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Assessing the impact of automatically derived depth phases on the determination of earthquake hypocentres – application to the South American subduction zone

Geophysical Journal International - Fri, 02/27/2026 - 00:00

SummaryAccurate earthquake hypocentres are fundamental to a wide range of geophysical studies, yet source depth remains poorly constrained in teleseismic earthquake catalogues. Near source surface reflections such as pP, sP, and sS (known as depth phases) provide critical information for resolving hypocentral depth, particularly for intermediate-depth earthquakes. The number of depth phases reported by global earthquake monitoring agencies has declined significantly in recent decades, potentially reducing the precision of resolved earthquake depths. To address this, we automatically detect P, pP, sP, S and sS phase arrivals using teleseismic ad-hoc arrays. We detect these phases for earthquakes in the South American Subduction Zone (SASZ) at depths of 40–350 km and between mb 4.7 to 6.5. The identified phases are integrated with the phases reported to the ISC Bulletin, and used to relocate earthquakes with ISCloc. We assess the impact of incorporating automatically detected, ad-hoc array-derived depth phases on earthquake relocations across the SASZ, and find an improvement in depth resolution for 88.8% of earthquakes. Using this enhanced catalogue we investigate the structure of the Wadati-Benioff zone, focusing on two significant earthquakes: the 2005 Mw 7.7 Tarapacá and 2019 Mw 8.0 Peru events. Finally, we successfully apply our methodology to deep focus earthquakes (350-700 km), which further define the deepest portion of the seismogenic slab. Our results demonstrate the potential for automatically detected, ad-hoc array-derived depth phases to substantially improve the accuracy of teleseismic earthquake hypocentres, and offer further constraint upon slab geometry and seismogenic structure.

Non-physical head wave identification in P-wave arrivals retrieved from seismic ambient noise at the Chémery underground gas storage (France)

Geophysical Journal International - Fri, 02/27/2026 - 00:00

SummaryThis study aims to retrieve P waves from seismic ambient noise recorded by a dense local broadband network at the ChÃ©mery underground gas storage site, where anticline deformation was previously identified through wells and seismic reflection survey. To this end, we propose a new approach for reconstructing P waves from ambient noise. We process the passive seismic data to reconstruct the body wave component of the empirical Green’s functions (EGF). The retrieved P-wave arrivals were identified and analyzed, revealing that in this dataset, the picked arrival times likely correspond to non-physical head waves rather than direct or diving P-wave arrivals between virtual sources and receivers. Numerical simulations support this idea of non-unique interpretation of the passively reconstructed P-wave arrivals. The simulations suggest the potential for mapping lateral heterogeneities above the critical refractor as a cumulative time-delay, although for this dataset the anticline-induced time-delay is likely within the measurement uncertainties. It is found that the dominance of non-physical head waves over diving waves is primarily due to the large distance from the network to ambient noise sources.

Deep carbon cycling in subduction zones: 1. Thermally-controlled metamorphic decarbonation of subducting oceanic slab

Geophysical Journal International - Fri, 02/27/2026 - 00:00

SummaryOceanic subduction zone is the dominant pathway for transporting carbon into the interior of the Earth, and thus plays a critical role in deep carbon cycling. Despite being recognized as a key mechanism for carbon release in subduction zones, the metamorphic decarbonation outflux and efficiency remain subjects of ongoing debate. The thermal structure of subduction zone is widely recognized as a primary dynamic control on metamorphic decarbonation, however, the quantitative relationship between metamorphic carbon outflux and simplified thermal parameters of subduction zones (here defined as φ= slab age × subduction velocity/100 in kilometer) remains poorly constrained. On the other hand, previous studies on metamorphic decarbonation have been conducted within two distinct scenarios: the P-T-dependent decarbonation (PTD) system versus P-T-H2O-dependent decarbonation (PTHD) system, yet a quantitative comparison between these two scenarios remains lacking. In order to investigate the metamorphic decarbonation behavior of subducting slab in the PTD versus PTHD systems, we develop a coupled thermo-petrological model by integrating the thermodynamic dataset of temperature-pressure-(H2O)-dependent CO2 content into the thermal model of subduction zones. Systematic numerical models indicate that the metamorphic carbon outflux in the PTHD system is about 50 per cent lower than that predicted in the PTD system. Meanwhile, the quantitative functional relationship has been built between the metamorphic carbon outflux and φ, which reveals that the decarbonation outflux and efficiency decrease exponentially with increasing φ in both systems. Under present-day widespread subduction thermal conditions with the φ values of around 30 km, both PTD and PTHD system models yield low metamorphic decarbonation efficiency, suggesting that a substantial proportion of slab carbon is likely retained in the slab and transported into the deeper mantle.