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Shear wave velocity image of an ophiolite nappe in New Caledonia and its implications for Eocene subduction initiation beneath Zealandia

Geophysical Journal International - Fri, 12/12/2025 - 00:00

SummaryTo better understand Eocene ophiolite emplacement and subduction initiation in northeastern Zealandia, we analysed ambient noise to image shallow (0–3 km) shear wave velocity structures of and beneath an ophiolite nappe in southern Grande Terre, New Caledonia. We assessed the uncertainties of each dispersion curve to obtain stable dispersion curves at short periods (<1 s) from a network of 17 seismic stations, whose average interstation distance is ∼15 km. We obtained 1D velocity profiles and interpolated them to generate 2D transects with a lateral resolution <2 km. Two velocity discontinuities were imaged at depths of 100 m and 400–700 m, representing surface regolith and the base of the ophiolite nappe, respectively. The ophiolite nappe is underlain by continental basement rocks in the centre of the island and sedimentary rocks near the east and west coasts. The base of the nappe shallows at ∼2.5° westward to the surface at its southwest flank. Based on the geometry of the ophiolite nappe, we suggest a down-going gravity-driven emplacement mechanism, and note similarities to allochthons in Reinga Basin and Raukumara Basin of northern New Zealand. The ophiolite nappe and underlying bedrock are more fractured on their east flank due to syn/post emplacement deformation, isostatic adjustment and present flexural bending.

Joint Source-Structure Full Waveform Inversion Using Distributed Acoustic Sensing

Geophysical Journal International - Fri, 12/12/2025 - 00:00

SummarySeismological inversion traditionally targets either source parameters, such as location and moment tensor, or structural parameters, such as velocity and anisotropy. However, the natural formulation of Full-Waveform Inversion, often used for high-resolution structural model estimation, is to jointly invert for source and structural parameters. The common practice of holding source parameters, after initial estimation, fixed throughout the inversion inherently leads to biased solutions of the structural model, and vice versa. Whereas a joint inversion suffers from severe non-uniqueness, we demonstrate that leveraging the large amounts of data available from Distributed Acoustic Sensing (DAS) can yield robust and unbiased estimations of source and structural parameters, provided an appropriate misfit function and optimisation scheme are used. We show how the size of the data space and eventual convergence can be improved by supplementing the phase misfit objective function with amplitude information. To this end, we formulate a new misfit function, the normalised envelope. To support native DAS data implementations, we calculate the adjoint sources for the new misfit function when defined directly on strain or strain-rate data. We also show how a new approach to preconditioning as part of the L-BFGS optimisation scheme allows for effective updates of all parameters in the same iteration, despite enormous differences in their relative importance. We test our approach in a challenging synthetic noisy 2D scenario, showing a considerable reduction in source parameter errors and an improved S-wave velocity model. We also show a 3D synthetic case with an idealised DAS recording array, demonstrating a significant reduction of source parameter errors using realistic initial estimates and structural model errors. We argue that the proposed methodology can be used to improve the quality of earthquake catalogues and high-resolution structural models in seismically active regions, especially at the local-to-regional scale. None the less, computational cost remains a major challenge of the method.

The shape of sand grains reveals the distance traveled by rivers

Phys.org: Earth science - Thu, 12/11/2025 - 21:24

A team from the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) has collaborated with researchers from the University of Málaga (UMA) and the University of Córdoba (UCO) on an article published in the Journal of Sedimentary Research, which examines the relationship between the shape of sand grains and the distance traveled in the Arlanzón River (Burgos) and the Guadalhorce River (Málaga).

How Earth's mantle locked away vast amounts of water in early magma ocean

Phys.org: Earth science - Thu, 12/11/2025 - 20:50

Some 4.6 billion years ago, Earth was nothing like the gentle blue planet we know today. Frequent and violent celestial impacts churned its surface and interior into a seething ocean of magma—an environment so extreme that liquid water could not exist, leaving the entire planet resembling an inferno.