Microearthquakes beneath the hydrothermal vent fields of the Endeavour segment of the Juan de Fuca Ridge: Insights into reaction zone processes

Abstract: From 2003-2006, a novel seismic network comprising seven short-period corehole seismometers and a broadband Guralp CMG-1T OBS was deployed using remotely operated vehicles in a subseafloor configuration on the Endeavour segment of the Juan de Fuca mid-ocean ridge. The seismic monitoring array was one part of a multi-disciplinary prototype NEPTUNE experiment designed to investigate the linkages between seismic deformation, hydrothermal fluxes, and microbial productivity along oceanic plate boundaries. The seismic network recorded high-quality data that illustrate the advantages of using an ROV to deploy seismometers in well-coupled configurations that are also away from the effects of ocean currents. A preliminary analysis of the first year of Keck seismic data was undertaken during a research apprenticeship class taught in the fall of 2004 at the University of Washington's Friday Harbor Laboratories. Eight post-baccalaureate students obtained a preliminary catalog of nearly 13,000 earthquakes on the Endeavour segment. Two of the apprentices conducted a second-pass analysis to refine the locations of ~3000 earthquakes that are within or near the network. Further analysis of these proximal earthquakes has focused on the application of cross-correlation and relative relocation techniques, the determination of focal mechanisms using P-wave first motions and P- to S-wave amplitudes ratios, and improved estimates of earthquake magnitudes. The results show that the entire Endeavour segment was seismically active during 2003-2004. Within the network, the earthquakes are located in clusters that are centered at ~2 km depth in the inferred location of the hydrothermal reaction zone immediately above a crustal magma chamber imaged by seismic reflection studies. The number of earthquakes below each hydrothermal vent field correlates with the heat flux measured in 2004 by other researchers suggesting that the earthquakes are closely related to the process of hydrothermal heat extraction. The local magnitudes vary from 2 and their distribution yields a high b value of 2.0 that is characteristic of magmatic and hydrothermal systems. The hypocenters are distributed vertically over a ~0.5 km, which is one to two orders of magnitude thicker than the thermal boundary necessary to support the observed heat fluxes. Between the High-Rise and Main Endeavour vent fields which have the highest measured heat fluxes of ~500 MW and ~250 MW, the focal mechanisms show a transition from ridge-perpendicular extension on two normal faults that dip steeply towards one another to ridge-perpendicular compression on either side over horizontal distances of a few hundred meters. The simplest explanation for the distribution of focal mechanisms is that they are the result of stresses associated with steady inflation of a crack-like magma sill. I argue that magma inflation plays a critical role in fracturing rock above the axial magma lens, thus allowing hydrothermal fluids to circulate within meters of the magma body and extract the high heat flux observed at the High Rise and Main Endeavour vent fields. -------------------------------------------------------------------- Séminaires Géosciences Marines --------------------------------------------------------------------