Patterns of faulting and seismicity in the mid-ocean ridge environment

Abyssal hill faults within the slow-spreading (∼20 mm/yr) mid-ocean ridge environment exhibit displacement-length ratios that are systematically larger than those observed within the fast-spreading environment (∼100 mm/yr). This can be explained by considering the importance of fault linkage in the lateral growth of abyssal hill faults and the limits imposed on maximum fault displacement by the lithospheric thickness within each setting. Within the superfast spreading environment (>140 mm/yr) of the southern East Pacific Rise (EPR), a pattern of inward-dipping master faults is observed, with smaller outward-dipping faults clustered within their hanging walls. This pattern is distinct from the more classic horst and graben morphology described on the fast-spreading northern EPR. The pattern documented on the southern EPR suggests inward-dipping master faults are more sharply curved or listric relative to their abyssal hill equivalents at fast-spreading rates. Boundary element modeling is used to examine the role of axial magma chamber inflation in generating the pattern of faulting observed along the southern EPR. An earthquake catalog derived from the detection of seismically generated T-waves is used to study the decay rate and spatial distribution of mid-ocean ridge aftershock sequences. Generally, aftershock sequences in the mid-ocean ridge setting can be well described by a modified Omori law. Decay constants (p-values) are slightly higher than those previously estimated from stacked sequences derived from the teleseismic record. Three transform fault sequences and one microplate thrust sequence exhibit p-values consistent with the global median value. There is some evidence for higher p-values associated with normal faulting mainshocks along the spreading centers. Although the June 1999 Endeavour segment sequence has been suggested previously to be of tectonic origin, the timing and distribution of earthquakes are inconsistent with the aftershock process. Allan factor, rescaled range, and coefficient of variation analysis are used to study the two-year temporal pattern of seismicity along the northern Mid-Atlantic Ridge. Earthquakes exhibit fractal-clustering behavior within this region.