Microseismicity of the Eastern Gakkel Ridge, Arctic Ocean

The mid-ocean ridge forms new oceanic lithosphere, which subsides, thickens, and moves away from the ridge axis. It is generally believed that the lithospheric thickness is dependent on spreading rate. At ultraslow-spreading ridges (<20 mm/yr), the lithosphere is expected to thicken substantially due to strong hydrothermal cooling and limited magma supply. However, this view has been challenged by the observed highly variable crustal thickness at the ultraslow-spreading Southwest Indian Ridge and Gakkel Ridge, where their lithospheric structures are poorly understood due to limited passive seismic observations. In particular, the Gakkel Ridge, located in the Arctic Ocean, is the slowest-spreading mid-ocean ridge in the world, but no onsite microseismicity has been reported due to severe ice conditions. The 2021 JASMInE cruise marked the first deployment of Ocean Bottom Seismometers (OBSs) array in the eastern part of Gakkel Ridge. 43 OBSs with spacings of 5-10 km were set up to record both air-gun source signals and natural seismic signals. These instruments were deployed along and across the ridge axis, with a focus on the volcanic area at 85°E, covering a range from 75°E to 102°E. Analysis of seismic data identified 234 microearthquakes that occurred continuously in August 2021, and ~50% of them have uncertainties of <10 km. Their focal depths are located no deeper than 13 km below the sea floor (bsf), with most events located at 0-10 km bsf. This depth range is much shallower compared to the microseismicity observed by seismic stations installed on the ice floes during the 2007 AGAVE expedition, where most events were found between 7-16 km deep. We reanalyzed the seismic data collected during the 2007 AGAVE expedition, and preliminary results indicate that the seismic phases have a very low signal-to-noise ratio, with poorly picked S-wave phases, which may result in the observed differences. Furthermore, the newly observed deepest depth of these seismic events is consistent with the 600°C isotherm as previously calculated, approximately 12.6 km bsf. It is unexpected that no earthquakes were recorded beneath the volcano center where explosive volcanic eruption was reported in 1999. Seismic source mechanism analysis reveals normal faulting near the volcano center, but no volcanic swarm-like events were observed. Instead, most earthquakes were concentrated near the segment end at around 88°E, likely associated with a normal fault inclined southward within the rift valley. In addition to the JASMInE cruise, a small seismic network consisting of five OBSs was deployed in August 2023 at the 100°E volcanic center. These instruments were operated on the seabed for approximately one week, but no microearthquakes were detected. These observations may suggest that, at ultraslow-spreading ridges, despite robust magma supply in magmatic segments, magmatic activity is not vigorous. Crust accretion and episodic volcanic eruptions appear to be short-lived, and for most of the time, the magmatic system remains in a period of seismic quiet.