On August 12, 2021 an earthquake sequence of several M_w > 7.3 earthquakes hit the central and southern South Sandwich trench. Due to its remote location and short interevent times, reported earthquake parameters varied significantly between different international agencies. We studied the complex rupture by combining different seismic source characterization techniques sensitive to different frequency ranges based on teleseismic broadband recordings from 0.001–2 Hz, including point and finite fault inversions and the back-projection of high-frequency signals. We also determined moment tensor solutions for 88 aftershocks. The rupture sequence initiated with an M_w 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface. Simultaneously a second shallow megathrust rupture was initiated, which propagated unilaterally to the south with a very slow rupture velocity of 1.2 km/s and varying strike following the curvature of the trench. The slow rupture covered nearly two thirds of the entire subduction, and with a M_w 8.2 released the bulk of the total moment of the sequence. Tsunami modelling indicates the inferred shallow rupture can explain the tsunami records. The southern segment of the shallow rupture overlaps with another activation of the deeper part of the megathrust equivalent to a M_w 7.6. The aftershock distribution confirms the extent and curvature of the rupture. Some mechanisms are consistent with the mainshocks, but many indicate also activation of secondary faults. Rupture velocities and radiated frequencies varied strongly between different stages of the rupture, which might explain the variability of published source parameters.