The mid-ocean ridge system is the longest mountain range in the world, running across all the worlds oceans. At these places, lithospheric plates drift apart and �18km3 of new oceanic crust is formed every year. The ultraslow spreading Southwest Indian ridge (SWIR) represents a global end-member of the mid-ocean ridge system, with a full spreading rate of �14mm/a (at 64 �E, 28 �S). It separates the Antarctic and African plates. At such little spreading rates reduced melt supplies are observed. Further observations suggest that the SWIR can be subdivided in segments which spread magmatically active and segments where amagmatic spreading processes are dominant. A conceptual model explaining these biassed melt regimes has been proposed, however, until now no striking evidence for this theory has been presented. In context of the expedition M101 with the research vessel FS Meteor in late 2013, 57 Ocean Bottom Seismometers (OBSs) have been recovered. The OBS devices recorded seismic data for a period of 13 months. I analysed 90GB of data of 8 OBS which have been deployed at a narrow area, the segment-8 at the SWIR. A STA/LTA trigger algorithm was used to extract 6,288 seismic events from the data. OBS clock drifts have been corrected applying a newly developed tool called ambient noise cross-correlation. Seafloor positions of the OBS have been calculated based on their deployment positions and under the assumption of uniform, constant and seasonally independent water currents which are free of tidal forces. I consecutively calculated 479 hypocentres of local earthquakes, using the earthquake location routine HYPOSAT and an one-dimensional velocity model. The results for the first time reveal strong evidence for a pronounced lithosphereasthenosphere boundary at which melt could be guided towards volcanic centres of ultraslow spreading ridges. I show that earthquakes at the magmatic segment-8 at occur as deep as 17km below the seafloor. Earthquake depths furthermore shallow towards the volcanic centre, in and beneath the volcano I find a pronounced seismic gap of a width of �20 km. I support the assumption of a cold, thick and brittle lithosphere. The results show, local seismicity studies with the aid of OBS networks represent a valuable tool to understand tectonic, magmatic and hydrothermal processes at ultraslow spreading ridges.