As part of a global study of the source characteristics and tectonic implications of large earthquakes on mid-ocean ridges, we report on the focal depths and mechanisms of the six largest earthquakes that have occurred in the last 20 years on the Arctic mid-ocean ridge system. For each earthquake we invert the long-period P and SH waveforms to estimate the parameters of the best fitting point source, including seismic moment, centroid depth, double-couple source orientation, and source time function. Three of the earthquakes occurred on the oceanic spreading center in the Eurasian Basin, along ridge segments spreading at half rates of 4--6 mm/yr. These events have mechanisms very similar to those of ridge crest earthquakes on the Mid-Atlantic Ridge: almost pure normal faulting on planes that dip at approximately 45¿ and strike parallel to the rift axis, moments of 4--5¿1024 dyn cm, centroid depths of 1--2 km beneath the seafloor, and water depths (inferred from the predominant period of water column reverberations) appropriate to epicentral locations within the median valley.

The remaining three earthquakes, also characterized by normal faulting, are associated with the continuation of the divergent plate boundary (2--3 mm/yr half rate) onto the continental shelf of the Laptev Sea, where the crust becomes transitional in nature. One of the largest known spreading center earthquakes (August 25, 1964, M0=1¿1026 dyn cm) occurred where the oceanic ridge intersects the outer edge of the continental slope. Waveform inversion for this event can resolve unilateral rupture from north to south (landward) along a fault at least 30 km in length. The preferred centroid depth is 5 km beneath the seafloor in crustal material with an unusually low shear velocity, but a centroid depth as great as 15 km cannot be ruled out. Two earthquakes beneath the continental shelf have significantly greater centroid depths (10--20 km) than mid-ocean ridge earthquakes, indicating a thicker brittle regime and a cooler thermal structure than are typical of oceanic spreading centers. The tectonic environment of these events is more representative of rifted continental lithosphere than of a mid-ocean ridge.