KNIPAS - Knipovich Ridge passive seismic experimentStudying active mid-ocean ridge spreading processes and lithospheric structure on segment scale

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KNIPAS belongs to the largest passive seismic experiments that have ever been conducted at a mid-ocean ridge (MOR). Along a 160 km long section of Knipovich Ridge, 28 Ocean Bottom Seismometers (OBS) recorded earthquakes over a period of one year. Earthquakes at MORs give insight into the active spreading processes but only the largest earthquakes can be captured by seismometers on land or by hydroacoustic arrays and their epicentre locations and source depths remain uncertain. Small-scale OBS studies give a more detailed image of spreading processes at smaller magnitudes. Recent OBS studies showed that ultraslow spreading MOR, to which Knipovich Ridge belongs, display distinctively different seismicity pattern than faster spreading ridges and therefore different spreading mechanisms. The widely spaced volcanoes of ultraslow MOR may seismically be very active. Amagmatic ridge sections, in contrast, produce only weak earthquakes and show a thick lithosphere with aseismic regions in the upper mantle that may point to serpentinisation reaching far deeper than previously known. It remains however unknown how relevant these locally studied spreading processes are on the scale of entire ridge segments.KNIPAS closes the gap between small-scale OBS and teleseismic studies. With an expected 9000 well-located earthquakes of magnitudes M 1-3 we will examine the thermal regime of the lithosphere, its thickness and structure and the prevalent deformation mode along an entire ridge segment. We will investigate if the topography of the base of the lithosphere is sufficient to warrant an upward flow of melts along an entire segment towards the volcanic centres where magma output exceeds by far the local melt production. In addition, we will examine how spreading works in the magmatic regions between the volcanoes and to what extent a potentially deep-reaching serpentinisation of the upper mantle influences the deformation mode. Furthermore it is currently not known if there are variations in the composition and physical properties of the upper mantle that may govern the long-lived segmentation of ultraslow spreading MOR in magmatic and amagmatic ridge sections.Due to its large spatial extent and the expected pronounced differences in crustal and lithospheric thickness and structure, KNIPAS furthermore forms an ideal data set to test and optimize passive seismic methods that only recently have been introduced in marine seismology. Receiver functions and the inversion of surface waves from teleseismic earthquakes and ambient noise will be used to study variations in seismic velocity and structure of the upper mantle along the ridge. We plan to further develop these methods for routine application to OBS networks and critically examine their resolution potential.