Episodic earthquake mechanisms and intervening seismicity during the 2018 summit collapse at Kilauea caldera

<p>The 2018 rift zone eruption of Kilauea volcano was accompanied by a remarkable and episodic collapse of its summit. Between May-August the eruption and collapse sequence included over 70,000 earthquakes (M≥0) and 54 major earthquakes (M≥5). We analyzed the seismicity in the Kilauea summit region and estimated seismic full moment tensors with their uncertainties for the 54 M≥5 events. These events occurred at almost daily intervals and were accompanied by intense seismicity which was concentrated between 0-3 km depths beneath the Halema‘uma‘u pit crater. The hypocenters reveal partial elliptical patterns (map view) that migrated downward by ∼200 m. The moment tensors reveal remarkably consistent mechanisms, with negative isotropic source types and localized uncertainties, and vertical P-axis orientations. From the moment tensors we derived Poisson’s ratios which are variable (ν = 0.1 − 0.3) for the first half of the collapse events and converged to ν ∼ 0.28 from June 26 onward.</p>

IMPROVED ANISOTROPIC LINEAR SOURCE FORMULATION FOR MULTIPHYSICS PROBLEMS

This work seeks to extend an existing formulation of the method of characteristics with linear source approximation for problems with dynamic cross sections. The previous formulation eliminated cross section dependence of precomputed coefficients for systems with an isotropic source. The method is extended to include a formulation for spatially flat anisotropic scattering that eliminates cross section dependence of precomputed coefficients without adding additional operations; increasing efficiency in multiphysics simulations where cross sections can be subject to change. The new formulation is implemented in the MPACT code and tested on two problems: 3D transport assembly calculations using MPACT’s 2D/1D method and a 3D assembly with T/H feedback using MPACT’s 2D/1D method coupled with COBRA-TF. This work demonstrates that the new linear source formulation allows for the number of mesh elements to be significantly reduced while maintaining accuracy, leading to shorter run-times for 3D cases with fixed cross sections, and substantial reduction of memory usage for 3D cases with fixed cross sections. The multiphysics calculations show similar runtimes for the same accuracy with significant reduction of memory. For similar accuracy, the method proved effective in reducingmemory usage by, on average, 30% for 3D problems and 21% for multiphysics problems.

Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d ≲ 10 pc) anomalously dense (ne ∼ 102 cm−3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (η UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10 μas, yielding an apparent brightness temperature for an isotropic source of ≳1014 K.

On the mass of bootstrapped Newtonian sources

We show that the bootstrapped Newtonian potential generated by a uniform and isotropic source does not depend on the one-loop correction for the matter coupling to gravity. The latter, however, affects the relation between the proper mass and the Arnowitt–Deser–Misner (ADM) mass and, consequently, the pressure needed to keep the configuration stable.

Dynamic rupture and seismic radiation in a damage-breakage rheology model

<p>We present simulations of dynamic ruptures in a continuum damage-breakage rheological model and waves radiated by the ruptures observed in the far field. The model combines aspects of a continuum viscoelastic damage framework for brittle solids with a continuum breakage mechanics for granular flow. The brittle instability is associated with a phase transition between a damaged solid with distributed cracks and a granular medium within the generated rupture zone. The formulation significantly extends the ability to model brittle processes in structures with complex volumetric geometries and evolving elastic properties, compared to the traditional models of pre-existing frictional surface(s) in a solid with fixed properties. A set of numerical simulations examines the sensitivity of dynamic ruptures, seismic source properties and radiated waves to material properties controlling the coupled damage-breakage evolution, the thickness and geometry of the damage zone, and fluidity of the granular material. The simulations are performed in two stages. First, details of the rupture process are simulated using adaptive fine grid model. The results of these simulations include source parameters such as rupture velocity, potency, stress and strain drop, heat generation, and others. In the second stage, the obtained velocity source function is used for simulating radiated seismic waves and synthetic seismograms sampled by stations around the rupture zone and in the far field.</p><p>Detailed comparisons between the simulated source properties and those obtained by analyzing the synthetic seismograms demonstrate the relations between different source processes and inferred seismic parameters (potency, strain drop, directivity, rupture velocity, corner frequency, and others). One main effect shown in these simulations emphasizes the important role of rock damage and granulation process generating dynamic expansion-compaction around the process-zone. This expansion-compaction process leads to isotropic source term, while shear motion that accumulates behind the propagating front produces deviatoric deformation and shear heating behind the rupture front. Changing through our simulations, source geometries, and fault zone properties, we demonstrate that the process-zone dissipation due to the damage-breakage mechanism, and the isotropic source component, significantly affect the radiation pattern, rupture directivity, S/P energy partitioning, seismic potency and moment, and more. The results are significant for understanding better the proper usage and limitations of methods applied within the observational framework of earthquake seismology.</p>

Multi-Sensor SAR Geodetic Imaging and Modelling of Santorini Volcano Post-Unrest Response

Volcanic history of Santorini over recent years records a seismo-volcanic unrest in 2011–12 with a non-eruptive behavior. The volcano deformation state following the unrest was investigated through multi-sensor Synthetic Aperture Radar Interferometry (InSAR) time series. We focused on the analysis of Copernicus Sentinel-1, Radarsat-2 and TerraSAR-X Multi-temporal SAR Interferometric (MT-InSAR) results, for the post-unrest period 2012–17. Data from multiple Sentinel-1 tracks and acquisition geometries were used to constrain the E-W and vertical components of the deformation field along with their evolution in time. The interpretation of the InSAR observations and modelling provided insights on the post-unrest deformation pattern of the volcano, allowing the further re-evaluation of the unrest event. The increase of subsidence rates on Nea Kameni, in accordance with the observed change of the spatial deformation pattern, compared to the pre-unrest period, suggests the superimposition of various deformation sources. Best-fitting inversion results indicate two deflation sources located at southwestern Nea Kameni at 1 km depth, and in the northern intra-caldera area at 2 km depth. A northern sill-like source interprets the post-unrest deflation attributed to the passive degassing of the magma intruded at 4 km during the unrest, while an isotropic source at Nea Kameni simulates a prevailing subsidence occurring since the pre-unrest period (1992–2010).