The subaqueous landslide cycle in south-central Chilean lakes: the role of tephra, slope gradient and repeated seismic shaking

Subaqueous landslides are common features at active and passive ocean margins, in fjords and lakes. They can develop on very gentle slope gradients (<2) and the presence of sandy tephra layers seems to facilitate the development of translational failure. Despite numerous investigations, it remains elusive how different slope preconditioning factors act and interact over time and how different triggering mechanisms can lead to slope failure. In settings of low to moderate seismicity, stratigraphic sequences with sublacustrine mass-transport deposits (MTDs) have successfully been used for constructing prehistorical earthquake catalogs. In high seismicity areas, it is inferred that not all strong earthquakes succeed in triggering landslides on the investigated slope segments, and MTD records do not fully represent their complete recurrence pattern. Here, we present the spatio-temporal distribution of MTDs in two large glacigenic Chilean lakes (Villarrica and Calafquén) based on a detailed seismic-stratigraphic analysis and several radiocarbon-dated piston cores (up to 14m long). We find a strong influence of slope gradient on the occurrence and volume of landslide events; i.e. most (small) landslides take place on slopes of 5-20, whereas the few large (potentially tsunamigenic) landslides exclusively occur on slopes of <4. Liquefaction of sandy tephra layers facilitates the development of thin (<0.5m) in-situ deformations during earthquake shaking. When sandy tephra layers get progressively buried, liquefaction becomes unlikely, but repeated excess pore pressure transfer to overlying units facilitates the development of translational sliding. The occurrence of voluminous landslides seems to follow a “landslide cycle” which starts with the deposition of a tephra layer and the development of in-situ deformations directly on top. Once the slope sequence reaches a critical thickness, the end of the cycle is indicated by incipient scarp development, and subsequent major sliding event(s). The duration of the landslide cycle is defined by the rate of gradual sedimentation, but may be affected by sudden geological events (e.g., volcanic eruptions), expediting the end of the cycle. Despite the many methodological challenges inherent to the construction of a MTD stratigraphy, we propose that well-dated multiple MTD events can be used as positive evidence to strengthen and specify the regional paleoseismic record, concerning the largest events in a high-seismicity region. This method is most successful when targeting the base of relatively steep slopes (5-20) with frequent, minor landsliding, and complementing this with seismic-stratigraphic analysis of fluid-escape features and correlation with distal turbidite records.(VLID)3242017Submitted versio

Earthquake-triggered remobilization of surficial slope sediment: a paradigm-shift in turbidite paleoseismology? : Identification, quantification and dating of cm-scale unconformities in lacustrine and oceanic slope sequences

Reliable knowledge on past earthquake recurrence is of key importance for seismic hazard assessment. However, short time-span of instrumental earthquake records relative to typical recurrence time of the largest earthquakes in a study area inhibits good estimations. Turbidite paleoseismology seeks to further extend the time-span of seismological observations through the study of turbidite records within lakes and oceans caused by earthquake-induced remobilization of slope sediment. For a good interpretation of these records, a thorough understanding of the processes under which sediment is remobilized is essential. However, many past studies simply assumed that remobilization took place by failure of thick subaquatic slope sequences which transformed into turbidity currents downslope. In contrast, recent research on basin cores of both lake and ocean showed that earthquake-triggered generation of turbidity currents occurred by remobilization of the upper few cm of surficial sediment, termed “surficial slope remobilization”. The goal of this study is to further develop our understanding of the mechanism and relevance of this process in both lakes and ocean margins. Slope sequences of both lacustrine (Lago Riñihue, Chile) and ocean (Japan Trench) settings were investigated. Different methods were chosen for pinpointing, quantification and dating of cm-scale unconformities: Stratigraphic correlation for Lago Riñihue and analysis by geochemical and geophysical proxies for the Japan Trench. In both settings surficial remobilization-related unconformities were identified and their erosional thickness quantified. Dating suggested good correlation between cm-scale erosion and historical earthquake records. Also, deformation structures were observed within the sediment of Lago Riñihue. A correlation between deformation occurrence and earthquake intensity was interpreted from our data. Former research hypothesized that earthquake-triggered surficial remobilization occurred over a uniform thickness on large slope segments. However, our results showed great variation in remobilization depth within individual slope segments. No clear correlation was observed between remobilization thickness and earthquake intensity. Deformation and remobilization depth did show linear correlation with slope angle increase for the lower slope angles of Lago Riñihue. The shear strength of Lago Riñihue and Japan Trench sediment was studied to further investigate the potential impact of seismic shaking on sediment consolidation, termed “seismic strengthening”. Sharp increases in shear strength were linked to individual earthquakes strongly suggesting that seismic shaking caused overconsolidation of the slope sediments. Further research is required to fully understand the process of surficial remobilization and its interconnection with soft sediment deformation and consolidation of sediment by seismic shaking.Ariana MolenaarUniversität Innsbruck, Masterarbeit, 2018(VLID)274445

Movie C.S1

This video contains a rotating 3D plot of PC1, PC2 and PC3 based on XRF data.</p

Supporting Information for Chapter 5 of the PhD thesis: "Natural Seismographs: A Stratigraphic Study on the Effects of Earthquakes on Subaqueous Surface Sediments" (2024)

This dataset contains the supporting information of Chapter 5 of my PhD thesis. It contains Datasets C.S1 and C.S2 as well as Movie C.S1.</p

Dataset C.S1

This file contains a csv file with Dataset C.S1 from my PhD thesis.</p

Dataset C.S2

This file contains a csv file with dataset C.S2 with data from my PhD thesis.</p

Variations in luminescence properties of quartz and feldspar from modern fluvial sediments in three rivers

Studies of modern sediments, their sedimentology and depositional processes are important for understanding the behaviour of the luminescence characteristics of quartz and feldspar in fluvial settings. Previous studies have shown large variations in OSL characteristics of quartz from different fluvial systems, while the IRSL and pIRIR signals from K-feldspar have been understudied. We test the effects of fluvial setting on luminescence characteristics by collecting modern (old) bedload sediments down the courses of three river systems with very different hydrological characteristics, geologic contexts, and catchment lithologies. The single grain (SG) and multi-grain aliquot (MGA) OSL (quartz) and IRSL and pIRIR (K-feldspar) properties of samples were measured and compared to better understand intra- and inter-fluvial system patterns in sensitivity, bleaching, and equivalent dose (De) distribution skewness and kurtosis. The quartz OSL and K-feldspar IRSL and pIRIR signal sensitivities increase with downstream transport distance of sediments, confirming previous studies (quartz) and showing that IRSL signals from K-feldspar also increase in response to reworking cycles. Increasing transport distance also results in better bleaching of the OSL signal from quartz samples (MGA and SG) due to more grains being exposed to sunlight. By contrast, the IRSL and pIRIR signals retain significant residuals in all samples, though 5¿15% of grains yield zero-dose De values and age modelling of SG data yields accurate burial dose estimates. Additionally, the skewness and kurtosis of SG OSL De datasets from one river increase with transport distance, with the best bleached samples exhibiting the highest skewness, thereby questioning the applicability of the skewness-value of a De dataset as an accurate indicator for partial-bleaching. Our data shows marked variability between (i) different river systems and (ii) the measured minerals, however consistent use of statistical models allows accurate De estimation in all contexts. Age modelling of SG data from K-feldspar, thus, provides a valuable tool for future fluvial research in regions where poor OSL characteristics prevent the use of quartz as a dosimeter

Datasets related to publication Molenaar et al. (2024) in Geochemistry, Geophysics, Geosystems

Supplementary data related to the publication Molenaar et al. (2024) in Geochemistry, Geophysics, Geosystems, titled "Shaken and Stirred: A Comparative Study of Earthquake-Triggered Soft-Sediment Deformation Structures in Lake Sediments". This supporting information (Text-file) provides additional information on the methods for sediment core analysis and ground motion calculations (Text S1) and more details on the bathymetry and age-depth models per lake (Text S2). For each lake, we provide a map with core sites and the lake location relative to epicenters of relevant earthquakes, figures showing age-depth models used for analysis and core correlation with SSDS as well as representative smear slide images. Datasets S1 and S2 are included as csv files and provide calculated shaking strengths associated to each of the seven earthquake-related SSDS along with basic information and all details for positive and negative evidence used in the figures presented in the main manuscript, respectively. Movie S1 presents a rotating 3D plot corresponding to subfigure 3b in the manuscript.</p

Disentangling factors controlling earthquake-triggered soft-sediment deformation in lakes

In-situ soft sediment deformation structures (SSDS) are commonly used as paleoseismic indicators in marine and lacustrine sedimentary records. Earthquake-related shear can deform sediment in the shallow subsurface through Kelvin-Helmholtz instability. The SSDS related to Kelvin-Helmholtz instability have been used to quan-tify shaking strength of past earthquakes. However, the relative importance of i) lithology and physical proper-ties, ii) potential basal shear surfaces (e.g. clastic deposits), iii) slope angle, and iv) seismic shaking strength (e.g. peak ground acceleration) for deformation related to Kelvin-Helmholtz instability remains poorly studied. Lake Rinihue (south-central Chile) is chosen as a natural laboratory for disentangling the effect of the aforemen-tioned factors because i) the sediment composition of background sediment varies downcore and ii) volcanogenic clastic deposits are abundant within the sedimentary sequence. A previous study at lake Rinihue identified 25 SSDS intervals induced by historical earthquakes of varying rupture extent in 17 sediment cores taken at slope angles ranging from-0.2 degrees to-4.9 degrees (i.e. 16 slope sites and 1 basin site). Our study shows that defor-mation mostly occurs directly above volcanogenic deposits (i.e. 72 % of SSDS intervals), suggesting that volcanogenic deposits promote earthquake-induced deformation by strain softening, liquefaction or water film formation. Deformation thickness of SSDS increases with higher slope angles (i.e. strong positive correlation). Ad-ditionally, deformation thickness commonly corresponds to the stratigraphic depth of the youngest preceding volcanogenic deposit, but for steeper slope angles stratigraphically older volcanogenic deposits can function as basal shear surface. Therefore, we suggest that deformation thickness is primarily regulated by gravitational stress (i.e. slope angle) and secondarily by the stratigraphic depth of volcanogenic deposits. The earthquakes related to strongest shaking caused almost exclusively SSDS with highest deformation degrees (i.e. folds and intraclast breccia) as well as largest spatial extent of SSDS, resulting in highest numbers of related SSDS in the investigated cores. Thinner SSDS have higher deformation degrees at a given shaking strength, as seismically -induced shear energy acts more effectively on thinner deforming sequences. Therefore, we suggest that deforma-tion degree is primarily controlled by shaking strength and secondarily modulated by the thickness of the deforming sequences. We infer that deformation thickness is not a reliable indicator of paleoseismic shaking strength as this relies on many preconditioning factors independent of shaking strength. On the other hand, deformation degree can be a good proxy for shaking strength also in settings with varying lithotypes and intercalated clastic event deposits, provided multiple cores are studied to avoid under-or overestimation of paleoseismic shaking strength. (c) 2022 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/)

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

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, Rinihue and Calafquen, 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 (Mw ca 9.5). In lake Rinihue, 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