Seismic velocity structure and seismicity at the Kairei hydrothermal vent field near the Rodriguez Triple Junction in the Indian Ocean

The Kairei hydrothermal vent field (KHF) is on the northeastern side of the first segment of the Central Indian Ridge and discharges H-2-rich hydrothermal fluids. To identify the source of these fluids based on the crustal and upper mantle structures and tectonic stress fields, we conducted a seismic refraction survey and observed natural seismicity in and around the KHF. We estimated 3D P-wave and S-wave velocity (V-P and V-S, respectively) structure models from airgun and natural seismic sources and determined 1861 hypocenters during a period of about two months. Beneath Yokoniwa Rise, V-P in a convex zone exceeded 6.0 km/s at similar to 2 km depth below the seafloor and exceeded 7.0 km/s at 3-4 km depth; we thus interpret the rise to be a non-transform offset massif formed by the uplift of deep rocks. We detected a seismic cluster in a normal fault zone 3-5 km northwest of the KHF. Many events in this cluster occurred at 2-5 km depth below the seafloor in an area of high V-P/V-S ratios (similar to 2.0) that are consistent with serpentinized peridotite, suggesting that seawater seeping downward through the fault zone has serpentinized peridotite at depth, thus becoming H-2-rich hydrothermal fluid. This high-V-P/V-S area extends laterally at depth, and other such areas are observed at <1 km below the seafloor near the KHF. This structural configuration suggests that the high-V-P/V-S areas might provide conduits for the hydrothermal fluids that issue from the vents of the KHF. At the first segment of the ridge, we detected distinct shallow and deep seismic clusters. Seismicity in the shallow cluster was intermittent whereas deeper seismicity was nearly continuous, possibly indicating distinct magmatic activities at each depth.