lengua de Vaca. The 2015 Chile tsunami provides an opportunity to document geologic evidence of tsunami inundation in a semi-arid, siliciclastic environment, thereby improving our ability to identify paleotsunami deposits in the context of relatively small values ( 20 cm uplift and 40 cm subsidence) of coseismic coastal vertical motion. We employ field, laboratory, and modeling methods to document the stratigraphical, sedimentological, and diatom characteristics of the 2015 Chile tsunami deposit in marshes bordering the Limari River (30.73°S) and Tongoy Bay (30.30°S). We exposed the tsunami deposit in 34 trenches along two 400 m-long coast-perpendicular transects fringing the Limari River and in 15 trenches along a 400 m coast-perpendicular transect in Tongoy Bay. In marshes at both sites, the tsunami deposited a well-sorted, grey medium-to-fine-grained sand on a brown silty soil. The tsunami deposit thins (from 50 cm to <1 cm) and fines (from medium- to fine-grained) inland. Up to four fining-upward intervals are present, consistent with multiple waves recorded by tide gauges and coastal residents. The 2015 Chile tsunami deposit contains a mixed diatom assemblage, including anomalous marine and brackish diatoms not found in the underlying soil. The relative abundance of marine diatoms in the deposit decreases with distance inland. Given the small values of coseismic coastal deformation, this sand layer may be the only onshore geologic record of the Mw 8.3 Illapel earthquake. Similar sediment characteristics are observed in an older sand bed at the Tongoy Bay site, suggesting that an older, previously undocumented tsunami inundated the marsh.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMNH51D..01D
- Keywords:
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- 3225 Numerical approximations and analysis;
- MATHEMATICAL GEOPHYSICSDE: 4332 Disaster resilience;
- NATURAL HAZARDSDE: 4341 Early warning systems;
- NATURAL HAZARDSDE: 4564 Tsunamis and storm surges;
- OCEANOGRAPHY: PHYSICAL