On August 12, 2021 a > 220 s lasting complex earthquake with Mw > 8.2 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 initiated simultaneously with a Mw 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 Mw 8.2 released the bulk of the total moment of the earthquake. 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 Mw 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.

Seismicity models are probabilistic forecasts of earthquake rates to support seismic hazard assessment. Physics-based models allow extrapolating previously unsampled parameter ranges and enable conclusions on underlying tectonic or human-induced processes. The Coulomb Failure (CF) and the rate-and-state (RS) models are two widely-used physics-based seismicity models both assuming pre-existing populations of faults responding to Coulomb stress changes. The CF model depends on the absolute Coulomb stress and assumes instantaneous triggering if stress exceeds a threshold, while the RS model only depends on stress changes. Both models can predict background earthquake rates and time-dependent stress effects, but the RS model with its three independent parameters can additionally explain delayed aftershock triggering. This study introduces a modified CF model where the instantaneous triggering is replaced by a mean time-to-failure depending on the absolute stress value. For the specific choice of an exponential dependence on stress and a stationary initial seismicity rate, we show that the model leads to identical results as the RS model and reproduces the Omori-Utsu relation for aftershock decays as well stress-shadowing effects. Thus, both CF and RS models can be seen as special cases of the new model. However, the new stress response model can also account for subcritical initial stress conditions and alternative functions of the mean time-to-failure depending on the problem and fracture mode.

The eruption frequency of geysers can be studied easily on the surface. However, details of the internal structure including possible water and gas filled chambers feeding eruptions and the driving mechanisms remain elusive. We recorded eruptions at Strokkur in June 2018 with a multidisciplinary network of seismometers, tiltmeter, video cameras and water pressure sensors to study the eruptive cycle, internal geyser structure and driving mechanisms in detail. An eruptive cycle at Strokkur always consists of 4 phases: the eruption (Phase 1), post-eruptive conduit refilling (Phase 2), gas filling of the bubble trap (Phase 3) and regular bubble migration and implosion at depth in the conduit (Phase 4). For a typical single eruption Phase 1 and 2 persist for 13.1 s. Phase 3 contains a 26.1 s long eruption coda of on average 19 seismic peaks spaced 1.5 s apart generated at 25 to 30 m depth, 13 to 23 m west of the conduit when the bubble trap refills with gas. Phase 4 starts on average 0.9 minutes after the beginning of the eruption and persists for 2.3 min. In this phase on average 8 large bubbles leave the bubble trap and implode at a spacing of 24.5 s at about 7 m depth in the conduit. The duration of the eruption and recharging phase linearly increases with the number of water fountains in close succession (Phase 1), likely due to a larger water, gas and heat loss from the bubble trap and conduit.