Crustal structure and seismicity of the ultraslow spreading Knipovich Ridge

The Knipovich Ridge situated in the North Atlantic Ocean represents an ultraslowspreading \nmember of the mid-ocean ridge system. Its spreading direction is highly \noblique (35° - 50° between the spreading axis and the normal to the ridge trend) with \na spreading rate of ~ 15 mm/yr. During the AWI expedition ARK-XXIV-3 a 125 km \nlong N-S orientated seismic refraction profile between 75° 30’ N and 77° 15’ N was shot \nalong the Knipovich Ridge valley. The obtained velocity distribution was used to locate \nlocal earthquakes which were acquired using a local OBS array. \nThe crustal thickness along the Knipovich Ridge varies from ~ 5.5 km below strong \nmagmatic centres and ~ 4 km between these centres in the amagmatic part. Just before \nthe southern strong magmatic centre the crust thins temporarily to 3.4 km. Seismic \nvelocities of the crust do mostly not exceed 6.6 km/s . The velocity distribution of the crust \nsuggests the absence of an oceanic Layer 3. The obtained upper mantle velocity is 7.6 km/s . \nDuring two days of processed local earthquake data 85 earthquakes were located with \na maximum magnitude of Ml= 2.7. The detection threshold of the local OBS array was \nMl= 1.3. The earthquake distribution is scattered along the ridge valley with slightly \nclustering. Two third of the earthquakes were located within the upper mantle which \nindicates that the upper mantle is not strongly serpentinised.