Seismic constraints on the evolution of hydrothermal circulation beneath Lucky Strike volcano, Mid-Atlantic Ridge

Lucky Strike volcano is the central edifice of the Lucky Strike segment, Mid-Atlantic Ridge. Its summit overlies an axial magma chamber (AMC), 3-3.8 km beneath the seafloor, and hosts one of the largest known deep-sea hydrothermal fields. Local seismicity beneath the hydrothermal field has been monitored since 2007 as a part of the EMSO (European Multidisciplinary Seafloor and water column Observatory)-Azores observatory by 5 OBSs with yearly redeployments. In a 12-year (2007-2019) earthquake catalog (noncontinuous), we observe continuous low magnitude seismicity (M_L ~ -1 to 0), focused mainly 0.5-2 km above the AMC, suggesting that thermal contraction of rocks, possibly combined to deformation induced by volume changes during hydrothermal alteration, at the base of a single limb along-axis hydrothermal cell is the primary source of this seismicity. We thus interpret the seismicity clusters, with horizontal extent 1200 to 1800 m², as zones of enhanced heat extraction, in the lower part of the hydrothermal downflow zone.

We present the evolution of this hydrothermally-induced seismicity over the 12 years of the catalog. We observe three lateral 400-800 m shifts of the main seismicity clusters. The first and second shifts are small and could be explained by a fortuitous combination of network-based biases, picking error and/or change in the shallow seismic velocity structure of the volcano. The third shift, occurring during a catalog gap between June 2013 and April 2015, is ~800 m eastward and corresponds to a change in the seismicity distribution from a patch above the AMC to a vertical pipe-like pattern, indicating a real change in the hydrothermal circulation. We propose that this shift is driven by recent magmatic injections above the AMC, and/or to the opening of new tectonic cracks, enhancing local permeability and allowing for more efficient cooling above the shallower region of the AMC roof.

We also observe three Higher Seismic Activity (HSA seismic rate > 18 events/week) periods: April-June 2009, August-September 2015, and April-May 2016. The 2009 HSA period was the most intense: it lasted ~13 weeks, starting with a relatively higher magnitude event (M_L = 1.7), and culminating in June after another higher magnitude (M_L = 1.8) event. Most of the events clustered 0 to 1 km above the AMC reflector, with a few deeper events (down to only 800 m below the AMC reflector) during the culmination period. Although we do not have focal mechanisms to test this hypothesis, we propose that this HSA period resulted from tectonic events opening enhanced local permeability channels for downgoing hydrothermal fluids, and leading to higher heat extraction by the hydrothermal system.