Highly Asymmetric Seismicity in a System of Tectonic Extension and Hydrothermal Venting at the Mohn-Knipovich Ridge Bend

In recent years hydrothermal vent systems were found in unexpectedly high abundance along ultraslow spreading ridges, despite their overall decreased magma supply. Thin oceanic crust and resulting shallow heat sources can drive hydrothermal fluid circulation and detachment faults can act as fluid pathways, resulting in e.g., serpentinization of the oceanic crust. So far, no long-term recording of seismicity around hydrothermal vent systems along ultraslow spreading ridges have been reported. Here, we present results from a ~1-year local Ocean Bottom Seismometer deployment between 2019 - 2020 at Loki’s Castle hydrothermal vent field (LCVF) along the Arctic Mid Ocean Ridge. LCVF is located at a water depth of ~2500m on top of the axial volcanic ridge (AVR) at the Mohn-Knipovich Ridge bend where spreading is highly asymmetric from west to east.

For the processing we use a combination of an automatic event detection algorithm (Lassie), a deep-learning phase picking model (PhaseNet) and partial manual re-evaluation of phase picks. Additionally, selected clusters of events are cross-correlated and relocated using hypoDD. The resulting earthquake catalogue consists of a total of 12368 events with 6719 manually re-evaluated and 5649 automatically picked events.

From the results we see that most of the plate divergence at the Mohn-Knipovich Ridge bend is accommodated by a young detachment fault west of the AVR. Most of the seismicity occurs between depths of ~2-8km in a bended band that steepens up to 70° with depth and follows the local topography. However, the described detachment fault differs from reported mature detachment faults at the Mid-Atlantic Ridge or Southwest Indian Ridge. Within the footwall we observe episodical, clustered seismicity with extensional faulting mechanisms, indicating that the detachment could be cross-cut by normal faults. Along strike, the seismicity of the fault plane appears highly heterogeneous, with the central part showing only sparse seismicity at depths below 3km while other segments show episodical shallow seismicity. Towards LCVF seismicity below the AVR increases and the maximum depth of earthquakes shallows by about ~2km. This could indicate the presence of a shallow heat source below LCVF as a driving factor for the hydrothermal circulation.