Die Herausforderung einer globalen Meeresbodenvermessung und das unterschätzte Potenzial der Datenakquise auf Transitstrecken

Nur etwa 20 % des Meeresbodens sind bislang mit Fächerecholoten kartiert worden. Größere zusammenhängende Flächen werden selten systematisch und hochauflösend kartiert. Stattdessen gibt es den Ansatz, dass Forschungsschiffe Bathymetriedaten auf Transitstrecken aufnehmen. Im Pilotprojekt »Unterwegs«-Forschungsdaten der DAM – Deutsche Allianz Meeresforschung tragen deutsche Forschungsschiffe dazu bei, weitere Teilstücke des Meeresbodens zu kartieren, um Datenlücken zu schließen. Die Bathymetriedaten werden im Datenarchiv PANGAEA veröffentlicht und von dort in das Portal Deutsche Meeresforschung integriert

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

On The Role of Volcanic Material in Submarine Landslide Initiation Processes

Along active continental margins earthquake shaking was singled out as one of the most important triggering mechanisms for submarine landslides, which may trigger a tsunami wave and have tremendous impact on coastal communities and infrastructure. It is known that seismic loading may lead to sudden strength loss in loosely deposited granular soils such as sands and silts due to liquefaction. Volcanic arcs provide a source of granular sediment along active margins through ejection of volcaniclastica. Volcanic ash is of sand to silt size, such that it is presumed to act preferentially as gliding plane in submarine landslides. However, volcanic ash differs significantly in its geotechnical properties compared to commonly studied sands and silts. This dissertation tackles the question to what extent physical properties of volcanic sands and silts affect their response to gravitational and seismic loading in comparison to common sands and silts. Advanced laboratory shear experiments (direct shear and triaxial shear) were conducted with the purpose to investigate the shear behavior of volcanic material. In generic laboratory studies it is shown that properties like angularity, roughness and particle strength (i.e. crushability) play a key role in sediment stability and majorly determine the shear behavior of volcanic ash at effective stress < 0.5 MPa. Furthermore, a case study was conducted on a subaqueous landslide located in the earthquake prone area of South-Central Chile. Recovered sediment cores show volcanic fall-out ash a few mm from the basal shear plane of a landslide that occurred ~ 6 k years ago. In a highly detailed analysis that gathers seismic profile data with CPT data and advanced geotechnical laboratory shear experiments, it is shown that the sole presence of volcanic ash was not sufficient to result in liquefaction failure due to earthquake shaking

Über die Rolle Vulkanischer Aschen in der Initiierung Submariner Hangrutschungen

Along active continental margins earthquake shaking was singled out as one of the most important triggering mechanisms for submarine landslides, which may trigger a tsunami wave and have tremendous impact on coastal communities and infrastructure. It is known that seismic loading may lead to sudden strength loss in loosely deposited granular soils such as sands and silts due to liquefaction. Volcanic arcs provide a source of granular sediment along active margins through ejection of volcaniclastica. Volcanic ash is of sand to silt size, such that it is presumed to act preferentially as gliding plane in submarine landslides. However, volcanic ash differs significantly in its geotechnical properties compared to commonly studied sands and silts. This dissertation tackles the question to what extent physical properties of volcanic sands and silts affect their response to gravitational and seismic loading in comparison to common sands and silts. Advanced laboratory shear experiments (direct shear and triaxial shear) were conducted with the purpose to investigate the shear behavior of volcanic material. In generic laboratory studies it is shown that properties like angularity, roughness and particle strength (i.e. crushability) play a key role in sediment stability and majorly determine the shear behavior of volcanic ash at effective stress < 0.5 MPa. Furthermore, a case study was conducted on a subaqueous landslide located in the earthquake prone area of South-Central Chile. Recovered sediment cores show volcanic fall-out ash a few mm from the basal shear plane of a landslide that occurred ~ 6 k years ago. In a highly detailed analysis that gathers seismic profile data with CPT data and advanced geotechnical laboratory shear experiments, it is shown that the sole presence of volcanic ash was not sufficient to result in liquefaction failure due to earthquake shaking

The enigmatic consolidation of diatomaceous sediment

Marine diatomaceous sediments are common along the polar belts and equator, but very little is known about their effect on sediment geotechnical properties and slope stability. Consolidation state analysis is frequently applied to derive past maximum overburden stress, to quantify over- or underconsolidation, or to infer excess pore water pressure, all relevant to assess risk of slope failure. Diatoms significantly alter geotechnical and other fundamental engineering properties usually observed in organic or inorganic sediment. The consolidation state of diatomaceous sediments is ambiguously discussed because geological evidence and laboratory data do not always correspond. A literature review revealed a near systematic overconsolidation of shallow diatomaceous sediments (< 100 mbsf) and normal or underconsolidation in deeper sediment sequences. One-dimensional compression tests are carried out on material sampled during the R/V POLARSTERN cruise ANT XXIX/4 to a landslide-prone area of the South Sandwich Trench, and on generic clayey-silt - diatomaceous earth sample mixtures. Results indicate that diatoms alter geotechnical properties to an extent that in situ stress conditions may not well be inferred from common consolidation state analysis. Undrained vane shear strength underestimates the in situ undrained shear strength and leads to underestimated normalized undrained shear strength ratios. Enhanced secondary compression with overburden and diatom content leads to a natural curvature of consolidation lines, the latter occasionally falsely interpreted as preconsolidation stress. The observations are furthermore dependent on the predominant diatom order. Moreover, inverse trends of porosity are not necessarily related to excess pore water pressure, but solely to a gradual increase of diatoms with depth

The role of sediment composition and behavior under dynamic loading conditions on slope failure initiation: a study of a subaqueous landslide in earthquake-prone South-Central Chile

Subaqueous slope failure mechanisms are still poorly understood partly because they are difficult to study due to the remote location of submarine landslides. Landslides in lakes are smaller in size and more readily accessible and therefore represent a good alternative to their marine counterparts. Lake Villarrica, located in South-Central Chile, experienced significant slope failure and serves here as an exemplary study area for subaqueous landslide initiation mechanisms in tectonically active settings. Coring and CPTU testing were undertaken with the MARUM free-fall CPTU deployed adjacent to the coring sites where all lithological units involved in the slope failure were sampled. Using geotechnical methods such as pseudo-static factor of safety analysis and cyclic triaxial testing, three types of soils (i.e., diatomaceous ooze, volcanic ash, and quick clay) were analyzed for their role in slope failure, and earthquake shaking was identified as the primary trigger mechanism. The investigated landslide consisted of two distinct phases. During the first phase, slope failure was initiated above a tephra layer. In the second phase, retrogression led to the shoreward extension of the slide scarp along a second failure plane located in a stratigraphically deeper, extremely sensitive lithology (i.e., quick clay). Results show that liquefaction of buried tephra layers was unlikely, but such layers might still have contributed to a reduction in shear strength along the contact area with the neighboring sediment. Furthermore, cyclic shaking-induced pore pressure in diatomaceous ooze may be similar to that in granular soils. We generally infer that failure mechanisms observed in this study are equally important for landslide initiation in submarine settings as diatomaceous ooze intercalated with volcanic ash may be abundantly present along active continental margins

Earthquake Impact on Active Margins: Tracing Surficial Remobilization and Seismic Strengthening in a Slope Sedimentary Sequence

Strong earthquakes at active ocean margins can remobilize vast amounts of surficial slope sediments and dynamically strengthen the margin sequences. Current process understanding is obtained from resulting event deposits and low‐resolution shear strength data, respectively. Here we directly target a site offshore Japan where both processes are expected to initiate, that is, at the uppermost part (15 cm) of a sedimentary slope sequence. Based on a novel application of short‐lived radionuclide data, we identified, dated, and quantified centimeter‐scale gaps related to surficial remobilization. Temporal correlation to the three largest regional earthquakes attest triggering by strong earthquakes (Mw >8). Also, extremely elevated shear strength values suggest a strong influence of seismic strengthening on shallow sediments. We show that despite enhanced slope stability by seismic strengthening, earthquake‐induced sediment transport can occur through surficial remobilization, which has large implications for the assessment of turbidite paleoseismology and carbon cycling at active margins.ISSN:0094-8276ISSN:1944-800