Hydrothermal seismicity beneath the summit of Lucky Strike volcano, Mid-Atlantic Ridge

We present two years (July 2007–July 2009) of earthquake locations from a local seismological network on the Lucky Strike volcano (Mid-Atlantic Ridge), whose summit hosts one of the most active and largest deep-sea hydrothermal fields known. Two clusters of small (ML<0.8) but continuous seismicity with well-defined lower depth limits extend north and south along-axis from the hydrothermal field. The lower limit of the northern cluster (79% of events) is a few hundred meters above the axial magma chamber (AMC) reflector, whereas the lower limit of the southern cluster (7% of events) is 600 m shallower. During a 3-month long event swarm in April–June 2009, the northern clusterʹs lower limit deepened by 50–100 m and two other event clusters were activated: one to the east of the hydrothermal vent fields (5% of events) and another beneath the fields (2% of events). We interpret the continuous background events in the northern and southern clusters as adjustments within a narrow weak axial region due to stress created by thermal contraction events at the bottom of the hydrothermal circulation zone or in the AMC. We interpret the swarm as an episode of hydrothermal penetration toward the AMC that may have been activated by a broader, AMC-level, contraction event. We propose that a narrow axial region of lower upper crustal porosity, combined with the sloped top of the AMC, generates along-axis hydrothermal circulation towards the vent fields. The hydrothermal circulation may in turn modify the AMC topography through enhanced cooling. The AMC reflectorʹs complex topography suggests that the volcano is in a waning phase in which the AMC has already been significantly altered since the latest magma emplacement.