What controls the remobilization and deformation of surficial sediment by seismic shaking? Linking lacustrine slope stratigraphy to great earthquakes in South-Central Chile.

Ariana Molenaar,Michael Strasser,Roberto Urrutia,Thomas Vandorpe,Marc De Batist,Gerald Degenhart,Jasper Moernaut,Maarten Van Daele,Mario Pino,Adam Mcarthur

doi:10.1111/sed.12856

Abstract

Remobilization and deformation of surficial subaqueous slope sediments create turbidites and soft sediment deformation structures, which are common features in many depositional records. Palaeoseismic studies have used seismically‐induced turbidites and soft sediment deformation structures preserved in sedimentary sequences to reconstruct recurrence patterns and – in some cases – allow quantifying rupture location and magnitude of past earthquakes. However, current understanding of earthquake‐triggered remobilization and deformation lacks studies targeting where these processes take place, the subaqueous slope and involving direct comparison of sedimentary fingerprint with well‐documented historical earthquakes. This study investigates the sedimentary imprint of six megathrust earthquakes with varying rupture characteristics in 17 slope sediment cores from two Chilean lakes, Riñihue and Calafquén, and evaluates how it links to seismic intensity, peak ground acceleration, bracketed duration and slope angle. Centimetre‐scale stratigraphic gaps ranging from ca 1 to 20 cm – caused by remobilization of surficial slope sediment – were identified using high‐resolution multi‐proxy core correlation of slope to basin cores, and six types of soft sediment deformation structures ranging from ca 1 to 25 cm thickness using high‐resolution three‐dimensional X‐ray computed tomography data. Stratigraphic gaps occur on slope angles of ≥2.3°, whereas deformation already occurs from slope angle 0.2°. The thickness of both stratigraphic gaps and soft sediment deformation structures increases with slope angle, suggesting that increased gravitational shear stress promotes both surficial remobilization and deformation. Seismic shaking is the dominant trigger for surficial remobilization and deformation at the studied lakes. Total remobilization depth correlates best with bracketed duration and is highest in both lakes for the strongest earthquakes (M w ca 9.5). In lake Riñihue, soft sediment deformation structure thickness and type correlate best with peak ground acceleration providing the first field‐based evidence of progressive soft sediment deformation structure development with increasing peak ground acceleration for soft sediment deformation structures caused by Kelvin‐Helmholtz instability. The authors propose that long duration and low frequency content of seismic shaking favours surficial remobilization, whereas ground motion amplitude controls Kelvin‐Helmholtz instability‐related soft sediment deformation structure development.

Highlights

Turbidites and soft sediment deformation structures (SSDS) are ubiquitous within many sedimentary records
Moernaut et al (2017) stated that surficial remobilization could explain the correlation of turbidite volume with seismic shaking intensity observed in some settings (Goldfinger et al, 2012; Moernaut et al, 2014; McHugh et al, 2016), because stronger shaking would remould deeper into the sediment resulting in higher turbidite volume
The three folds observed in this lake correspond to these two earthquakes – two to the 1737 CE and one to the ca 1466 CE earthquake. This proposed method of stratigraphic correlation to well-dated seismo-turbidite records proved successful as centimetre-scale gaps and SSDS were detected in slope sequences and linked to six well-documented megathrust earthquakes

Summary

Introduction

Turbidites and soft sediment deformation structures (SSDS) are ubiquitous within many sedimentary records (turbidites: e.g. Bouma, 1962; Lowe, 1982; Mutti, 1992; SSDS: e.g. Lowe, 1975; Allen, 1982; Maltman, 1994). Full exploitation of palaeoseismic turbidite and SSDS records hinges on a thorough understanding of the effect of: (i) different ground motion characteristics; (ii) sediment properties; (iii) slope morphology; and (iv) seismic site effects on causative processes like surficial remobilization and deformation. Surficial remobilization describes the earthquake-induced remoulding and subsequent transportation of a centimetre-scale veneer of surficial subaqueous slope sediment (e.g. McHugh et al, 2016, 2020; Moernaut et al, 2017; Molenaar et al, 2019; Ikehara et al, 2020; Schwestermann et al, 2020). Whereas surficial remobilization is caused by seismically-induced transient stresses at the sediment–water interface (Moernaut et al., 2017; Gomberg, 2018), submarine landsliding strongly depends on preconditioning factors, such as build-up of critical overburden stress, availability of weak layers and excess pore pressure, in addition to a trigger mechanism such as earthquake shaking (Locat & Lee, 2002). Seismicallyinduced surficial remobilization might allow for quantification of palaeoseismic intensity through the volumetric analysis of seismo-turbidites

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