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  1. Abstract

    The 2021MW6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼. The relatively low moment scaled radiated energyof 1.5 × 10−5and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.

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  2. Abstract

    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 2021MW8.2 Chignik earthquake occurred just northeast of the 22 July 2020MW7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismicPandSHwaves, 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 magnitudeMSof 8.1 for the 2021 earthquake is smaller thanMS = 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.

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  3. Abstract

    The Shumagin seismic gap along the Alaska Peninsula experienced a major,MW7.8, 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 teleseismicPandSHwaves, 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.

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  4. Abstract

    On 29 July 2021, anMW8.2 thrust‐faulting earthquake ruptured offshore of the Alaska Peninsula within the rupture zone of the 1938MW8.2 earthquake. The spatiotemporal distribution of megathrust slip is resolved by jointly inverting regional and teleseismic broadband waveforms along with co‐seismic static and high‐rate GNSS displacements. The primarily unilateral rupture expanded northeastward, away from the rupture zone of the 22 July 2020MW7.8 Shumagin earthquake. Large slip extends along approximately 175 km, spanning about two third of the estimated 1938 aftershock zone, with well‐bounded depth from 20 to 40 km, and up to 8.6 m slip near the hypocenter. The rupture terminated in the eastern portion of the 1938 aftershock zone in a region of very large geodetic slip deficit where peak slip appears to have occurred in the 1938 rupture. The 2021 and 1938 events do not have similar slip distributions and do not indicate persistent asperities.

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  5. Abstract

    The Kalapana, Hawaii,MW7.7 earthquake on November 29, 1975 generated a local tsunami with at least 14.3 m runup on the southeast shore of Hawaii Island adjacent to Kilauea Volcano. This was the largest locally generated tsunami since the great 1868 Ka'u earthquake located along‐shore to the southwest. Well‐recorded tide gauge and runup observations provide a key benchmark for studies of statewide tsunami hazards from actively deforming southeast Hawaii Island. However, the source process of the earthquake remains controversial, with coastal landsliding and/or offshore normal or thrust faulting mechanisms having been proposed to reconcile features of seismic, geodetic, and tsunami observations. We utilize these diverse observations for the 1975 Kalapana earthquake to deduce a compound faulting model that accounts for the overall tsunamigenesis, involving both landslide block faulting along the shore and slip on the island basal décollement. Thrust slip of 4.5–8.0 m on the offshore décollement produces moderate near‐field runup but controls the far‐field tsunami. The slip distribution implies that residual strain energy was available for the May 4, 2018MW7.2 thrust earthquake during the Kilauea‐East Rift Zone eruption. Local faulting below land contributes to geodetic and seismic observations, but is non‐tsunamigenic and not considered. Slip of 4–10 m on landslide‐like faults, which extend from the Hilina Fault Zone scarp to offshore shallowly dipping faults reaching near the seafloor, triples the near‐field tsunami runup. This compound model clarifies the roles of the faulting components in assessing tsunami hazards for the Hawaiian Islands.

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  6. Abstract

    The Queen Charlotte‐Fairweather Fault (QC‐FF) system off the coast of British Columbia and southeast Alaska is a highly active dextral strike‐slip plate boundary that accommodates ∼50 mm/yr of relative motion between the Pacific and North America plates. NineMW ≥ 6.7 earthquakes have occurred along the QC‐FF system since 1910, including aMS(G‐R)8.1 event in 1949. Two recent earthquakes, the October 28, 2012 Haida Gwaii (MW7.8) and January 5, 2013 Craig, Alaska (MW7.5) events, produced postseismic transient deformation that was recorded in the motions of 25 nearby continuous Global Positioning System (cGPS) stations. Here, we use 5+ yr of cGPS measurements to characterize the underlying mechanisms of postseismic deformation and to constrain the viscosity structure of the upper mantle surrounding the QC‐FF. We construct forward models of viscoelastic deformation driven by coseismic stress changes from these two earthquakes and explore a large set of laterally heterogeneous viscosity structures that incorporate a relatively weak back‐arc domain; we then evaluate each model based on its fit to the postseismic signals in our cGPS data. In determining best‐fit model structures, we additionally incorporate the effects of afterslip following the 2012 event. Our results indicate the occurrence of a combination of temporally decaying afterslip and vigorous viscoelastic relaxation of the mantle asthenosphere. In addition, our best‐fit viscosity structure (transient viscosity of 1.4–2.0 × 1018 Pa s; steady‐state viscosity of 1019 Pa s) is consistent with the range of upper mantle viscosities determined in previous studies of glacial isostatic rebound and postseismic deformation.

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  7. Abstract

    The eastern portion of the Shumagin gap along the Alaska Peninsula ruptured in anMW7.8 thrust earthquake on 22 July 2020. The megathrust fault space‐time slip history is determined by joint inversion of regional and teleseismic waveform data along with co‐seismic static Global Navigation Satellite System (GNSS) displacements. The rupture expanded westward and along‐dip from the hypocenter, located adjacent to the 1938MW8.2 Alaska earthquake, with slip and aftershocks extending into the gap about 180 to 205 km, respectively, at depths from 15 to 40 km. The deeper half of ~75% of the Shumagin gap experienced faulting. However, the patchy slip is significantly less than possible accumulated slip since the region's last major rupture in 1917, compatible with geodetic seismic‐coupling estimates of 10‐40% beneath the Shumagin Islands. The rupture terminated in the western region of very low seismic coupling. There was a regional decade‐scale decrease in b‐value prior to the 2020 event.

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  8. Abstract

    The number of aftershocks increases with mainshock size following a well‐defined scaling law. However, excursions from the average behavior are common. This variability is particularly concerning for large earthquakes where the number of aftershocks varies by factors of 100 for mainshocks of comparable magnitude. Do observable factors lead to differences in aftershock behavior? We examine aftershock productivity relative to the global average for all mainshocks () from 1990 to 2019. A global map of earthquake productivity highlights the influence of tectonic regimes. Earthquake depth, lithosphere age, and plate boundary type correspond well with earthquake productivity. We investigate the role of mainshock attributes by compiling source dimensions, radiated seismic energy, stress drop, and a measure of slip heterogeneity based on finite‐fault source inversions for the largest earthquakes from 1990 to 2017. On an individual basis, stress drop, normalized rupture width, and aspect ratio most strongly correlate with aftershock productivity. A multivariate analysis shows that a particular set of parameters (dip, lithospheric age, and normalized rupture area) combines well to improve predictions of aftershock productivity on a cross‐validated data set. Our overall analysis is consistent with a model in which the volumetric abundance of nearby stressed faults controls the aftershock productivity rather than variations in source stress. Thus, we suggest a complementary approach to aftershock forecasts based on geological and rupture properties rather than local calibration alone.

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  9. Abstract

    A vigorous shallow earthquake sequence along the southern coast of Puerto Rico commenced on 28 December 2019 in a region with little prior large seismicity. The largest event in the sequence (MW = 6.4), struck on 7 January 2020 and involved normal faulting. It produced extensive damage in southern Puerto Rico and power disruption across the island. Nearby strong ground motions and static offsets from GPS stations along with teleseismic recordings are inverted for the kinematic rupture process of the mainshock. The ~15‐km‐long rupture is spatially concentrated, with most slip between 3 and 13 km deep and peak slip of ~1.6 m. The static stress drop is high, ~19 MPa, with the rupture locating in the eastern section of a ~30‐km‐long band of seismicity bisected by a near‐orthogonal lineation. Complex faulting and high stress in the intraplate region appears to be responsible for the high earthquake productivity.

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  10. Abstract

    Major earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon, but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and/or having high aftershock productivity with diffuse distribution. This includes the 21 December 2010MW7.4 Ogasawara (Bonin), 11 April 2012MW8.6 Indo‐Australia, 23 January 2018MW7.9 Off‐Kodiak Island, and 20 December 2018MW7.3 Nikol'skoye (northwest Pacific) earthquakes. Major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks. Outer trench slope extensional faulting can involve complex distributed sequences, particularly when activated by great megathrust ruptures such as 11 March 2011MW9.1 Tohoku and 15 November 2006MW8.3 Kuril Islands. Intense faulting sequences also occur near subduction zone corners, with many fault geometries being activated, including some in nearby oceanic lithosphere, as for the 29 September 2009MW8.1 Samoa, 6 February 2013MW8.0 Santa Cruz Islands, and 16 November 2000MW8.0 New Ireland earthquakes. The laterally varying plate boundary stresses from heterogeneous locking, recent earthquakes, or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere. Preexisting lithospheric structures and fabrics exert secondary influences on the faulting. Intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near‐critical fault systems rather than localized, single‐fault failures, both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull.

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