Seismogenic behavior of symmetric vs. detachment-dominated sections of the Northern Mid-Atlantic Ridge

Abstract:

Normal faults are essential in shaping the seafloor formed at slow-spreading mid-ocean ridges, and information on their behavior on short (seismic cycle) time scales is limited. Here we combine catalogs of hydro-acoustically recorded [Bohnenstiehl et al., 2000; 2002] and teleseismic earthquakes to analyze the modes of seismic moment release along the Northern Mid-Atlantic Ridge between the Marathon (12ºN) and Oceanographer (35ºN) Fracture Zones. Along about 50% of the ridge axis, tectonic extension is taken up by steep, symmetric conjugate faults that bound a well-defined axial valley. By contrast, along the remaining 50% most of the tectonic strain is accommodated by large-offset detachment faults [Escartín et al., 2008].

Upon declustering the catalogs to remove major aftershock sequences, we estimate seismicity rates and infer seismic moment release along the ridge axis by converting hydro-acoustic source level to moment magnitude. The shape of the resulting cumulative moment release curves is then investigated at individual ridge sections defined by tectonic interpretation of seafloor morphology. Qualitatively, the seismogenic behavior of symmetric sections appears more stick slip-like (with large events separated by a few years of relative quiescence) than that of detachment-dominated sections, which feature more frequent, smaller events.

We quantify these differences by measuring relevant parameters (e.g., background seismicity rates, b-values...) along different tectonic ridge sections, as well as along sliding windows with no prior knowledge of the seafloor morphology. Finally, we assess possible relations between the observed contrasted seismogenic behavior and the systematic variations in fault geometry and rheology (dip, offset, cross-axis and down-dip extent, presence of weak minerals in the fault zone) inferred from seafloor observations along slow-spreading ridges.