Geophysical Journal International

SummaryThe ability to detect low frequency sounds from distant energetic events depends on the temperature and wind structure of the atmosphere. Thus, from time to time surface-based acoustic detectors may not be able to capture sounds arriving from certain directions. However, the temperature minimum at the tropopause may create an acoustic duct called the “AtmoSOFAR” channel that could transmit acoustic waves laterally – but perhaps not to the ground. If true, then elevated sensors such as those borne aloft by balloons would record the signatures even in regions where ground based sensors cannot. This has been difficult to prove because high altitude acoustic sources are rare and balloon deployments are sporadic. This work describes the detection and characterization of powerful acoustic waves generated during the launch and terminal explosion of the SpaceX Starship rocket on 20 April 2023 using a pair of microbarometers on a stratospheric balloon. The signals traveled through the AtmoSOFAR channel, carrying information about the size and nature of their source. This channel also appears to leak some acoustic energy to the ground, in agreement with previous studies. The acoustic yield of the Starship terminal explosion was on the order of 103 tons TNT equivalent, which agrees with the estimated fuel load of the vehicle to about a factor of 2, but is two orders of magnitude larger than optical estimates. These results support an earlier study that claimed lateral transmission of sound from a smaller rocket through the AtmoSOFAR channel. The transmission of source information through the AtmoSOFAR channel motivates its use for monitoring other natural and anthropogenic events using balloon-borne sensors. This may become increasingly important as more and more private and government entities conduct spacecraft launches and reentries. It may also provide a means of monitoring clear air turbulence and other sound-generating atmospheric phenomena at a distance.

SummaryThe Sichuan-Yunnan region is a crucial area for studying the deformation and tectonic evolution of the lithosphere within the Tibetan Plateau. However, a significant controversy exists about the spatial distribution of the low-viscosity zones in its mid-lower crust. Herein, we utilized a combination of topography, geoid height, surface heat flow, and Rayleigh wave phase velocity dispersion curves to ascertain the lithospheric temperature, seismic wave velocity, and density structure in this region. By correlating the inverted velocity and density structures with laboratory measurements of rock velocity and density, we further deduced the lithospheric lithology structure and rheological property of the Sichuan-Yunnan region. Our findings suggest that the lower crust of this region is predominantly composed of felsic granulite. The lower crust of the Qiangtang Block, the Chuan-Xi Block, the Dian-Zhong Block west of the Lvzhijiang fault, and the IndoChina Block exhibit extensive areas with low-viscosity characteristics (<1021 Pa·s). In contrast, the Sichuan Basin, the Eastern Himalayan Syntaxis, and the central region of the Emeishan Igneous Province are characterized by high strength. We argue that the collision between the Indian and Eurasian plates led to the thickening of the Qiangtang Block's crust, producing a large low-viscosity area within the mid-lower crust. The delamination of the IndoChina lithosphere may cause the upwelling of mantle material, thereby weakening the lithosphere of the Dian-Zhong Block west of the Lvzhijiang fault and the IndoChina Block. This study delineates the spatial distribution of low-viscosity zones within the mid-lower crust of the Sichuan-Yunnan region, offering a foundational rheological model that can be instrumental for subsequent seismological and dynamic analyses.

SummaryApplications of spectral induced polarization (SIP) require electrodes that maintain hydrologic contact with surrounding soils to capture small electrical responses, often observed as phase shifts in milliradians. For unsaturated soils, electrodes must overcome the increased electrical contact impedance due to reduced pore fluid. Traditional designs use a ceramic membrane electrode (CME) with a water reservoir and metal conductor, requiring periodic maintenance to retain electrolytic solution. For field applications where maintenance is impractical, alternative designs are needed. This study evaluated a new electrode design (silica flour electrode, SFE) alongside a CME design. SFEs use packed silica flour to store water via capillary forces against a metal conductor. The study examined both designs in three variably saturated soils at soil suctions up to 700 mbar and soil water contents below 1 per cent, with SIP measurements across 0.01 to 10,000 Hz frequencies. SFEs match CMEs at high frequencies and perform better at lower frequencies, without requiring ongoing maintenance, making them ideal for field use. In water-only experiments, CMEs produced errors and high noise below 1.5 Hz, whereas SFEs were more accurate. However, CMEs performed better above 300 Hz. In fine sand, SFEs performed better due to the relatively lower contact impedance as compared to CMEs. Both electrode types performed comparably in silty sand and silt loam soils, although CMEs required ongoing maintenance, suggesting potential for long-term reliability issues.

SummaryThe finite-difference method (FDM), limited by uniform grids, often encounters severe oversampling in high-velocity regions when applied to multi-scale subsurface structures, leading to reduced computational efficiency. A feasible solution to this issue is the use of non-uniform grids. However, previous discontinuous grid approaches required careful consideration of interpolation operations in transition regions, while single-block continuous grids lacked flexibility. This paper proposes a novel approach using multi-block stretched grids with positive and negative singularities to achieve non-uniform grids, the numerical simulation of seismic waves is realized by combining it with the curvilinear grid finite-difference method (CGFDM). Our method facilitates seamless information exchange between coarse and fine grids without additional interpolation or data processing and allows for flexible grid configurations by adjusting singularity pairs.The effectiveness of our approach is verified through comparisons with the generalized reflection/transmission method (GRTM) and the finite-element method (FEM). Numerical experiments demonstrate the method's reliable accuracy and significant reduction in grid points compared to uniform grids. Although the stability of our method has not been rigorously mathematically proven, we demonstrate that the algorithm remains applicable for sufficiently long simulations to address realistic scenarios.

SummaryFor a weakly anisotropic medium, Rayleigh and Love wave phase speeds at angular frequency ω and propagation azimuth ψ are given approximately by V(ω, ψ) = A0 + A2ccos 2ψ + A2ssin 2ψ + A4ccos 4ψ + A4ssin 4ψ. Earlier theories of the propagation of surface waves in anisotropic media based on non-degenerate perturbation theory predict that the dominant components are expected to be 2ψ for Rayleigh waves and 4ψ for Love waves. This paper is motivated by recent observations of the the 2ψ component for Love waves and 4ψ for Rayleigh waves, referred to here as “unexpected anisotropy”. To explain these observations, we present a quasi-degenerate theory of Rayleigh-Love coupling in a weakly anisotropic medium based on Hamilton’s Principle in Cartesian coordinates, benchmarking this theory with numerical results based on SPECFEM3D. We show that unexpected anisotropy is expected to be present when Rayleigh-Love coupling is strong and recent observations of Rayleigh and Love wave 2ψ and 4ψ anisotropy can be fit successfully with physically plausible models of a depth-dependent tilted transversely isotropic (TTI) medium. In addition, when observations of the 2ψ and 4ψ components of Rayleigh and Love anisotropy are used in the inversion, the ellipticity parameter ηX, introduced here, is better constrained, we can constrain the absolute dip direction based on polarization measurements, and we provide evidence that the mantle should be modeled as a tilted orthorhombic medium rather than a TTI medium. Ignoring observations of unexpected anisotropy may bias the estimated seismic model significantly. We also provide information about the polarization of the quasi-Love waves and coupling between fundamental mode Love and overtone Rayleigh waves in both continental and oceanic settings. The theory of SV-SH coupling for horizontally propagating body waves is presented for comparison with the surface wave theory, with emphasis on results for a TTI medium.