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1. Rupture Directivity of the 2021 M _W 6.0 Yangbi, Yunnan Earthquake

   https://doi.org/10.1029/2022JB024321  

   Gong, Wenzheng; Ye, Lingling; Qiu, Yuxin; Lay, Thorne; Kanamori, Hiroo (September 2022, Journal of Geophysical Research: Solid Earth)

   Abstract The 2021M_W6.0 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. Energy Partitioning, Dynamic Fragmentation, and Off‐Fault Damage in the Earthquake Source Volume

   https://doi.org/10.1029/2021JB022616  

   Johnson, Scott E.; Song, Won Joon; Vel, Senthil S.; Song, Bo Ra; Gerbi, Christopher C. (November 2021, Journal of Geophysical Research: Solid Earth)

   Abstract Seismological fracture or breakdown energy represents energy expended in a volume surrounding the advancing rupture front and the slipping fault surface. Estimates are commonly obtained by inverting ground motions and using the results to model slip on the fault surface. However, this practice cannot identify contributions from different energy‐consumption processes, so our understanding of the importance of these processes comes largely from field‐ and laboratory‐based studies. Here, we use garnet fragment size data to estimate surface‐area energy density with distance from the fault core in the damage zone of a deeply exhumed strike‐slip fault/shear zone. Estimated energy densities per fragmentation event range from 2.87 × 10³to 2.72 × 10⁵ J/m³in the outer and inner portions of the dynamic damage zone, respectively, with the dynamic zone being inferred from the fractal dimensions of fragment size distributions and other indicators. Integrating over the ∼105 m width of the dynamic damage zone gives fracture surface‐area energy per unit fault area ranging from a lower bound of 6.63 × 10⁵ J/m²to an upper bound of 1.63 × 10⁷ J/m²per event. This range overlaps with most geological, theoretical, and kinematic slip‐model estimates of energy expenditure in the source volume for earthquakes characterized by seismic moments >10¹⁷ N·m. We employ physics‐based fragmentation models to estimate equivalent tensile strain rates associated with garnet fragmentation, which range from 5.42 × 10²to 1.04 × 10⁴ s⁻¹per earthquake in the outer and inner portions of the dynamic damage zone, respectively. Our results suggest that surface‐energy generation is a nonnegligible component of the earthquake energy budget. 

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3. High-rate very-long-period seismicity at Yasur volcano, Vanuatu: source mechanism and decoupling from surficial explosions and infrasound

   https://doi.org/10.1093/gji/ggab533  

   Matoza, Robin S.; Chouet, Bernard A.; Jolly, Arthur D.; Dawson, Phillip B.; Fitzgerald, Rebecca H.; Kennedy, Ben M.; Fee, David; Iezzi, Alexandra M.; Kilgour, Geoff N.; Garaebiti, Esline; et al (January 2022, Geophysical Journal International)

   SUMMARY Yasur volcano, Vanuatu is a continuously active open-vent basaltic-andesite stratocone with persistent and long-lived eruptive activity. We present results from a seismo-acoustic field experiment at Yasur, providing locally dense broad-band seismic and infrasonic network coverage from 2016 July 27 to August 3. We corroborate our seismo-acoustic observations with coincident video data from cameras deployed at the crater and on an unoccupied aircraft system (UAS). The waveforms contain a profusion of signals reflecting Yasur’s rapidly occurring and persistent explosive activity. The typical infrasonic signature of Yasur explosions is a classic short-duration and often asymmetric explosion waveform characterized by a sharp compressive onset and wideband frequency content. The dominant seismic signals are numerous repetitive very-long-period (VLP) signals with periods of ∼2–10 s. The VLP seismic events are ‘high-rate’, reoccurring near-continuously throughout the data set with short interevent times (∼20–60 s). We observe variability in the synchronization of seismic VLP and acoustic sources. Explosion events clearly delineated by infrasonic waveforms are underlain by seismic VLPs. However, strong seismic VLPs also occur with only a weak infrasonic expression. Multiplet analysis of the seismic VLPs reveals a systematic progression in the seismo-acoustic source decoupling. The same dominant seismic VLP multiplet occurs with and without surficial explosions and infrasound, and these transitions occur over a timescale of a few days during our field campaign. We subsequently employ template matching, stacking, and full-waveform inversion to image the source mechanism of the dominant VLP multiplet. Inversion of the dominant VLP multiplet stack points to a composite source consisting of either a dual-crack (plus forces) or pipe-crack (plus forces) mechanism. The derived mechanisms correspond to a point-source directly beneath the summit vents with centroid depths in the range ∼900–1000 m below topography. All mechanisms suggest a northeast trending crack dipping relatively shallowly to the northwest and indicate a VLP source centroid and mechanism controlled by a stable structural geologic feature beneath Yasur. We interpret the results in the framework of gas slug ascent through the conduit responsible for Yasur explosions. The VLP mechanism and timing with infrasound (when present) are explained by a shallow-buffered top-down model in which slug ascent is relatively aseismic until reaching the base of a shallow section. Slug disruption in this shallow zone triggers a pressure disturbance that propagates downward and couples at the conduit base (VLP centroid). If the shallow section is open, an explosion propagates to the surface, producing infrasound. In the case of (the same multiplet) VLPs occurring without surficial explosions and weak or no infrasound, the decoupling of the dominant VLPs at ∼900–1000 m depth from surficial explosions and infrasound strongly indicates buffering of the terminal slug ascent. This buffering could be achieved by a variety of conditions at or directly beneath the vents, such as a high-viscosity layer of crystal-rich magma, a debris cap from backfill, a foam layer, or a combination of these. The dominant VLP at Yasur captured by our experiment has a source depth and mechanism separated from surface processes and is stable over time. 

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4. The evolution of continuum polarization in type II supernovae as a diagnostic of ejecta morphology

   https://doi.org/10.1051/0004-6361/202347808  

   Dessart, Luc; Hillier, D John; Leonard, Douglas C (April 2024, Astronomy & Astrophysics)

   The linear polarization of the optical continuum of type II supernovae (SNe), together with its temporal evolution is a promising source of information about the large-scale geometry of their ejecta. To help access this information, we undertook 2D polarized radiative transfer calculations to map the possible landscape of type II SN continuum polarization (P_cont) from 20 to 300 days after explosion. Our simulations were based on crafted 2D axisymmetric ejecta constructed from 1D nonlocal thermodynamic equilibrium time-dependent radiative transfer calculations for the explosion of a red supergiant star. Following the approach used in our previous work on SN 2012aw, we considered a variety of bipolar explosions in which spherical symmetry is broken by material within ~30° of the poles that has a higher kinetic energy (up to a factor of two) and higher⁵⁶Ni abundance (up to a factor of about five, allowing for⁵⁶Ni at high velocity). Our set of eight 2D ejecta configurations produced considerable diversity inP_cont(λ~ 7000 Å), although its maximum of 1–4% systematically occurs around the transition to the nebular phase. Before and after this transition,P_contmay be null, constant, rising, or decreasing, which is caused by the complex geometry of the depth-dependent density and ionization and also by optical depth effects. Our modest angle-dependent explosion energy can yield aP_contof 0.5–1% at early times. Residual optical-depth effects can yield an angle-dependent SN brightness and constant polarization at nebular times. The observed values ofP_conttend to be lower than obtained here. This suggests that more complicated geometries with competing large-scale structures cancel the polarization. Extreme asymmetries seem to be excluded. 

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5. A Luminous Red Supergiant and Dusty Long-period Variable Progenitor for SN 2023ixf

   https://doi.org/10.3847/2041-8213/ace618  

   Jencson, Jacob E.; Pearson, Jeniveve; Beasor, Emma R.; Lau, Ryan M.; Andrews, Jennifer E.; Bostroem, K. Azalee; Dong 董, Yize 一泽; Engesser, Michael; Gomez, Sebastian; Guolo, Muryel; et al (July 2023, The Astrophysical Journal Letters)

   Abstract We analyze pre-explosion near- and mid-infrared (IR) imaging of the site of SN 2023ixf in the nearby spiral galaxy M101 and characterize the candidate progenitor star. The star displays compelling evidence of variability with a possible period of ≈1000 days and an amplitude of Δm≈ 0.6 mag in extensive monitoring with the Spitzer Space Telescope since 2004, likely indicative of radial pulsations. Variability consistent with this period is also seen in the near-IRJandK_sbands between 2010 and 2023, up to just 10 days before the explosion. Beyond the periodic variability, we do not find evidence for any IR-bright pre-supernova outbursts in this time period. The IR brightness ( $M_{K_s}=-10.7$mag) and color (J−K_s= 1.6 mag) of the star suggest a luminous and dusty red supergiant. Modeling of the phase-averaged spectral energy distribution (SED) yields constraints on the stellar temperature ( $T_{\mathrm{eff}}={3500}_{-1400}^{+800}$K) and luminosity ( $\log L/L_{\odot }=5.1\pm 0.2$). This places the candidate among the most luminous Type II supernova progenitors with direct imaging constraints, with the caveat that many of these rely only on optical measurements. Comparison with stellar evolution models gives an initial mass ofM_init= 17 ± 4M_⊙. We estimate the pre-supernova mass-loss rate of the star between 3 and 19 yr before explosion from the SED modeling at $\dot{M}\approx 3\times {10}^{-5}$to 3 × 10⁻⁴M_⊙yr⁻¹for an assumed wind velocity ofv_w= 10 km s⁻¹, perhaps pointing to enhanced mass loss in a pulsation-driven wind. 

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