In 1999 an unusually strong, teleseismically registered earthquake swarm at the volcanic complex at 85°E/85°N marked the onset of a spreading episode at Gakkel ridge with deep submarine explosive volcanism. At this site, the rift valley hosts distinctive volcanic features which cluster on ridge-parallel faults or fissures. Activity persisted at least until 2001, seafloor imagery in 2007 showed fresh lava at volcanic cones. Three arrays of four seismometers each which were mounted on ice floes, drifted 16 days in July 2007 with a drift speed of up to 1 km/h over a region of 60 km x 70 km. We recorded over 300 local earthquakes. Of these, we used 124 earthquakes which were recorded by two or more arrays for a local tomography. They yield a dense ray coverage of the centre of the rift valley and the rift valley walls. We chose a 1D velocity starting model by selecting the best performing out of 90 randomly selected models and used the resulting hypocentres as starting locations for the tomographic algorithm. We complemented the 1D-velocity model with a 3D-water layer (vp=1500 m/s) whose thickness varies between 2.8 and 4.3 km to take the bathymetry in the survey region into account. Here we present the results of a seismic tomography for three-dimensional P-velocity structure in the crust and upper mantle with the algorithm FMTOMO: Most of the earthquakes cluster at a depth of 11 to 15 km, directly beneath or closely around the centrally located volcanic cones, with only slight activity in the surrounding area. The velocity variations indicate reduced seismic velocities (up to 0.2 km/s variation) in the centre of the rift valley around the site of most recent activity, stretching down from the upper crust to the assumed Moho at 11 km depth. A region of increased seismic velocities (up to 0.2 km/s variation) lies directly to the southeast, also in the middle of the rift valley.