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Strong tsunami excitation from slow rupture of shallow subduction zone faults is recognized as a key concern for tsunami hazard assessment. Three months after the 22 July 2020 magnitude 7.8 thrust earthquake struck the plate boundary below the Shumagin Islands, Alaska, a magnitude 7.6 aftershock ruptured with complex intraplate faulting. Despite the smaller size and predominantly strike-slip faulting mechanism inferred from seismic waves for the aftershock, it generated much larger tsunami waves than the mainshock. Here we show through detailed analysis of seismic, geodetic, and tsunami observations of the aftershock that the event implicated unprecedented source complexity, involving weakly tsunamigenic fast rupture of two intraplate faults located below and most likely above the plate boundary, along with induced strongly tsunamigenic slow thrust slip on a third fault near the shelf break likely striking nearly perpendicular to the trench. The thrust slip took over 5 min, giving no clear expression in seismic or geodetic observations while producing the sizeable far-field tsunami.

The last decade has seen the rapid emergence of nonhydrostatic modeling as an advanced tool for studies of tsunami processes and source mechanisms that warrants a critical assessment of the state of the art and value-added features in relation to contemporary approaches. Inclusion of depth-averaged vertical velocity and nonhydrostatic pressure in the nonlinear shallow-water equations enables description of long-wave dynamics in quasi three-dimensional flows. The governing equations involve first-order derivatives, but retain higher-order properties, as in the Boussinesq-type approach. The commonly-used staggered finite difference scheme continues to provide the surface elevation and horizontal velocity, which in turn are updated by the nonhydrostatic pressure evaluated from a Poisson-type equation. In addition to having dispersion properties complementary to the governing equations, the numerical framework allows implementation of time-varying seafloor excitation from earthquake rupture, a shock-capturing scheme for discontinuous flows, and a multilevel two-way nested grid system for dispersive and shock waves. A series of numerical and laboratory benchmarks as well as a case study of the 2011 Tohoku tsunami illustrate the model capabilities in describing tsunami generation, dispersion, shoaling, bore formation, and separation-driven currents with high precision across a wide range of temporal and spatial scales for general application. These capabilities have an important role in resolving effects of detailed earthquake rupture patterns and providing accurate tsunami impact predictions with implications for warning guidance, hazard assessment, and seismological research. [ABSTRACT FROM AUTHOR]

The dominant type of vibration in wind turbine blades is flapwise vibration, which causes fatigue loading owing to the out-of-plane alternating load, with blade breakage occurring when the load is below the strength limit. Consequently, the mechanism and distribution of alternating-load generation must be understood to reduce the fatigue load. The strain data in the flapwise direction at the characteristic position of the operating wind turbine were collected to learn the characteristics of the spanwise distribution of the strain power spectral density (PSD) amplitude, to explore the fatigue damage-prone area and the generation mechanism, and to reveal the influence of the rotational speed on the spanwise distribution of the strain. It was evident that linked torsional and edgewise vibrations were predominant and that the strain PSD amplitude of the 1P loading component was nonlinear because of the Brazier effect in the transition region. The fatigue failure susceptible region of the rotating blade is more accurately identified. Except for the 0.10 R position, the blade 1P component 0.70–0.75 R spanwise region is more prone to failure. The rotation speed increases the dynamic fluctuation of spanwise strain. [ABSTRACT FROM AUTHOR]

A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re‐rupturing part of the 1938 rupture zone. The 2021 MW 8.2 Chignik earthquake occurred just northeast of the 22 July 2020 MW 7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismic P and SH waves, regional Global Navigation Satellite System (GNSS) displacements, and near‐field and far‐field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60‐s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9‐m slip in an ∼170‐km‐long patch. The 40–45 km down‐dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large‐slip zones, with a concentration of activity up‐dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface‐wave magnitude MS of 8.1 for the 2021 earthquake is smaller than MS = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidi Islands, where the 2021 event did not rupture. Plain Language Summary: The Pacific plate underthrusts the North American plate along the Alaska‐Aleutian subduction zone producing great earthquakes along most of the plate boundary. The region, that ruptured in 1938, hosted another great earthquake with magnitude 8.2 on 29 July 2021 offshore of Chignik, adjacent to a magnitude 7.8 event in 2020 beneath the Shumagin Islands. Using seismic wave recordings, Global Navigation Satellite System static displacements, and tsunami wave recordings at regional and distant locations, we determine the space‐time evolution of slip during the 2021 rupture process. The rupture extended northeastward ∼170 km, ending just short of the Semidi Islands. The slip is concentrated on the megathrust, from 32‐km to 40‐km deep. Shallow slip at depths <25 km is precluded by the tsunami observations, confining all rupture to beneath the shallow continental shelf. Most aftershocks are up‐dip of the large‐slip regions with a concentration in the shallow region of the southwestern rupture zone. Comparisons of observations and rupture models with the 1938 event indicate distinct slip distributions and seismic energy release, with the 1938 event being somewhat larger and extending further northeast than the 2021 rupture. This provides strong evidence that the 2021 event is not a simple repeat failure of the 1938 rupture. Key Points: The slip distribution of the 2021 Chignik MW 8.2 earthquake is constrained by joint analysis of seismic, geodetic, and tsunami recordingsDuring the ∼60‐s long rupture, patchy slip of up to 12.9 m occurred at depths of 32–40 km on the megathrust below the continental shelfThe 2021 slip pattern differs from the 1938 MW 8.2 rupture, with larger slip in the west and no slip in the easternmost 1938 rupture zone [ABSTRACT FROM AUTHOR]

The Shumagin seismic gap along the Alaska Peninsula experienced a major, MW 7.8, interplate thrust earthquake on 22 July 2020. Several available finite‐fault inversions indicate patchy slip of up to 4 m at 8–48 km depth. There are differences among the models in peak slip and absolute placement of slip on the plate boundary, resulting from differences in data distributions, model parameterizations, and inversion algorithms. Two representative slip models obtained from inversions of large seismic and geodetic data sets produce very different tsunami predictions at tide gauges and deep‐water pressure sensors (DART stations), despite having only secondary differences in slip distribution. This is found to be the result of the acute sensitivity of the tsunami excitation for rupture below the continental shelf in proximity to an abrupt shelf break. Iteratively perturbing seismic and geodetic inversions by constraining fault model extent along dip and strike, we obtain an optimal rupture model compatible with teleseismic P and SH waves, regional three‐component broadband and strong‐motion seismic recordings, hr‐GNSS time series and static offsets, as well as tsunami recordings at DART stations and regional and remote tide gauges. Slip is tightly bounded between 25 and 40 km depth, the up‐dip limit of slip in the earthquake is resolved to be well‐inland of the shelf break, and the rupture extent along strike is well‐constrained. The coseismic slip increased Coulomb stress on the shallow plate boundary extending to the trench, but the frictional behavior of the megathrust below the continental slope remains uncertain. Plain Language Summary: Several studies on the 22 July 2020 MW 7.8 thrust event have included finite‐fault inversions, yielding generally consistent slip models. However, none of these studies have fully considered the seismic and geodetic data together with the tsunami measurements at tide gauges and deep‐water pressure sensors. We select two representative slip models obtained from prior seismic and geodetic inversions for tsunami modeling and find that they give distinct tsunami predictions despite having only a minor shift of slip by ∼20 km relative to shelf break. Large differences in tsunami excitation occur when seafloor deformation extends to the continental slope under deeper water. We perform iterative inversion of seismic and geodetic observations and forward modeling of tsunami signals to obtain an optimal slip model compatible with teleseismic waves, regional seismic recordings, hr‐GNSS time series and static offsets, along with tsunami recordings at DARTs and tide gauges. Precise placement of the slip distribution beneath the continental shelf controls the strength and timing of seaward tsunamis, with waveforms varying nonlinearly with seafloor deformation extending across the shelf break. The earthquake increased Coulomb stress on the shallow megathrust, but uncertainty in the shallow frictional behavior leaves it unclear whether a future large event can occur there. Key Points: Published finite‐fault models for the 2020 MW 7.8 earthquake predict varying tsunami signals despite having similar slip patternsThe tsunami excitation is very sensitive to the absolute placement of slip relative to the shelf break and along strikeIteration of slip inversion parameters and tsunami predictions optimizes the model to fit all data well with slip only beneath the shelf [ABSTRACT FROM AUTHOR]