Seismicity along the ultraslow spreading Knipovich Ridge near the Logachev Seamount

Hot material from the asthenosphere rises along mid oceanic ridges. The processes behind the magma supply at fast and slow spreading ridges are fairly known. However, for ultraslow spreading ridges, various theories exist, how the magma supply and the plumbing system looks like, but geophysical evidence supporting these models is lacking. The predominantly used model describes mantle upwelling cells spaced every 85 – 100 km, which generate magmatic robust segment centers and feed magmatically weaker segments by lateral melt migration. The upwelling ductile magma creates a localized hotter mantle area. In 2009 during the Polarstern expedition ARK – XXIV/3 a network of broadband ocean bottom seismometers was deployed for 10 days between 75°50' N and 76°50' N along the ultra – slow spreading Knipovich Ridge. In this time period over 900 local earthquakes were recorded. The local velocity model provided by seismic refraction data in this region was used to localize these microearthquakes with Hyposat. The events were selected after their root- mean -square value, their error ellipses and their greatest azimuthal angle without observation (GAP) to ensure a high quality of the event location. Furthermore, the stability of the hypocenters was tested with different localization programs. After the identification of highly reliable events the tectonic structures become visible. We interpret a locally high concentration of earthquakes in the crust as an indication of a detachment fault. However of greater interest is a zone, where no earthquakes occur in the uppermost mantle underneath the Logatev Seamount. Since earthquakes reflect the released energy of a sudden stress drop caused by brittle failure, the absence of this failure suggest a higher temperature in this area. The lack of crustal and upper mantle seismicity in a clearly defined area beneath the seamount may thus yield a first convincing evidence for the existence of mantle upwelling cells underneath a magmatic center