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

Abstract

We determined the rupture sequence of the 12 August 2021 M_w 8.2 South Sandwich Island earthquake which appears to be a complex sequence in both time and space. Notable tsunamis were recorded by tide gauges at global distances. Given the complexity of this event, we conducted a multiple subevent inversion on broadband seismograms, to resolve its complex variations of fault geometry, location, depth, and temporal characteristics. We found that the rupture initiated as a regular deep thrust earthquake; it then ruptured shallower and triggered a silent and dominantly slow subevent extending ∼200 km to the south, and ended with two other regular subevents. The total duration is ∼260 s, unusually long for an M_w 8.2 event. Our result is qualitatively consistent with other moment tensor solutions and the deviant m_B–M_w and M_S–M_w relations, and provides a more quantitative space-temporal pattern of this unusual sequence.