Large megathrust earthquakes on the subduction interface extend from near-trench to depths and display very different depth-varying slip behaviors (Lay et al., 2012). Large earthquakes that rupture the shallowest portion of the subduction interface (<15 km) can generate devastating tsunamis, but they appear to rupture slowly with inefficient excitation of short-period seismic waves disproportionately to their seismic moment and tsunami. These earthquakes are "tsunami earthquakes" (Kanamori, 1972). At deeper depths (15-50 km), large thrust earthquakes have faster rupture velocities and stronger radiation of short-period seismic energy with inefficient tsunami generation. Their contrasting rupture characteristics are well interpreted by two distinct types of fault properties; the slow slip of shallow tsunami earthquakes is commonly attributed to weak sediments and low rigidity of the upper plate (Bilek & Lay, 1999;Prada et al., 2021;Sallarès & Ranero, 2019), while the brittle failures of unstable fault patches explain the fast deeper earthquakes.On 12 August 2021, a great earthquake (M w > 8) struck the South Sandwich Island region of the south Atlantic Ocean (Figure 1a). This event occurred close to the South Sandwich trench, where the South American plate subducts beneath the South Sandwich plate at a velocity of 7 cm/year (Pelayo & Wiens, 1989). A remarkable observation of this earthquake is its far reaching-tsunamis. The tsunamis spread to the north Atlantic, Pacific, and Indian Oceans, where tide gauges measured peak amplitudes of ∼20 cm at over 10,000 km distance from the source (Figure S1 in Supporting Information S1). Although modeling these tide gauge observations is challenging because of the lack of detailed bathymetry data between the source and gauges, the observed tsunamis at global distances appear to suggest that the South Sandwich Island earthquake could be categorized as a regular shallow tsunamigenic earthquake.However, the South Sandwich Island earthquake seems to have extended to large depths with a complex temporal history. The early report (PDE) from the National Earthquake Information Center (NEIC) of the US Geological Survey listed two events within 3 min: (a) NEIC1,
