Coastal lake records of past tsunamis in South-Central Chile

Abstract

Overlying the subducting Nazca Plate, Chile’s coastline is notoriously prone to megathrust earthquakes and associated tsunamis, as illustrated by the 2010 Maule (Mw 8.8) and 1960 Valdivia (Mw 9.5) events. Despite numerous geophysical and paleoseismic studies, many questions remain about the timing, location and rupture extent of great tsunamigenic earthquakes. To study past tsunami inundation, coastal lakes may form complementary sedimentary archives to classical coastal plain studies as they have more accommodation space and a better preservation potential for tsunami deposits. Moreover, tsunami inundation may lead to prolonged changes in the lake system that are reflected in its sedimentary record. Here we present an overview of published and ongoing research on four coastal lake systems in Chile, i.e. Lake Gemelas West, Lake Huelde, Lake Cucao and Lake Huillinco. Due to their different basin and barrier morphologies, distance to the ocean and temporally-changing ocean connectivity, we expect significant differences in how tsunami inundation is recorded in their sediments. We used the 1960 tsunami deposit in these systems as a reference for which we constrain its sedimentological characteristics and spatial distribution within the lakes. Geochemistry of the pre- and post-1960 sediments is compared based on XRF scans, carbon isotopes and microfacies analysis. This comparison shows that lakes close to the Pacific (i.e. Lake Cucao, Huelde and Gemelas West) contain typical tsunami sands sometimes with mud-rip up clasts, whereas the inland Lake Huillinco exhibits a drastic change in inorganic geochemistry (e.g. a sudden increase in S) that persists until present. We propose that a large amount of salt water propagated in Lake Huillinco, leading to a permanent stratified water body, anoxic bottom water conditions and the preservation of varves. Similar long-lived impacts are also inferred for prehistoric tsunami events in Lake Huelde, where post-tsunami sediments often show varves during a few decades, whereas pre-tsunami sediments are typically homogenous. This study shows that besides tsunami sands, tsunami inundation can also produce long-lived changes in coastal lake systems that are recorded in their sedimentary archive and can be used to infer past tsunami occurrence.