Minor Lithospheric Mantle Earthquakes Beneath the South Indian Shield: Implications for Continental Mantle Rheology

We report two continental lithospheric mantle (CLM) earthquakes of M W 3.8 and 3.0 beneath the South Indian Shield, which occurred on 25 and 27 July 2021, respectively. Probabilistic location, waveform modeling, and depth phase analysis confirm that both earthquakes occurred at a depth of 45 ± 2 km along the western margin of the Cuddapah Basin (CB). Receiver function modeling reveals that the local Moho depth is at 33 ± 2 km, which implies that the earthquakes occurred ∼12 km below the Moho. Besides mantle earthquakes, 314 earthquakes ( M L 0.0–4.3) occurred throughout the entire crust beneath this region between 2008 and 2022. The source mechanisms of M L ≥ 3.0 crustal and mantle earthquakes show dominant strike‐slip and oblique mechanisms, respectively, on high‐angle faults, striking NW–SE or NE–SW, with pressure axes aligning with the India‐Eurasia convergence direction. The average spectral stress drops for both crustal and mantle earthquakes are indistinguishable. Geotherm modeling indicates temperatures of 522 ± 33°C at the source depth, aligning with the view that the CLM can be seismogenic up to temperatures of ∼600°C. The results suggest that the entire crust and about ∼25 km of the upper mantle form a single seismogenic layer beneath the CB, while the mantle earthquakes concentrate along the lithosphere thickness gradient where the layer is at its weakest. The presence of CLM earthquakes beneath the South Indian Shield and Himalayan foreland might indicate that the strength of the Indian lithospheric mantle is optimal—sufficient to facilitate transmission of horizontal stresses without preventing seismogenesis.