Description: G33A-0852 The convergent continental margin off Central Chile displays one of the steepest forearc reliefs on earth: the distance from coast to the deformation front, i.e. from 0 to more than 5,000 m water depths just spans less than 100 km. The steep slope is characterized by voluminous submarine landslides, large slumps and deeply incised canyons. On February 27, 2010 this area was shaken heavily by one of the largest earthquakes ever recorded. How did this mega-event affect the seafloor morphology? Did it re-shape the submarine landscape? Did it create new slumps and slides on the continental slope? Were pre-existing fault and slide scarps modified? The pre-event geomorphology is well displayed in a detailed bathymetric map based on a compilation of data from more than 10 cruises with German RV Sonne and Meteor, Chilean RV Vidal Gormaz and British RRS James Cook to the area since 1995. In the framework of the "Rapid Response" project SIOSEARCH (Scripps Institution of Oceanography's Survey of the Earthquake and Rupture Offshore Chile) the same area was surveyed immediately after the earthquake by US RV Melville. Very detailed bathymetric maps were compiled from data of the new Kongsberg EM122 multibeam system onboard RV Melville. Both datasets allow for an unprecedented "before" and "after" comparison of the morphology of the part of the continental slope that was hit by the earthquake. Both datasets were carefully processed applying the same algorithms to achieve comparable high-resolution maps. The high data density allowed to create digital terrain models on a grid with cell sizes as small as 50 m down to a water depth of 5000 m. Additional information from the backscatter and acoustic imagery recordings was also taken into account. So far, a thorough inspection and comparison of pre- and post-event morphology revealed surprisingly small changes, however processing and interpretation of the data is still going on.

Description: Kongsberg (EM120, EM1002) and ELAC (SB3050) multibeam systems of low to medium frequencies and various subbottom profilers were used to analyze the seafloor of the Baltic Sea between twenty and one hundred meter water depth. The working areas are characterized by soft mud allowing for significant penetration by both subbottom and multibeam signals, especially if lower frequencies were used. Locally shallow gas was found transforming the low-reflectivity mud acoustically into a strong volume scatterer. Single beam subbottom profiles across these shallow gas areas show distinct blanking effects below one and four meters below the seafloor. We demonstrate that low frequency multibeam systems are ideally suited to map those shallow gas areas over the entire swath of 140°. First the depth of the working areas was successfully determined with the shallow to mid-water 95kHz multibeam system. No backscatter anomaly was found while crossing the transition zone between mud and gas-bearing mud. In contrast a 12kHz survey over the same location reveals several meters deeper soundings. The resulting bathymetric data mimics the subbottom morphology of a till structure rather than the seafloor. The reason is strong penetration into the mud up to ten meters, even though the system was manually optimized for correct bottom detection. This makes the 12kHz system prone to subsurface mapping of strong reflectors within very soft sediments. High scattering gas bubbles embedded in the mud could be mapped by backscatter anomalies and misinterpretation of the shallow gas front as bottom echoes occurred. Angular range backscattering strength analysis suggests distinct differences between gassy and non-gassy areas and demonstrates the sensitivity of the low frequency multibeam sounder on free gas even on the very outer beams of the swath. The data is groundtruthed by subbottom profiling and geochemical sampling both indicating free gas. Even small gas pockets of only a few meters extension can be resolved demonstrating the advantages of high resolution and large coverage multibeam mapping compared to single beam surveys. Similar results were gathered using a mobile 50kHz system. (a) Backscatter amplitude chart of EM120. The red rectangle focuses on a transition zone between blue color/no-shallow-gas and red color/shallow-gas area; the inlet shows amplitude data from the 95kHz system not showing any transition. (b) PARASOUND subbottom data. The transition zone (red arrow) between shallow gas and no shallow gas plots exactly at the same location as seen in the multibeam data (a).

Description: T13A-2185 Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. We use morphological structures in multibeam bathymetric data across the forearc to identify the extent of the seismogenic zone (SZ). The updip limit of the SZ is associated with a distinct slope break at the seawordmost part of the outer arc high off Java and the Lesser Sunda islands. In contrast, the slope break is rather indistinctive off large parts of Sumatra. The inner wedge shows differences along the Indonesian margin. Uniform trench-parallel ridge structures lie off Java and Lesser Sunda islands, whereas non-uniform trench-parallel outer arc high structures consisting of several broad tectonic ridges off Sumatra shape the seaward part of the inner wedge. The landward termination of the inner wedge ridge structure and a shallow upper plate mantle at a depth range of ~15-25 km at Java and the Lesser Sunda islands coincide with the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. Here, the downdip limit of the seismogenic zone is situated at depths of ~30-40 km, which coincides predominantly with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/island arc upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault. Off Sumatra the SZ is up to more than twice as wide as off Java and the Sunda islands, enlarging the unstable regime off Sumatra and thus the risk of sudden stress release in a great earthquake.

Description: C23B-0603 Ilulissat Icefjord in West-Greenland is the fastest and most productive iceberg calving area outside Antarctica. Changes in climate exert a first-order control on the recession of the icefront and the calving of the icebergs. Glacial and geological processes related to iceberg calving and transport shape the morphology of the seafloor in the area characteristically. Revealing the morphology by high-resolution bathymetric mapping helps to understand these processes. During a cruise with RV Maria S. Merian in summer 2007 large parts of the area were mapped with Kongsberg EM120 and EM1002 multibeam systems. This data was complemented by a survey using a portable Seabeam 1180 multibeam system temporarily mounted on the small local vessel Smilla which could navigate through areas inside the icefjord inaccessible to large research vessels. A comprehensive image of the morphology of the area was achieved by compiling and merging both datasets. Different morphological features such as ridges, shaped like drumlins and valleys which could be connected to channel systems, directing debris flows to a deposition centre characterize the central part of the survey area. Here, a series of prominent circular features 80m to 150m in diameter and up to 30m deep have been found and are interpreted as pockmarks. A parasound sediment echosounder profile across one of the pockmarks documents the absence of the upper sedimentary unit inside. Furthermore, a blank zone in the central part indicates uprising fluids or gas. The northeast - southwest alignment of the pockmarks points to a formation related to slides, faults, and iceberg furrows. The depth of their occurrence indicates a formation by dissociating gas hydrates. The most recent active pockmarks are located in the centre and the northeastern end of the depression in a depth of 395m. The gas hydrate stability zone in arctic regions tapers out at around 400m at 3° bottom water temperature which coincides with the values measured with a CTD close to this position. The decreasing age from southwest to northeast could be explained by changing water temperature coupled to sea level rises. The gas hydrate stability zone would migrate upward with rising sea level.