Local seismicity in the obliquely spreading setting of Fram Strait constrained from ocean bottom seismometers: Implications for fluid flow and methane seepage

Le résumé fourni par la source

The Fram Strait opening is associated with a complex stress regime that results from the oblique relation between two ultra-slow spreading mid-ocean ridges, the Molloy ridge (MR) and the Knipovich Ridge (KR), offset by the Molloy Transform Fault (MTF). Gas-charged thick sedimentary deposits developed over both oceanic and continental crust. Sedimentary faulting reveals recent stress transfer into the sub-surface. However, the mechanisms by which stress accommodates across the west Svalbard margin and its effect on fluid flow and seepage dynamics remain poorly understood. An analysis of earthquake occurrence and focal mechanisms can shed light on the present state of tectonic forces in the area, their origin and potential influence on nearby faults. Conventional studies using land instrumentation provide incomplete seismological records even for such comparatively land proximal settings, due to still large distances to the nearest permanent observatories and a poor azimuthal coverage. We deployed 10 ocean bottom seismometers (OBS) for 11 months between 2020-2021 about 10 km north of the northern termination of KR to investigate patterns of stress transfer off the ridge and the influence on the sedimentary system. OBSs are spaced by about 10 km around an area characterized by fault-related seepage and sedimentary slumps visible on the bathymetry. Using partially automated routines we built a catalogue of local earthquakes and computed their epicenters and magnitudes. Earthquake locations roughly follow the plate boundaries and better focus seismicity along their bathymetric imprint versus the land observations. Along the MTF, we observe that the earthquakes are concentrated southwards on the North American plate and seismicity across the west-Svalbard margin is limited. A large number of earthquakes extend beyond the MTF and KR corner and concentrate at a bathymetric depression, adjacent to the recently revised continental-oceanic transition boundary. Focal mechanisms from past observations show a gradual change from strike-slip movement along the MTF to extensional faulting at the corner. The distribution of earthquakes correlates with highly faulted sedimentary overburden interpreted in high resolution seismic data, and with major structures in gravity and magnetic maps. This suggests an efficient stress release at the plate boundary and little to no transfer northward from the KR termination onto the Eurasian plate. We detected only a few events recorded along the Vestnesa contourite drift and on the continental shelf. These earthquakes may indicate reactivation of crustal faults under the weight of thick sedimentary deposits or other processes such as glacial isostacy. The inferred stress distribution in the region has implications for understanding fault-related gas transport and methane seepage at Arctic margins.

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