The 2021 South Sandwich Island M_w 8.2 Earthquake: A Slow Event Sandwiched Between Regular Ruptures

Abstract

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,

Seismic and Tsunamigenic Characteristics of a Multimodal Rupture of Rapid and Slow Stages: The Example of the Complex 12 August 2021 South Sandwich Earthquake

“…We find a strong agreement with the results of Jia et al. (2022) from both their W‐phase and multievent inversions. The CMT W‐phase inversion (subevent B) yields a comparable cumulative duration, magnitude, thrust mechanism, and centroid location, but with a steeper dip of the preferred nodal plane (43° ± 16° versus 14°), which could be caused by larger time windows and a broader frequency band used in our case.…”

“…Mechanisms, locations, and times of A and D are consistent with subevents E1, E2, E4, and E5 by Jia et al. (2022). Our inversion of subevent A as an extended rupture reproduces E1 and E2 with similar location, slightly longer duration and also larger magnitude ( M w 7.6 compared to cumulative M w 7.4), as it fits a longer part of the first rupture signal.…”

“…A multiple event inversion combining centroid moment tensors (MTs) and a simplified extended rupture model applied to the complex rupture (Jia et al., 2022) yielded a total of five subevents, where the dominant subevent indicated very shallow and very slow southward rupture propagation with a rupture velocity of ≈1 km/s. Their results thus indicate a typical tsunami earthquake behavior (as explained by Bilek and Lay (2018)) with a tsunamigenic slow rupturing event in the shallow conditionally stable domain of a subduction megathrust.…”

“…This may explain the unusual variety of focal mechanism solutions and magnitude estimates between different agencies as USGS, GEOFON, and GCMT (Table 1). GCMT first reported the doublet as two separate earthquakes with M w 8.3 and 7.9 for the first and second events, respectively (see e.g., Jia et al., 2022, for the originally distributed GCMT estimates), but later switched to a single long duration (300 s) earthquake with M w 8.3. The solutions of USGS and GEOFON assume an earthquake doublet with magnitudes of M w 7.5–7.7 for the first and M w 8.0–8.2 for the second subevent.…”

“…The 12 August 2021 rupture represents the largest moment release along the SST in the instrumental period. The simultaneous occurrence of fast and slow rupture modes as stated by Jia et al (2022) is rarely observed so clearly, but led to an increased complexity of the rupture process of the earthquake. This complexity makes it challenging to reconstruct the rupture processes and estimate its tsunamigenic potential from standard seismological analysis approaches, even more so as no near field GNSS observations are available.…”