Seismic Structure and Crustal Accretion Along an Intermediate-rate Mid-Ocean Ridge Segment
Weekly, Robert Todd
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Epicenters and magnitudes for 36,523 earthquakes recorded along the Endeavour segment between August 2003 and October 2006 are automatically determined using a local ocean-bottom seismometer (OBS) network. The catalog is dominated by two swarm sequences in January and February 2005 in the vicinity of the Endeavour overlapping spreading center, which included earthquakes in West Valley, the northern portion of the Endeavour segment, southwest Endeavour Valley and the Endeavour vent fields. These swarms are attributed to volcanism including a dike intrusion on the northern Endeavour in February 2005 and smaller diking events on the propagating tip of the West Valley segment in both swarms. The dike on the northern Endeavour propagated to the south, which is inconsistent with magma sourced from the axial magma chamber beneath the elevated central portion of the segment. Following the swarms, seismic activity on the Endeavour segment decreased on average to ~15% of pre-swarm values and almost ceased at the segment ends. I infer that a six-year non-eruptive event that started with a swarm in 1999 and finished with the 2005 swarms ruptured the entire segment and relieved plate-spreading stresses. The inferred coupling between the 1999 and 2005 events, the observation of extensive precursory activity prior to the 2005 swarms, and the interaction between seismically active regions during the swarms is consistent with static triggering with delays influenced by viscoelastic relaxation, hydraulic diffusion and magma withdrawal and replenishment. The isotropic and anisotropic P-wave velocity structure of the upper oceanic crust on the Endeavour Segment of the Juan de Fuca Ridge is studied using refracted travel time data collected by an active-source, three-dimensional tomography experiment. The isotropic velocity structure is characterized by low crustal velocities in the overlapping spreading centers (OSCs) at the ends of the segment. These low velocities are indicative of pervasive tectonic fracturing and persist off-axis, recording the history of ridge propagation. Near the segment center, velocities within the upper 1 km show ridge-parallel bands with low velocities on the outer flanks of topographic highs. These features are attributed to localized thickening of the volcanic extrusive layer from eruptions extending outside of the axial valley that flow down the fault-tilted blocks that form the abyssal hill topography. On-axis velocities are relatively high beneath the hydrothermal vent fields due to the infilling of porosity by mineral precipitation. Lower velocities are observed beneath the most vigorous vent fields in a seismically active region above the axial magma chamber and may reflect increased fracturing and higher temperatures. Seismic anisotropy is high on-axis but decreases substantially off-axis over ~8 km (0.3 Ma). This decrease coincides with an increase in seismic velocities at depths greater than 1 km and is attributed to the infilling of cracks in the sheeted dike layer by mineral precipitation associated with near-axis hydrothermal circulation. The orientation of the fast-axis of anisotropy is ridge-parallel near the segment center but curves near the segment ends reflecting the tectonic fabric within the OSCs.