The 15 January 2022 submarine volcanic eruption of Hunga Tonga-Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid-oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30-min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T-wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray-theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental-mode T-wave eigenfunction is maximal. Adjusted for geometric spreading, the cross-correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T-wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.
Hydrologic observations and experimental studies indicate that inelastic dilation from coseismic fault damage can cause substantial pore pressure reduction, yet most near-fault hydromechanical models ignore such inelastic effects. Here, we present a 3-D groundwater flow model incorporating the effects of inelastic dilation based on an earthquake dynamic rupture model with inelastic off-fault deformation, both on pore pressure and permeability enhancement. Our results show that inelastic dilation causes mostly notable depressurization within ∼1 km off the fault at shallow depths (<3 km). We found agreement between our model predictions and recent field observations, namely that both sides of the fault can experience large-magnitude (∼tens of meters) water level drawdowns. For comparison, simulations considering only elastic strain produced smaller water level changes (∼several meters) and contrasting signs of water level change on either side of the fault. The models show that inelastic dilation is a mechanism for coseismic fault depressurization at shallow depths. While the inelastic dilation is a localized phenomenon which is most pronounced in the fault zone, the pressure gradients produced in the coseismic phase have a broader effect, increasing fluid migration back into the fault zone in the postseismic phase. We suggest field hydrologic measurements in the very-near-field (<1 km) of active faults could capture damage-related pore pressure signals produced by inelastic dilation, helping improve our understanding of fault mechanics and groundwater management near active faults.
We develop a simple explanation for Gutenberg-Richter (G-R) size scaling of earthquakes on a single fault. We discretize the fault and consider all possible contiguous ruptures at that level of discretization. In this static model, we assume that slip scales with rupture length, and that the rupture rates at each point along the fault are consistent with an a priori long-term slip rate. These simple assumptions define an (under-determined) non-negative least squares inverse problem. Each solution to this inverse problem is a set of earthquake rates that matches the slip-rate constraint. We use a Markov Chain Monte Carlo (MCMC) algorithm to uniformly sample the solution space assuming constant slip rates along the fault. At finer discretizations, deviations from G-R behavior decrease, which is consistent with an entropic pressure toward G-R solutions. When the fault is discretized into 10 or more segments, random solutions found by the MCMC algorithm have G-R size scaling, even though there are trivial solutions that, for example, have earthquakes of only one size. This is because there are simply far more solutions that have G-R scaling; as the problem size increases, the strong degeneracy of G-R solutions results in other solutions becoming improbably rare. Also, the entropically favored G-R distribution has a -value of approximately 1, which agrees with measured -values in real earthquake catalogs.
The crystal structure of Earth's solid inner core is fundamental to understanding the chemical composition and dynamical evolution of the core. However, despite extensive research, the structure still remains controversial with competing hypotheses regarding the stability of various Fe phases (e.g., bcc, fcc, and hcp). In this paper, we review the studies on the crystal structure of Fe under inner core conditions, and find, in line with previous work, that the main challenges come from the small energy differences between these structures. This has led to a variety of different conclusions across varying theoretical methods and precision, including ab initio, force field and machine learning methods. To address this problem, we employ a Bain path thermodynamic integration approach to reach consistent conclusions among different methods; we find that bcc Fe is mechanically stable but thermodynamically less stable under inner core conditions. Using the energetics from the Bain path method as a benchmark, we establish the requirements for converging free energy calculations through a two-phase thermodynamic modeling approach. These calculations confirm that the hcp phase is the most stable, exhibiting the highest melting temperature regardless of the method used. This unified conclusion on the hcp phase as the stable crystal structure provides a robust foundation for future studies on Earth's core.
The mechanisms responsible for intracontinental basaltic volcanism are not well understood. The Cenozoic long-lived (∼30 Myr) and diffuse intraplate volcanism in central Mongolia is an ideal natural lab to address this issue. Here we present a high-resolution lithospheric 3-D shear-wave velocity model using ambient noise tomography with data mainly from two dense seismic arrays. Our model shows strong lithospheric heterogeneities beneath central Mongolia, correlating well with the occurrence of Cenozoic basaltic volcanism. Specifically, relatively thick, high-velocity lithospheric mantle is imaged beneath the central Hangai Dome and the Hövsgöl region where Precambrian basements have been suggested to exist, which likely represent relicts of old cratonic lithosphere. Step changes in lithospheric thickness formed at their peripheries due to heterogenous lithospheric thinning or modification likely caused by recent deep mantle upwelling. More interestingly, most basaltic volcanism in central Mongolia is located at or near these strong lateral gradients of lithospheric thickness, suggesting the important role of small-scale convection in their formation due to lithospheric thickness undulations. Taken together, the Cenozoic intraplate volcanism in central Mongolia was likely controlled by inherited lithospheric heterogeneities and mantle upwelling.
Intraplate volcanism has long been linked to deep mantle plumes. However, recent studies showed that intraplate magmatism can originate from transition zone dynamics, where lower-mantle plumes might be ponding, creating a Thermal Boundary Layer (TBL). Inspired by intraplate volcanoes in Eifel, Massif Central and Hainan that are distributed near tips of stagnant slabs imaged at transition zone depth, we hypothesize that subducted slabs might destabilize the TBL and trigger upper mantle plumes (secondary plumes), leading to intraplate volcanism. So far, the generation of such secondary plumes and the influence of slabs on plumes remain poorly understood. In this study, we perform 2D upper-mantle geodynamic models with a TBL imposed at 670 km depth interacting with a slab of an intra-oceanic subduction zone. The effects of various slab geometries (rollback, rollover and intermediate), TBL temperature and heating time are tested. Our models show that slabs of all geometries can destabilize the TBL, initiating secondary plumes ahead of and behind the slab. All plumes are deflected by the slab-induced mantle flow and a sinking slab may even suppress plumes beneath it. However, a higher TBL temperature and a longer pre-subduction heating duration may increase buoyancy flux of secondary plumes, making them more resistant to slab-driven flow. Under all conditions explored in this study, the strength of secondary plumes produced in our models is comparable to that of the Eifel plume. This paper elucidates slab-plume interaction and their impact on intraplate volcanism with applications to the Eifel, Massif Central and Hainan volcanic areas.
Accurate earthquake location is of fundamental importance for understanding seismogenic processes, revealing the Earth's interior structure, and mitigating seismic hazard. However, precisely determining the depth of an earthquake is often challenging due to the severe trade-off between focal depth and origin time, especially in the absence of nearby seismic stations. To address this challenge, we have developed an integrative procedure for reliably and efficiently identifying the sP depth phase in local and regional seismic records. After picking the traveltimes of first-arriving P, S waves and sP depth phases, we sequentially refine earthquake hypocenter (longitude, latitude and depth) and origin time within a Bayesian inversion framework. The efficacy of the proposed depth phase identification procedure and earthquake location method is validated through the analysis of small-to-moderate aftershocks that occurred within 2 months of the 2019 Mw 7.1 Ridgecrest earthquake. Our study shows that including depth phases can significantly reduce location uncertainty in depth by a factor of five. Moreover, the results achieved by jointly using first arrivals and depth phases are less dependent on the background velocity model, enabling more accurate location estimates for 86.6% of the examined earthquakes. In regions northwest of the mainshock nucleation area, the base of the seismogenic zone is located generally below 10 km, likely sandwiching a much shallower brittle-to-ductile transition zone (<4 km) beneath the Coso geothermal site. This locally abrupt change in rock rheology may modulate the rupture propagation of large earthquakes.
Imaging the Earth's thermochemical structure is crucial for understanding its dynamics and evolution. Moreover, the increased demand for critical minerals and geothermal energy driven by the energy transition has intensified the need for reliable subsurface models. Multi-Observable Thermochemical Tomography (MTT) is a simulation-based, probabilistic inversion platform designed to harness the combined sensitivities of multiple geophysical data sets and thermodynamic modeling. It produces internally consistent estimates of the Earth's interior as probability distributions, offering a powerful means for uncertainty quantification. Here, we present an updated MTT formalism and assess its benefits and limitations to image the thermochemical structure of the lithosphere-asthenosphere system. Individual and combined sensitivities of different observables to parameters of interest (e.g., temperature, composition, crustal architecture) are explored using challenging synthetic models. Our findings demonstrate that a judicious combination of observables can retrieve complex thermochemical structures relevant to greenfields exploration. We then apply MTT to study two cratonic regions of geological and economic significance. In the Superior Craton, we jointly invert receiver functions, gravity anomalies, gravity gradients, geoid anomalies, Rayleigh-wave dispersion curves, absolute elevation and surface heat flow. In the North Australian Craton, we incorporate new data from the AusArray and add teleseismic P- and S-phase travel times to the data sets. The imaged lithospheric architectures provide new insights into the tectonic evolution of these two regions and the physical meaning of geophysical signatures. Additionally, these models offer unique proxies to guide exploration efforts for clean energy and critical minerals and serve as reference models for future high-resolution studies.
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