Dependence of seismic coupling on normal fault style along the Northern Mid‐Atlantic Ridge

Abstract

AbstractWhile normal faults are essential in shaping the seafloor formed at slow spreading mid‐ocean ridges, information on their behavior on short (seismic cycle) time scales is limited. Here we combine catalogs of hydro‐acoustically and teleseismically recorded earthquakes to characterize the state of seismic coupling along the Northern Mid‐Atlantic Ridge (MAR) between 12°N and 35°N. Along this portion of the MAR axis, tectonic extension is either taken up by steep conjugate faults that outline well‐defined ridge‐parallel abyssal hills, or dominantly by a large‐offset detachment fault on one side of the axis. We investigate variations in seismicity and seismic moment release rates across 30 ridge sections that can be clearly characterized either as abyssal hill or detachment bearing. We find that detachment‐bearing sections are associated with significantly greater seismicity and moment release rates than abyssal hill‐bearing sections but show variability that may reflect the along‐axis extent of individual detachment faults. Overall, the measured seismic moment release rates fail to account for the long‐term fault slip rates. This apparent seismic deficit could indicate a mixed‐mode of fault slip where earthquakes only account for ∼10–30% of offset buildup at abyssal hill faults, while the rest is accommodated by some form of transient aseismic creep. We find this seismic coupling fraction to be significantly greater (∼40–60%) at individual detachment systems, which is somewhat at odds with the common inference that detachment faults can sustain long‐lived localized strain because they are weak. We therefore propose alternative interpretations for seismic coupling based on dynamic friction theory.