Multi-disciplinary assessment of the August 12, 2021, South Sandwich earthquake doublet

On August 12, 2021, an earthquake doublet with a cumulative magnitude M_W 8.0 – 8.3 hit the South Sandwich Trench in the South Atlantic where the South American plate is subducted beneath the Sandwich microplate. Significant differences in location, depth, and magnitude are reported by international agencies. Discrepant results might be due to the short inter-event time of ~150 s between both subevents and the lack of local and regional data.

We apply a multi-disciplinary approach to clarify the source processes and characterize different features of the doublet. Our centroid solutions of the mainshocks, separated by ~290 km, confirm the overall southward rupture directivity. The predominant thrust mechanisms, with different strike directions, suggest the activation of a bent portion of the slab. We estimate a cumulative magnitude of M_wc 7.65 inverted from body waves in the frequency band 0.01 – 0.03 Hz. Our magnitude estimate is substantially smaller than the one reported, e.g., by Global CMT, suggesting that a significant part of the moment has been released at lower frequency as a slow slip process. It is verified by a W-phase inversion in the frequency band 0.005 - 0.01 Hz with a resulting magnitude M_ww of 8.0. The iterative deconvolution and stacking method (IDS) resolves high slip patches located in the area of the two mainshock centroids. High-frequency back-projection results confirm the unilateral southward rupture propagation. Complex fault and slab geometries do not significantly improve the fit, providing no clear evidence for the activation of secondary faults. Centroid moment tensors, estimated for 87 aftershocks between August 12, 2021 and August 31, 2021, support the identification and characterization of activated fault segments.