Description: New oceanic crust is formed the mid-oceanic ridges (MOR), where the lithospheric plates drift apart. The structure and characteristics of MOR differ with spreading rates being either fast (80-100mm/a), intermediate (55-70mm/a), slow (20-55mm/a), or ultraslow spreading ridges (〈20mm/a). In comparison with faster spreading ridges the ultraslow spreading ridges are poorly investigated, their spreading mechanisms and parameters for generating volcanic centres are relatively unknown. Local earthquakes analyses can be used for a better understanding of the thermal and mechanical condition of the lithosphere at MOR. The Southwest Indian ridge (SWIR) separates the Antarctic and African plates and with a full spreading rate of 14mm/a belongs to the ultraslow spreading ridges. The segment-8 volcano (65.60E, 27.65S) is located at the north eastern part near the Rodriguez triple junction. I processed nine months of continuous seismic data recorded by the first local network of eight Ocean Bottom Seismometers (OBS) in this area. After automatic event identification, arrivals of P- and S-phases were hand-picked. Locations of 2996 hypocentres could be calculated using the linear location algorithm Hyposat. Furthermore, I used the relative location algorithm hypoDD to improve the standard earthquake location and additional waveform cross-correlation of Pphases to increase the accuracy of phase onset times. With detailed analyses of the parameters for the inversion with hypoDD I could relocated 2083 events. I received a sharp image of seismicity confined to the rift valley with local magnitudes in the range -0.9 up to 3.8. Relative locations are on average accurate to each other within 0.025 km. I identified spatial clusters around the segment-8 volcano with considerable background seismicity throughout the recording period. Receding from the centre of the volcano, focal depths increase continuously to 〉20 km, defining the boundary between brittle-ductile behaviour of the lithosphere. The results show a noticeable zone with a radius of 15km without any occurrences of earthquakes located beneath the central volcano. This aseismic zone has a remarkable low-velocity anomaly, which indicates increased temperatures. Along-axis, I find another aseismic zones wich I charactirized as serpentized peridotis because of the losses of the shear strength. With the impressiv depths of the hypocenters I find evidence that supportes the idea of a cold, thick and brittle Lithosphere of ultraslow spreading ridges. Depending on the model from Standish et al. [2008], I could established the hypothetical conception of a pronounced Lithosphere- Asthenosphere-Boundary (LAB) which provides the focussing melt supply over large distances towards the isolated volcanic centers along-axis.

Description: What are the challenges and peculiarities of deploying seismic instruments at the abyss of the stormy Southern Ocean? How can local earthquakes help us to understand the complex magmatic and amagmatic processes of creating pristine oceanic lithosphere? Reporting from two recent experiments at the Southwest Indian Mid-ocean Ridge the above questions will be tackled and results of seismicity and local earthquake tomography studies will be presented.

Description: The area in north-east Siberia, adjacent to the Laptev Sea of the Arctic Ocean, inherits a complex geological setting, which is to date not fully understood. Located north to the Laptev Sea, the ultraslow spreading Gakkel Ridge is propagating into a continental rift system – the Laptev Sea Rift. This rift separates the North American plate from the Eurasian plate and presents a rare opportunity to investigate mechanisms of recent continental breakups. In general, divergent plate boundaries are accompanied by magmatic and earthquake activities. The earthquake activity at the Gakkel Ridge shows a sharp image of seismicity, confined to the rift valley extends to the continental shelf of the Laptev Sea. Though, the Laptev Sea region indicates less and more diffuse seismicity and an absence of magmatic activity. So far, the local (micro-)seismicity of the Laptev Sea region is poorly described, due to a lack of local seismological stations and data sets. In summer 2016, as part of the project SIOLA, 25 seismological stations were temporarily installed in the Lena Delta. A network of 12 seismometers were located in the Lena Delta along the active seismic Olenek Fault Zone and another 13 stations as a seismological array close to Tiksi. Over a period of nine months, these instruments continuously recorded seismic data, to investigate the geodynamic processes of the Laptev Sea Rift by analysing the local seismicity. Furthermore, new data recently collected by members of the project and expand the existing data base. We will report the progress of the project and show some data and preliminary results.

Description: At mid-ocean ridges volcanism generally decreases with spreading rate but surprisingly massive volcanic centres occur at the slowest spreading ridges. These volcanoes can host unexpectedly strong earthquakes and vigorous, explosive submarine eruptions. Our understanding of the geodynamic processes forming these volcanic centres is still incomplete due to a lack of geophysical data and the difficulty to capture their rare phases of magmatic activity. We present a local earthquake tomographic image of the magma plumbing system beneath the Segment 8 volcano at the ultraslow-spreading Southwest Indian Ridge. The tomography shows a confined domain of partial melt under the volcano. We infer that from there melt is horizontally transported to a neighbouring ridge segment at 35 km distance where microearthquake swarms and intrusion tremor occur that suggest ongoing magmatic activity. Teleseismic earthquakes around the Segment 8 volcano, prior to our study, indicate that the current magmatic spreading episode may already have lasted over a decade and hence its temporal extent greatly exceeds the frequent short-lived spreading episodes at faster opening mid-ocean ridges.