摘要：

The eastern Southwest Indian Ridge (SWIR) is characterized by low melt supply and this melt is focused on axial volcanos, so that the intervening regions of the ridge spread in a nearly amagmatic fashion, producing extensive ultramafic seafloor and flip-flop detachment faults (Cannat et al., 2006; Sauter et al., 2013). Schlindwein and Schmid (2016) studied microseismicity near an axial volcano at 65°30'E and found that maximum earthquake depth increased from ~7 km near the volcano, to 20 km in a magma-starved region at 65°10'E. They also reported a seismic gap at depths of 0-10 km in this magma-starved region. Here, we report on two microseismicity experiments in a magma-poor to magma-starved region of the ridge around 64°30'E with 14-OBSs deployed for 8 days during gaps in an active seismic survey (SEISMOSMOOTH cruise; Oct 2014) and with 6-OBSs deployed for 19 days (ROVSMOOTH cruise; Dec 2016). Serpentinized peridotites make up most of the seafloor in this region, with patches of hummocky basalts in the western part of the area (Sauter et al., 2013). The presently active detachment fault has been imaged seismically with a dip of ~50° down to ~5 km, and the seismic crust is ~5 km (Momoh et al., 2017). Our purpose is to relocate the recorded earthquakes in order to constrain the tectonic activity in the axial detachment context and the thickness of the brittle lithosphere. 289 microearthquakes of local magnitudes between -1.1 and 2.9 were located using NonLinLoc and relocated using HypoDD with a 1-D P wave velocity model from Momoh et al. (2017) and a Vp/Vs ratio of 1.7. Relocated events nearly evenly occur in the footwall (31%), in the hanging wall (39%), and in detachment fault region (30%). Most events occur in the western, more volcanically active part of the survey area. A seismic swarm occurred in the footwall and fault plane of this volcanic area during Dec 22-23, with progressively deeper events that could tentatively be attributed to a diking event. Footwall events probably correspond to bending of the lithosphere. Hanging wall events could be related to minor normal faults observed in seafloor maps and seismic reflection images (Momoh et al., 2017 and 2020). The deepest relocated earthquakes occur near the fault zone about 13-14 km below seafloor, suggesting that the 64°30'E magma-poor region has a thinner brittle lithosphere than proposed by Schlindwein and Schmid (2016) for the adjacent region at 65°10'E.

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