TY  - JOUR
A1  - Metz, M.
A1  - Vera, F.
A1  - Carrillo Ponce, A.
A1  - Cesca, S.
A1  - Babeyko, A.
A1  - Dahm, T.
A1  - Saul, J.
A1  - Tilmann, F.
T1  - Seismic and Tsunamigenic Characteristics of a Multimodal Rupture of Rapid and Slow Stages: The Example of the Complex 12 August 2021 South Sandwich Earthquake
Y1  - 2022-11-07
VL  - 127
IS  - 11
JF  - Journal of Geophysical Research: Solid Earth
DO  - 10.1029/2022JB024646
PB  - 
N2  - On 12 August 2021, a >220 s lasting complex earthquake with M<sub>w</sub> > 8.2 hit the 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 to 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 initiated simultaneously with a rupture equivalent to a M<sub>w</sub> 7.6 thrust earthquake in the deep part of the seismogenic zone in the central subduction interface and a shallow megathrust rupture, 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 zone length, and with M<sub>w</sub> 8.2 released the bulk of the total moment of the whole earthquake. Tsunami modeling 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 M<sub>w</sub> 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.
N2  - Plain Language Summary: The earthquake of 12 August 2021 along the deep‐sea trench of the South Sandwich Islands in the South Atlantic reached a magnitude of 8.2 and triggered a tsunami. The automatic earthquake parameter determination of different agencies showed very different results shortly after the earthquake and partially underestimated the tsunami potential of the earthquake. A possible reason was the complex rupture process and that the tsunami was generated by a long and shallow slow slip rupture sandwiched between more conventional fast slip subevents at its northern and southern ends. In addition, the fault surface, which extended over 450 km, was highly curved striking 150°–220°. We investigated the different components of the seismic wavefields in different frequency ranges and with different methods. The analysis shows how even complex earthquakes can be deciphered by combining analyzing methods. The comparison with aftershocks and the triggered tsunami waves confirms our model that explains the South Sandwich rupture by four subevents in the plate boundary along the curved deep‐sea trench. Here, the depth, rupture velocities, and slip on each segment of the rupture vary considerably. The method can also be applied to other megathrust earthquakes and help to further improve tsunami warnings in the future.
N2  - Key Points: A combination of multiple approaches, inversion setups, and frequency ranges deciphered the complex earthquake of 2021 South Sandwich. The rupture consisted of four subevents with the largest occurring as a shallow slow rupture parallel to the South Sandwich Trench. Forward modeling proves that the large, shallow thrust subevent caused the recorded tsunami.
UR  - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/11223
ER  -