Jan H. Behrmann

Submarine Mass Wasting Off Southern Central Chile: Distribution and Possible Mechanisms of Slope Failure at an Active Continental Margin

Around 5–6% of the convergent continental margin of Southern Central Chile (33–42°S) is shaped by a variety of submarine mass wasting processes. We use swath bathymetric data covering >90% of the continental slope to map and investigate mass wasting-related seafloor features. In total, 62 submarine landslides are found that we separate into four categories (slides related to canyons, slides on open slopes, lower slope collapses and giant slope failures) with different failure mechanisms, preconditioning factors and time scales.

Size-Frequency Relationship of Submarine Landslides at Convergent Plate Margins: Implications for Hazard and Risk Assessment

We use complete inventories of submarine landslides from the Middle America (MA) and the Central Chile (CC) trench and forearc systems to analyze the size-frequency relationship of such structures on active continental slopes. The MA forearc is characterized by subduction erosion, and the CC forearc has had an accretionary tectonic history since the Late Neogene. Both are end-member types of convergent margins around the world. Both margin segments have been mapped by high-resolution swath bathymetry at strike lengths of about 1,300 km (MA) and 1,000 km (CC). The Middle America forearc has 143 discernible slides with sizes ranging from 0.38 to 1,426 km2. Offshore Central Chile, the 62 mapped slides are 0.9–1,285 km2 in size. Slide localization is markedly different at both margin types. While they also vary strongly along strike of the individual margin, depending on forearc slope gradient, kinematic coupling between plates, or topographic structure of the downgoing plate, the size-frequency relationships are remarkably similar. This allows quantification of the incidence of a submarine slide of given size per margin segment. The relationships hold for slide sizes from 10 to 1,000 km2, with the cut-off defined by slide size (smaller slides) and sample size (larger slides). As slide traces of 100–300 km2 size are obliterated by tectonic deformation after about 200,000 years, recurrence rates for slides of a given size can be estimated. This offers a chance to assess hazard and risk resulting from such events. It is suggested that it takes 20 to 200 plate boundary earthquakes to set off a medium-sized (>10 km2) or larger slump or slide.

Submarine slides, Gulf of Mexico continental slope: insights into transport processes from fabrics and geotechnical data

Ursa Basin on the Gulf of Mexico continental slope is a site of extremely fast sedimentation, building thick sequences of underconsolidated and overpressured muds and clays. Frequent sliding created mass transport deposits (MTD). In a study of strength, frictional behaviour, and fabrics of IODP Expedition 308 drillcores we find that mass transport is governed by very low friction coefficients and peak shear strengths of the sediments. The majority of the samples shows velocity weakening, enabling runaway instabilities in the sediment once deformation has started. While sediments at the bases of MTD seem to strengthen by the sliding, those below the bases remain weak, constraining a hazard for slide reactivation. Submarine sediment sliding leaves a strong and irreversible imprint, changing fabric geometries, and reducing the pore space. This is a transport phenomenon leading to expulsion of large amounts of pore fluids during sliding. MTD transport is probably as cohesive bodies, defining a considerable geohazard potential.

Fahrtbericht / Cruise Report SO244/1 - GeoSEA: Geodetic Earthquake Observatory on the Seafloor, Antofagasta (Chile) - Antofagasta (Chile), 30.10.2015 - 24.11.2015

The majority of M>8.5 active plate boundary earthquakes has hypocenters located beneath the oceans in subduction zones. Post-hoc analysis shows that most of the surface deformation related to co-seismic stress release takes place on the seafloor, in many cases unleashing major tsunamis. The structure and morphology of the seafloor and shallow subbottom thus stores crucial information on sub-seafloor processes, such as permanent deformation by seismic slip or aseismic creep within the overriding plate and earthquake and tsunami generation. We have mapped the seafloor seaward of the Northern Chilean coast between about 19°S and 22°S down to the Northern Chile deep sea trench, using the ship-based Multibeam Echosounder EM122, Parasound, and AUV (autonomous underwater vehicle) – in selected subareas - at sufficient resolution to identify active tectonic fault structures and submarine mass wasting structures, to quantitatively assess young and active deformation of the overriding plate in the area, and quantify the extent of recent catastrophic downslope mass movements of sediment. Furthermore, the investigations serve as a site survey for the deployment of the GeoSEA seafloor geodetic array, to be installed immediately after completion of this cruise. The investigations were made timely by the 1st April 2014 Pisagua M=8.2 earthquake, that ruptured the plate interface in the northern part of the area of investigation. The central and southern parts are located in the last remaining locked seimotectonic segment along the Chilean active margin. In addition to providing the first data base for geomorphological and tectonic interpretation of geo-processes at the seafloor, the bathymetric mapping done during SO244 Leg 1 will provide an important data reference for possible post-earthquake surveys once this seismotectonic segment will have broken in the future.