From 2012 to 2013 two networks of ocean bottom seismometers were installed on the Southwest Indian Ridge (SWIR) spreading axis at contrasting segments that feature end-members in terms of magmatic and amagmatic lithospheric accretion. In total, several thousands of local earthquakes at magnitudes of 0.5 – 3.0 Ml could be located at both sites that allow insights at unprecedented detail into the lithosphere structure and modes of deformation at ultraslow spreading ridges. Here we present key results of both experiments. Beneath the network deployed at the amagmatic Oblique Supersegment frequent earthquakes down to 35 km depth imply an unusually thick and cold lithosphere. The shallow lithosphere down to 15 km is devoid of earthquakes, suggesting hydrothermal circulation and serpentinization to reach these depths, causing an entirely aseismic mode of deformation. We further observe a strong relation between sea floor petrology, topography and the distribution of microearthquakes. Beneath the network deployed at the magmatic SWIR Segment-8 volcano a prominent aseismic zone was observed implying elevated temperatures and seismicity data provide strong indications for ongoing diking activity. Local earthquake tomography indicates a zone of partial melt. The undulating brittle-ductile transition identified at both sites along the ridge axis provides geophysical evidence for a pronounced topography of the Lithosphere-Asthenosphere-Boundary that allows melts to flow along-axis upslope towards magmatic centers.