Microseismicity studies yield insight into the active rifting processes of ultraslow-spreading ridges

We present the results of a pilot study of the microearthquake activity of Gakkel ridge and explain our newly started comprehensive and comparative microearthquake study of ultraslow-spreading ridges.During the international and multidisciplinary Arctic Mid-Ocean Ridge Expedition (AMORE) in 2001, the Alfred Wegener Institute for Polar and Marine Research undertook a first attempt at recording the seismic activity of Gakkel Ridge using drifting ice-floes as platforms for small-aperture seismological arrays. The unconventional technique proved successful and we recorded and localised numerous small earthquakes in the Western Volcanic Zone (WVZ) near 4°W, the Sparsely Magmatic Zone (SMZ) near 16°E and in the Eastern Volcanic Zone (EVZ) near the volcanic center at 85° which showed a long-lasting and strong swarm of teleseismically recorded earthquakes and a submarine eruption in 1999.The microearthquake activity is preferentially concentrated on exposed terrain like the steep rift valley walls or volcanic ridges. We note higher levels of activity on the northern rift valley wall than on the southern rift valley wall in the WVZ and the SMZ. We further made a rare in-situ observation of a submarine eruption by recording a swarm of explosive seismoacoustic signals. These sounds originate at the seafloor of the large volcanic center near 85°E in an area where a massive hydrothermal plume was detected in the water column.The pilot study proved the feasibility of microearthquake studies in ice-covered oceans but the limited data set and localisation accuracy hindered a detailed geological interpretation. We therefore initiated an improved and systematic microseismicity survey of ultraslow-spreading ridges. The plans for this project will be presented.