| Summary: | On September 16, 2015, a Mw 8.3 earthquake struck offshore of the north-central Chile coast with a fault-rupture length of approximately 150 km. The earthquake triggered a tsunami that impacted 500 km of 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 (30.0°S) with run-up heights >10 m at a few exposed locations between Limarí (30.7°S) and Coquimbo. The tsunami provided an invaluable opportunity to examine the nature of tsunami deposit evidence in a semi-arid, siliciclastic environment, where settings suitable for the preservation of tsunami sedimentation are scarce, thereby improving our ability to identify such evidence in the geologic record. Using before-and-after-tsunami satellite imagery and post-tsunami coastal surveys, we targeted one of the few low-energy depositional terrestrial environments in the tsunami-affected area that had a high potential to preserve the 2015 tsunami deposit and older events: the Pachingo marsh in Tongoy Bay (30.3°S). We employed field and laboratory methods to document the 2015 tsunami deposit and discovered sedimentological evidence of previous tsunami inundation of the site.
The 2015 tsunami deposit and an older sand bed ~10 cm lower in the stratigraphy exhibit similar sedimentological characteristics. Both sand beds are composed of poorly to moderately sorted, gray-brown, fine- to medium-grained sand and are distinct from underlying and overlying organic-rich silty sediments. The sand beds are thinner (from ~20 cm to <1 cm) and finer (from medium- to fine-grained sand) at more inland locations, and fine upward. However, the older sand bed extends over 150 m farther inland than the 2015 tsunami deposit. To explore the differences in the offshore ruptures that generated the tsunamis that deposited each sand bed, we employed an inverse sediment transport model (TSUFLIND). Our field survey, sedimentological data, and modeling results infer that the older sand bed preserved at the Pachingo marsh field site was produced by a larger tsunami than the 2015 tsunami. Anthropogenic evidence (copper smelter waste) along with Cs137 and Pb210 dating constrains the magnitude and age of the older sand bed to the last 130 years. Based on historical analysis of recent tsunamis that impacted the Pachingo marsh region, we infer a widespread tsunami in 1922 is the best candidate for depositing the older sand bed at our site, providing first geologic evidence of pre-2015 tsunami inundation along the north-central Chile coast. === Master of Science === On September 16, 2015 a segment of the Chilean subduction zone ruptured off the coast of north-central Chile producing a magnitude 8.3 earthquake. The earthquake created a tsunami that impacted 500 km of coastline and created waves measuring 4.5 m on the tide gauge at
Coquimbo (30.0°S) with waves reaching inland to heights >10 m at a few exposed locations. The 2015 event in north-central Chile provided a unique opportunity to study tsunami deposits in semi-arid, sand-dominated environment where preservation of tsunami deposits within coastal sediments is rare, helping improve our ability to identify this type of evidence in the sedimentary record. Using before-and-after-tsunami satellite imagery and post-tsunami coastal surveys, we targeted a marsh in the tsunami-affected area that was capable of preserving the 2015 tsunami deposit and older events: the Pachingo marsh in Tongoy Bay (30.3°S). We employed field and laboratory methods to document the 2015 tsunami deposit and discovered evidence of previous tsunami overwash at the site within the sediments of the marsh. Our field work observations and sedimentary data revealed that in addition to the 2015 tsunami deposit, the site contained an older, anomalous sand bed that we infer to be a tsunami based on its similarity to the modern deposit at the site. Our modeling results suggest that the older sand bed was deposited by a tsunami larger than the 2015 tsunami. Based on historical analysis of recent tsunamis that impacted the Pachingo marsh region, we infer a tsunami in 1922 is the best candidate for depositing the older sand bed at our site. This discovery provides the first evidence of tsunami sediments from pre-2015 tsunami inundation along the north-central Chile coast.
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