Contrasted Seismogenic Behavior of Magmatic vs. Tectonic Segments along Indian Ocean Spreading Centers

<jats:p>Mid-Ocean Ridges (MORs) accommodate plate divergence through magmatic emplacement and fault slip, only a fraction of which is expressed seismically. Quantifying these contributions requires accurate earthquake catalogs, which are difficult to obtain at remote MORs because of the limited sensitivity and location accuracy of distant land seismometers. This can be remedied with the use of hydrophone networks, which greatly expand coverage and location accuracy at the expense of information on source mechanism. Here we leverage autonomous hydrophone records from the OHA-SIS-BIO &amp; IMS networks (2010–2022) to relocate teleseismic events from both the ISC and GCMT catalogs along the ultraslow Southwest (SWIR), slow Central (CIR) and intermediate-spreading Southeast (SEIR) Indian ridges. From an initial set of 10,270 events detected across the Indian Ocean, we retain 2,356 wellrelocated MOR earthquakes. We visually classify spreading segments along our study area according to their morphology, from magmatically-to tectonically-dominated. We then propagate uncertainties on earthquake location, magnitude and focal-mechanism through a Monte-Carlo workflow that yields seismic moment budgets per MOR and segment class. We find that 2 tectonically-dominated segments consistently release more seismic moment per unit length than magmatic segments on all three ridges. Relocated seismicity also concentrates much closer to the ridge axis on the CIR and SEIR (≤ 15 km) than previously reported, while the SWIR retains a broader off-axis distribution (≤ 30 km). Upon fitting earthquake distributions with a tapered Gutenberg-Richter law, we estimate coupled seismogenic thicknesses that decrease from 1300 m to 70 m with increasing spreading rate and magma supply. Correlations between seafloor spreading styles and seismogenic behavior are consistent with those previously inferred at the slow-spreading Mid-Atlantic Ridge. Hence, we propose a general model where MOR normal faults accrue offset by a mix of seismic and aseismic slip modulated by local magma supply.</jats:p>