Stratigraphic evidence of two historical tsunamis on the semi-arid coast of north-central Chile

On September 16, 2015, a M_w 8.3 earthquake struck the north-central Chile coast, triggering a tsunami observed along 500 km of coastline, between Huasco (28.5°S) and San Antonio (33.5°S). This tsunami provided a unique opportunity to examine the nature of tsunami deposits in a semi-arid, siliciclastic environment where stratigraphic and sedimentological records of past tsunamis are difficult to distinguish. To improve our ability to identify such evidence, we targeted one of the few low-energy, organic-rich depositional environments in north-central Chile: Pachingo marsh in Tongoy Bay (30.3°S).

We found sedimentary evidence of the 2015 and one previous tsunami as tabular sand sheets. Both deposits are composed of poorly to moderately sorted, gray-brown, fine-to medium-grained sand and are distinct from underlying and overlying organic-rich silt. Both sand beds thin (from ∼20 cm to <1 cm) and="" fine="" landward,="" and="" show="" normal="" grading.="" the="" older="" sand="" bed="" is="" thicker="" and="" extends="" over="" 125 m="" further="" inland="" than="" the="" 2015="" tsunami="" deposit.="" to="" model="" the="" relative="" size="" of="" the="" tsunamis="" that="" deposited="" each="" sand="" bed,="" we="" employed="" tsunami="" flow="" inversion.="" our="" results="" show="" that="" the="" older="" sand="" bed="" was="" produced="" by="" higher="" flow="" speeds="" and="" depths="" than="" those="" in="" 2015.="" anthropogenic="" evidence="" along="">¹³⁷Cs and ²¹⁰Pb dating constrains the age of the older tsunami to the last ∼110 years. We suggest that the older sand bed was deposited by the large tsunami in 1922 CE sourced to the north of our study site. This deposit represents the first geologic evidence of a pre-2015 tsunami along the semi-arid north-central Chile coast and highlights the current and continuing tsunami hazard in the region.