LAUSR

21 Beneath ultra-slow spreading ridges, the oceanic lithosphere remains poorly understood. Using 22 recordings from a temporary array of ocean bottom seismometers, we here report a ~17-days23 long microearthquake study on two segments (27 and 28) of the ultra-slow spreading Southwest 24 Indian Ridge (49.2° to 50.8° E). A total of 214 locatable microearthquakes are recorded; seismic 25 activity appears to be concentrated within the west median valley at segment 28 and adjacent 26 nontransform discontinuities (NTDs). Earthquakes reach a maximum depth of ~20 km beneath 27 the seafloor, and they mainly occur in the mantle, implying a cold and thick brittle lithosphere. 28 The relatively uniform brittle/ductile boundary beneath segment 28 suggests that there is no 29 focused melting in this region. The majority of earthquakes are located below the Moho 30 interface, and a 5-km-thick aseismic zone is present beneath segment 28 and adjacent NTDs. At 31 the Dragon Flag hydrothermal vent field along segment 28, the presence of a detachment fault 32 has been inferred from geomorphic features and seismic tomography. Our seismicity data show 33 that this detachment fault deeply penetrates into the mantle with a steeply dipping (~65°) 34 interface, and it appears to rotate to a lower angle in the upper crust, with ~55° of rollover. There 35 is a virtual seismic gap beneath magmatic segment 27, which may be connected to the presence 36 of an axial magma chamber beneath the spreading centre as well as focused melting; in this 37 scenario, the increased magma supply produces a broad, elevated temperature environment 38 which suppresses earthquake generation. 39