Microseismicity of the ultraslow-spreading Gakkel ridge, Arctic Ocean: a pilot study

S U M M A R Y The active mid-ocean ridge of the Arctic Ocean, named Gakkel ridge, is the slowest spreading ridge of the global system of mid-oceanic ridges with full spreading rates declining from about 12.5 to 6 mm yr−1 from west to east. Geological models of seafloor spreading predict a decreasing intensity of magmatic processes with decreasing spreading rate. In summer 2001, the multidisciplinary Arctic Mid-Ocean Ridge Expedition (AMORE2001) discovered robust magmatism at western Gakkel ridge, an amagmatic section further east and pronounced volcanic centres at eastern Gakkel ridge. During AMORE2001, an attempt was made at recording the microearthquake activity of the ridge which allows important insights into the character and dynamics of active crustal accretion at the ridge axis. Due to the permanent ice cover of the Arctic Ocean, the use of ocean-bottom seismometers bears the risk of instrument and data loss. In this pilot study, we used for the first time drifting ice floes as platforms for small seismological arrays. The arrays consisted of four three-component seismometers equipped with GPS devices and arranged as a triangle with a central seismometer and a side length of about 1 km. Three such arrays were deployed in different rift segments and recorded the seismic activity continuously for 5–11 days at a sampling rate of 100 Hz. The array technique allowed to distinguish clearly between icequakes and earthquakes and to localize the earthquake source to within few kilometres or less depending on epicentral distance. We intensively discuss the detection capabilities and the location accuracy of this single array on a drifting ice floe. Earthquake magnitudes could not be calculated in our pilot study but are estimated to be significantly smaller than magnitude 2 by comparison with a regional earthquake of known magnitude. Furthermore, we analyse the characteristics of the recorded seismic events ranging from long waveforms of regional events to short local events with reverberations in the water column. All of the arrays recorded numerous microearthquakes in the central rift valley and on its flanks which we interpret as tectonic earthquakes. A swarm of microearthquakes was localized with high accuracy underneath the crest of a volcanic ridge in the rift valley and is proposed to have magmatic origin. The pilot study was thus successful in detecting, localizing and interpreting microearthquakes below magnitude 2 at Gakkel ridge. However, we suggest improvements of the method for a comprehensive microearthquake survey of Gakkel ridge which should aim at an understanding of active magmatism and faulting at ultraslow-spreading ridges.