Seismicity and structure of a magmatic accretionary centre at an ultraslow spreading ridge: The volcanic centre at 85°E/85°N, Gakkel Ridge

Mid-ocean ridges are divergent plate boundaries. Yet, there are important differences between these individual ridges, which seem to be linked to the rate with which they spread apart. The appearance of 'ultraslow' spreading ridges is drastically different from faster spreading ridges. Yet, only few surveys have so far been conducted. In 1999, an exceptional earthquake sequence was teleseismically registered. It was unusual in the magnitudes involved, the number of events registered, and the duration of activity (9 months). It originated at 85°E/85°N at a volcanic centre in the eastern part of Gakkel Ridge. I analyzed this sequence, relocating the entire teleseismic earthquake sequence with three different localization algorithms. I also analyzed a 16-day local dataset which had been collected in 2007, using seismometer arrays installed on ice floes which drifted over the survey area. It shows the seismological aftermath of the spreading episode of 1999 at Gakkel Ridge, resultin in the first local earthquake tomography ever done at an ultraslow spreading centre.