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SUMMARY In 1999, an unusual earthquake swarm at the 85°E/85°N volcanic centre on the ultraslow spreading Gakkel Ridge, Arctic Ocean, was detected teleseismically. The swarm lasted over 9 months and counted 252 events with mb≥ 3.1. It represents the strongest and largest ever recorded mid-ocean ridge earthquake swarm, and it occurred at a site where spreading rates are only about 10 mm yr-1. We relocated the earthquake swarm comparing the performance of three different localization algorithms: (1) the absolute least-squares routine HYPOSAT, (2) the absolute probabilistic routine NonLinLoc and (3) the relative least-squares routine Mlocate. The epicentres as calculated by each algorithm mostly did not agree within their error ellipses. Thus, the choice of location algorithm proved more critical than, for example, the choice of a local velocity model. We compiled a set of well-localized events which closely agree in at least two routines, mostly Mlocate and NonLinLoc. We conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events. Our geological interpretation revealed three phases in swarm activity: In the first phase from January 17 up to February 1 fracturing of the crust took place, either as a result of or enabling magmatic intrusion. Seismicity in the second phase from February 2 to April 6 expanded along- and across axis. It showed signs of magmatic interaction, but a clear dyke migration pattern is absent. At the beginning of the third phase, a distinct break in the event rate suggested a change in the physical process, either an adjustment of the stress field to the new regime or a transition to an effusive stage. [ABSTRACT FROM AUTHOR]

With its remote location in the ice-covered Fram Strait, Lena Trough is a poorly known segment of the global mid-ocean ridge system. It is a prominent member of the ultra-slow spreading mid-ocean ridges but its spreading mechanisms are not well understood. We relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in Lena Trough and the adjacent Spitsbergen Fracture Zone (FZ). During two cruises with RV Polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of Lena Trough. We could identify and localize microseismic events which we assume to be present in the entire rift valley. In contrast, our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along Lena Trough with earthquakes occurring predominately along the western valley flanks of Lena Trough. In 2009 February/March, several high-magnitude earthquakes peaking in an M 6.6 event occurred in an outside-corner setting of the Spitsbergen FZ. This is the strongest earthquake which has ever been recorded in Fram Strait and its location at the outside-corner high of the ultra-slow spreading ridge is exceptional. Comparing the seismicity with the magnetic anomalies and high-resolution multibeam bathymetry, we divide Lena Trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the South Lena FZ. We propose that a complex interaction between the former De Geer Megashear zone, which separated Greenland from Svalbard starting at Late Mesozoic/Early Cenozoic times, and the developing rift in the southern Lena Trough resulted an increasing eastward dislocation towards the Spitsbergen FZ between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain. [ABSTRACT FROM AUTHOR]