Upper Crustal Vp/Vs Ratios at the Endeavour Segment, Juan de Fuca Ridge, From Joint Inversion of P and S Traveltimes: Implications for Hydrothermal Circulation

The Endeavour segment of the Juan de Fuca Ridge is one of the most active and long-lived hydrothermal areas of the mid-ocean ridge system. However, the permeability structure that gives rise to long-term venting at well-established fields, such as the High Rise, Main Endeavour, and Mothra fields, is not fully understood. Here we jointly invert Pg and Sg traveltimes from a seismic refraction experiment conducted at the Endeavour segment using P-to-S coupling constraints. We then calculate porosity and crack density as a function of crack aspect ratio by applying the differential effective medium theory to the seismic velocities. At 1.4-km depth, average Vp similar to 5 km off axis increases by similar to 0.4 km/s compared to the ridge axis. The average Vp/Vs has a minimum of similar to 1.75 on the ridge axis and increases to a maximum of similar to 1.84 off axis. The inferred porosity and crack density distributions show that the proportion of thick versus thin cracks decreases from the ridge axis to the flanks, since theoretical models indicate that Vp/Vs increases going from thick to thin cracks (aspect ratio decreasing from 0.1 to 0.01). The dominant presence of thick cracks on the axis may provide long-term conduits for upflow in high-temperature hydrothermal circulation potentially forming the vent fields. The increased proportion of thin cracks on the flanks, coupled with the increased seismic velocity, indicates a decrease in permeability caused by progressive clogging of thick cracks due to mineral precipitation likely in the downflow zone of hydrothermal circulation.