Accurately determining the seismic structure of the deep crust of continents is crucial for understanding the geological record and continental dynamics. However, traditional surface wave methods often face challenges in solving the trade-offs between elastic parameters and discontinuities. In this work, we present a new approach that combines two established inversion techniques, receiver function H-ᵰ5; stacking and joint inversion of surface wave dispersion and receiver function waveforms, within a Bayesian Monte Carlo (MC) framework to address these challenges. As demonstrated by the synthetic test, the new method greatly reduces trade-offs between critical parameters, such as the deep crustal Vs, Moho depth, and crustal Vp/Vs ratio. This eliminates the need for assumptions regarding crustal Vp/Vs ratios in joint inversion, leading to a more accurate outcome. Furthermore, it improves the precision of the upper mantle velocity structure by reducing its trade-off with Moho depth. Additional notes on the sources of bias in the results are also included. Application of the new approach to USArray stations in the Northwestern US reveals consistency with previous studies and also identifies new features. Notably, we find elevated Vp/Vs ratios in the crystalline crust of regions such as coastal Oregon, suggesting potential mafic composition or fluid presence. Shallower Moho depth in the Basin and Range indicates reduced crustal support to the topography. The uppermost mantle Vs, averaging 5 km below Moho, aligns well with the Pn-derived Moho temperature map, offering the potential of using Vs as an additional constraint to Moho temperature and crustal thermal properties.

We use seismic ambient noise recorded by dense ocean bottom nodes (OBNs) in the Gorgon gas field, Western Australia, to compute time-lapse seafloor models of shear-wave velocity. The extracted hourly cross-correlation (CC) functions in the frequency band 0.1 – 1 Hz contain mainly Scholte waves with very high signal to noise ratio. We observe temporal velocity variations (dv/v) at the order of 0.1% with a peak velocity change of 0.8% averaged from all station pairs, from the conventional time-lapse analysis with the assumption of a spatially homogeneous dv/v. With a high-resolution reference (baseline) model from full waveform inversion of Scholte waves, we present an elastic wave equation based double-difference inversion (EW-DD) method, using arrival time differences between the reference and time-lapsed Scholte waves, for mapping temporally varying dv/v in the heterogeneous subsurface. The time-lapse velocity models reveal increasing/decreasing patterns of shear-wave velocity in agreement with those from the conventional analysis. The velocity variation exhibits a ~24-hour cycling pattern, which appears to be inversely correlated with sea level height, possibly associated with dilatant effects for porous, low-velocity shallow seafloor and rising pore pressure with higher sea level. This study demonstrates the feasibility of using dense passive seismic surveys for quantitative monitoring of subsurface property changes in the horizontal and depth domain.

We present a class of “ellipsoidal rotation matrices” which can be used to characterise tectonic plate motion; where geocentric Cartesian coordinates travel along paths tangential to the ellipsoid. We contrast them with conventional Euler pole plate motion models which are more closely aligned with spherical coordinate systems and inherently induce a change in geodetic ellipsoidal height. We demonstrate the use of each in the Indo-Australian tectonic plate setting, which is known to move approximately 7 cm/yr in a north-northeast direction. Geocentric Datum of Australia 2020 (GDA2020) coordinates are “plate-fixed” static coordinates obtained using a conventional Euler pole plate motion model to align time dependent coordinates with the 2014 realisation of the International Terrestrial Reference Frame (ITRF) at the epoch 2020.0. We show that this Euler pole plate motion model can introduce ellipsoidal height velocities of up to -0.2 mm/yr. This is small but systematic, so pertinent for consideration with high accuracy vertical land motion studies using GDA2020 coordinates. We further investigate the comparative statistical accuracy of conventional Euler pole and the ellipsoidal models with respect to characterising plate motion captured in high quality GNSS data.

Accessible seafloor minerals located near mid-ocean ridges are noticed to mitigate projected metal demands of the net-zero energy transition, promoting growing research interest in quantifying global distributions of seafloor massive sulfides (SMS). Mineral potentials are commonly estimated using geophysical and geological data that lastly rely on additional confirmation studies using sparsely available, locally limited, seafloor imagery, grab samples, and coring data. This raises the challenge of linking in-situ confirmation data to geophysical data acquired at disparate spatial scales to obtain quantitative mineral predictions. Although multivariate datasets for marine mineral research are incessantly acquired, robust, integrative data analysis requires cumbersome workflows and experienced interpreters. Here, we introduce an automated two-step machine learning approach that integrates automated mound detection with geophysical data to merge mineral predictors into distinct classes and reassess marine mineral potentials for distinct regions. The automated workflow employs a U-Net convolutional neural network to identify mound-like structures in bathymetry data and distinguishes different mound classes through classification of mound architectures and magnetic signatures. Finally, controlled source electromagnetic data is utilized to reassess predictions of potential SMS volumes. Our study focuses on the Trans-Atlantic Geotraverse (TAG) area, which is amid the most explored SMS area worldwide and includes 15 known SMS sites. The automated workflow classifies 14 of the 15 known mounds as exploration targets of either high- or medium-priority. This reduces the exploration area to less than 7% of the original survey area from 49 km2 to 3.1 km2.

Seismic anisotropy parameters (φ, δt) were estimated between 2016 and 2021 using records from the Colombian National Seismological Network (CNSN) stations and related to regional tectonic features in NW South America. To achieve it, we studied the polarization of S-wave phases of local events (associated with Nazca-South America and Caribbean-South America subduction processes) and SKS phases of teleseismic events. The local events used in this study are deeper than 70 km and Mw≥4.5. The teleseismic events have epicentral distances between 90°-130° and Mw≥6.5. We found seismic anisotropy linked to structural control in the crust, suggesting a dominant S-wave polarization inside the faults rather than polarization due to the regional tectonic field stress. Erratic patterns of φ due to the possible presence of fluids in the crust were interpreted, and orientations of asthenospheric flow under the subducting slabs were inferred with a generalized convergence-oriented SW-NE under NW South America.

A characteristic feature of the main phase of geomagnetic storms is the dawn-dusk asymmetric depression of low- and mid-latitude ground magnetic fields, with largest depression in the dusk sector. Recent work has shown, using data taken from hundreds of storms, that this dawn-dusk asymmetry is strongly correlated with enhancements of the dawnside westward electrojet and this has been interpreted as a 'dawnside current wedge' (DCW). Its ubiquity suggests it is an important aspect of stormtime magnetosphere-ionosphere (MI) coupling. In this work we simulate a moderate geomagnetic storm to investigate the mechanisms that give rise to the formation of the DCW. Using synthetic SuperMAG indices we show that the model reproduces the observed phenomenology of the DCW, namely the correlation between asymmetry in the low-latitude ground perturbation and the dawnside high-latitude ground perturbation. We further show that these periods are characterized by the penetration of mesoscale bursty bulk flows (BBFs) into the dawnside inner magnetosphere. In the context of this event we find that the development of the asymmetric ring current, which inflates the dusk-side magnetotail, leads to asymmetric reconnection and dawnward-biased flow bursts. This results in an eastward expansion and multiscale enhancement of the dawnside electrojet. The electrojet enhancement extends across the dawn quadrant with localized enhancements associated with the wedgelet current systems of the penetrating BBFs. Finally, we connect this work with recent studies that have shown rapid, localized ground variability on the dawnside which can lead to hazardous geomagnetically induced currents.

Feature tracking is an efficient method for estimating glacier velocity by identifying the surface displacement between image pairs through maximum normalized cross-correlation (NCC). However, this method may misidentify displacement when noise is present in one or both images or when natural causes change glacier morphology. To improve accuracy, we developed the Google Earth Engine Glacier Velocity (GEEG-Vel) estimation method, which utilizes image enhancement and multi-image pair NCC maximization. GEEG-Vel results are further filtered using PyFilter, a Python routine that improves the glacier velocity estimation by utilizing velocity pairs obtained from GEEG-Vel. The combination of GEEG-Vel and PyFilter provides an efficient and accurate approach for glacier velocity estimation across various types of datasets, including optical and SAR data. We compared the results with the ITS_LIVE glacier velocity for the same period (2013-2018), and the mean velocity difference for each year was less than 10 m/year. Our study demonstrates that the combination of GEEG-Vel and PyFilter provides a reliable and accurate approach for glacier velocity estimation, which can be useful for monitoring the dynamics of glaciers and their response to climate change.

Geophysical granular flows generate seismic signals known as 'slidequakes' or 'landquakes', with low-frequency components whose generation by mean forces is widely used to infer hazard-relevant flow properties. Many more such properties could be inferred by understanding the fluctuating forces that generate slidequakes' higher frequency components and, to do so, Arran et al. (2021, https://doi.org/10.1029/2021JF006172) (A21) compared the predictions of pre-existing physical models to the forces exerted by laboratory-scale flows. However, A21 was unable to establish whether the laboratory flows exhibited basal slip, and the conditions for applying its results are therefore unclear. Here, we describe discrete-element simulations that examined the fluctuating forces exerted by steady, downslope-periodic granular flows on fixed, rough bases that prevented basal slip. We show that, in its absence, A21's results do not hold: simulated basal forces' power spectra have high-frequency components more accurately predicted using mean shear rates than using depth-averaged flow velocities, and can have intermediate-frequency components which we relate to chains of prolonged inter-particle contacts. We develop a 'minimal model', which uses a flow's collisional properties to even more accurately predict the high-frequency components, and empirically parametrize this model in terms of mean flow properties. Finally, we demonstrate that the bulk inertial number determines not only the magnitude ratio of rapidly fluctuating and mean forces on a unit basal area, consistent with A21, but also the relative magnitudes of the high and intermediate-frequency force components.Plain Language SummaryAny geophysical granular flow-such as a landslide, rockfall, or debris flow-exerts fluctuating forces that cause the ground to vibrate, in a 'slidequake' that can provide useful information about the flow. Here, we examine simulated slidequakes: computer models of the individual particles within idealized flows, the collisions between them, and the rapidly fluctuating forces they exert on the flow's base. By recording particle and collision properties throughout the flow, we examine pre-existing models for the fluctuating forces; develop, test, and simplify a new model; and relate ratios between forces to an 'inertial number' that characterizes different flows. Our results differ from those of laboratory experiments that previously investigated slidequakes, but the two sets of results can be combined to provide information about real geophysical flows.

We analyzed 49,592 teleseismic receiver functions recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast-marching method and a high-resolution 3-D velocity model (FWEA18) derived from full waveform inversion in computing P-to-S conversion times to better image the 410-km and 660-km discontinuities. Our results indicate that the common-conversion-point stacking of receiver functions using 3-D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410-km with a few small uplifted patches, and showed that the 660-km beneath the western Yangtze Craton is depressed by 10-25 km, which is likely caused by the stagnant Paleo-Pacific slab. The 660-km beneath the southern Cathaysia Block has a 5-15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ~150 km and located beneath the central Pearl River Mount Basin near Hong Kong. We speculate the topographic low occurs within the Hainan plume with a temperature excess of ~300-400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410-km. The southwest channel is likely the source that feeds the Hainan volcanoes.

Submarine volcano monitoring is vital for assessing volcanic hazards but challenging in remote and inaccessible environments. In the vicinity of Kita-Ioto Island, south of Japan, unusual M~5 non-double-couple volcanic earthquakes exhibited quasi-regular repetition near a submarine caldera. Following the 2008 earthquake, a distant ocean bottom pressure sensor recorded a distinct tsunami signal. In this study, we aim to find a source model of the tsunami-generating earthquake and quantify the pre-seismic magma overpressure within the caldera’s magma reservoir. Based on the earthquake’s atypical focal mechanism and efficient tsunami generation, we hypothesize that submarine trapdoor faulting occurred due to highly pressurized magma. To investigate this hypothesis, we establish a mechanical earthquake model that links pre-seismic magma overpressure to the size of the resulting trapdoor faulting, by considering stress interaction between a ring-fault system and a reservoir of the caldera. The model reproduces the observed tsunami waveform data. Our estimates indicate trapdoor faulting with large fault slip occurred in the critically stressed submarine caldera accommodating pre-seismic magma overpressure of ~10 MPa. The model infers that the earthquake partially reduced magma overpressure by 10–20%, indicating that the magmatic system maintained high stress levels even after the earthquake. Due to limited data, uncertainties persist, and alternative source geometries of trapdoor faulting could lead to estimate variations. These results suggest that magmatic systems beneath calderas are influenced much by intra-caldera fault systems. Monitoring and investigation of volcanic tsunamis and earthquakes help to obtain quantitative insights into submarine volcanism hidden under the ocean.

A document by Sergio León-Ríos. Click on the document to view its contents.

North of the Denali Fault, the collision between the Yakutat block with North America is accommodated by a fold-thrust belt that gives rise to the northern Alaska Range foothills. At the western end of the belt, the Kantishna Hills anticline hosts prominent microseismicity and surface deformation, together interpreted as active folding of the Kantishna Hills anticline above a midcrustal detachment. Here, we test for such a detachment by using anisotropy-aware receiver functions to image fabric contrasts within the crust and comparing the depths of such contrasts to seismicity statistics. Seismic stations near the crest of the Kantishna Hills anticline and near its southern flank show a single strong contrast in dipping fabric at depths of 12 and 13 km, near where the microseismicity clusters at depth and consistent with a detachment plane beneath the fold. A minimum in b-value at 10-13 km depth is consistent with seismicity on the detachment, compatible with the imaged anisotropic contrast, while off-fault seismicity is shallower, deeper, and limited to smaller magnitudes. South-dipping imbricate thrusts in schist characterize the northern Alaska Range foothills structure and support our interpretation of the observed anisotropy as reflecting SSW-SSE-dipping foliation above a detachment at ~10-13 km depth that may exploit existing crustal weaknesses along more subtle fabric contrasts observed in the seismically quiescent region north of the actively deforming belt.

Advances in micro-scale imaging techniques, such as X-ray microtomography, have provided new insights into a broad range of porous media processes. However, direct imaging of flow and transport processes remains challenging due to spatial and temporal resolution limitations. Here, we investigate the use of dynamic three-dimensional neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in-situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in-situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl2 and Na2CO3, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co-flow experiment). We use the contrast in neutron attenuation from time-lapse tomograms to derive three-dimensional fluid velocity field by using an inversion technique based on the advection-dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co-flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. These findings suggest that neutron imaging is a promising technique to investigate dynamics processes in porous media.

The Queen Charlotte plate boundary marks a transpressional system between the Pacific and North American plates, extending from offshore Haida Gwaii in Canada into southeastern Alaska. Using continuous seismic waveforms from temporary and permanent seismic networks from 1998–2020, we produced a comprehensive catalog of ~50,000 earthquakes across the region near Haida Gwaii. We used an automated processing technique of auto-regressive phase detection and onset estimation to obtain the initial seismic catalog, integrated existing catalogs, inverted for 3D velocity structure using data from the most well constrained period, and relocated the entire catalog using the new 3D velocity model. We investigate the seismically active sections of the transcurrent Queen Charlotte fault (QCF), noting that little seismicity locates directly along the bathymetrically defined QCF trace. Instead, the seismicity illuminates a complex system of multiple segmented structures, featuring variable geometries along strike. Clustered shallow seismicity could indicate active shallow faults within the highly deformed Queen Charlotte terrace. Few aftershocks appear on the thrust plane of the 2012 Mw 7.8 Haida Gwaii earthquake except near its inferred intersection with the QCF between 15 and 20 km depths, suggesting elevated residual stress. Deep (up to ~20 km) crustal seismicity below central Haida Gwaii aligned parallel to the strike of the thrust plane may manifest the landward underthrusting of the Pacific plate. We also explore the possibility of coseismic strike-slip rupture on the QCF during the 2012 earthquake. Our results provide insights into postseismic strain accommodation and partitioning across this complex oblique transpressive system.

Subsurface tidal analysis requires only continuous pressure monitoring data and therefore can be a cost-effective technique for estimating aquifer properties. The tidal behavior of a well in a semiconfined aquifer can be described by a diffusion equation that includes a leakage term. This approach is valid for thin aquifers, as long as the overlying layer has low permeability relative to the main aquifer. However, in cases where the aquifer is not thin and the permeability of the overlying layer is not low, using the existing solutions based on these approximations may lead to unsatisfactory outcomes. Alternative solutions for both vertical and horizontal wells were obtained by solving the standard diffusion equation, with leakage expressed as a boundary condition. Furthermore, a nondimensional number was derived mathematically, which forms the basis for a quantitative criterion to assess the applicability of the existing solutions. In the case of a vertical well, the existing solution exhibits acceptable error only if the nondimensional number is less than 0.245. Our new solution extends this upper limitation to 0.475. However, when the number is greater than 0.475, both the existing solution and our new solution are invalid due to the invalid uniform flowrate assumption. For a horizontal well, when the number is less than 0.245, the existing solution is suitable with acceptable error. Our new solution effectively overcomes this limitation. Finally, the new solution was applied to the case of the Arbuckle aquifer to demonstrate the improved validity of the new solution compared to the existing one.

In the central Tibetan Plateau, an east-west trending band of basins is developed. How such topography formed and the underlying geodynamic processes are still in debate. Magnetotelluric data were collected across the Lunpola basin to study the crustal structure beneath central Tibet. Phase tensors and 3-D inversion are employed to obtain the electrical resistivity model. Our model clearly portrays conductive sedimentary layers beneath the basins with average resistivity of 2.0 Ω·m. The low-resistivity mid-to-lower crust is revealed beneath the Lunpola basin with bulk resistivity of 20 Ω·m and fluid fraction of 1.3-3.0%, which would be attributed to partial melting. Compared to the significant conductive crust in southern Tibet, the crustal rheology is less well developed beneath central Tibet. We propose that the asthenospheric flow beneath central Tibet is responsible for the crustal partial melting and drives the eastward escape of the continental lithosphere in a rigid block fashion.

The $b$-value in earthquake magnitude-frequency distribution quantifies the relative frequency of large versus small earthquakes. Monitoring its evolution could provide fundamental insights into temporal variations of stress on different fault patches. However, genuine $b$-value changes are often difficult to distinguish from artificial ones induced by temporal variations of the detection threshold. A highly innovative and effective solution to this issue has recently been proposed by van der Elst (2021) by means of the b-positive estimator, which is based on analyzing only the positive differences in magnitude between successive earthquakes. Here, we demonstrate the robustness of the estimator, which remains largely unaffected by detection issues due to the properties of conditional probability. We illustrate that this robustness can be further improved by considering positive differences in magnitude, not only between successive earthquakes but also between different pairs of earthquakes. This generalized approach, defined as the “b-more-positive estimator,” enhances efficiency by providing a precise estimate of the $b$-value while including a larger number of earthquakes from an incomplete catalog. However, our analysis reveals that the accuracy of the $b$ estimators diminishes when earthquakes below the completeness threshold are included in the catalog. This leads to the paradoxical observation that greater efficiency is achieved when the catalog is more incomplete. To address this, we introduce the “b-more-incomplete estimator”, where the b-more-positive estimator is applied only after artificially filtering the instrumental catalog to make it more incomplete. Our findings show the superior efficiency of the b-more-incomplete method.

Repeating earthquakes repeatedly rupture the same fault asperities, which are likely loaded to failure by surrounding aseismic slip. However, repeaters occur less often than would be expected if these earthquakes accommodate all of the long-term slip on the asperities. Here we assess a possible explanation for this slip discrepancy: partial ruptures. On asperities that are much larger than the nucleation radius, a fraction of the slip could be accommodated by smaller ruptures on the same asperities. We search for partial ruptures of repeating earthquakes in Parkfield using the Northern California earthquakes catalogue. We find 3991 individual repeaters which have 4468 partial ruptures. The presence of partial ruptures suggests that asperities of repeating earthquakes are much larger than the nucleation radius. However, we find that partial ruptures could accommodate only around 25% of the slip on repeating earthquake patches. A 25% increase in the slip budget can explain only a small portion of the long recurrence intervals of repeating earthquakes.

We reprocessed and interpreted Chang’E-4 Lunar Penetrating Radar (LPR) data collected until 14th February 2023, exploiting a new Deep Learning-based algorithm to automatically extract reflectors from a processed radar dataset. The results are in terms of horizon probability and have been interpreted by integrating signal attribute analysis with orbital imagery. The approach provides more objective results by minimizing the subjectivity of data interpretation allowing to link radar reflectors to their geological context and surface structures. For the first time, we imaged dipping layers and at least 20 shallow buried crateriform structures within the regolith using LPR data. We further recognized four deeper structures similar to craters, locating ejecta deposits related to a crater rim crossed by the rover path and visible in satellite image data.

Fluid-filled cracks sustain a slow guided wave (Krauklis wave or crack wave) whose resonant frequencies are widely used for interpreting long period (LP) and very long period (VLP) seismic signals at active volcanoes. Significant efforts have been made to model this process using analytical developments along an infinite crack or numerical methods on simple crack geometries. In this work, we develop an efficient hybrid numerical method for computing resonant frequencies of complex-shaped fluid-filled cracks and networks of cracks and apply it to explain the ratio of spectral peaks in the VLP signals from the Fani Maoré submarine volcano that formed in Mayotte in 2018. By coupling triangular boundary elements and the finite volume method, we successfully handle complex geometries and achieve computational efficiency by discretizing solely the crack surfaces. The resonant frequencies are directly determined through eigenvalue analysis. After proper verification, we systematically analyze the resonant frequencies of rectangular and elliptical cracks, quantifying the effect of aspect ratio and crack stiffness ratio. We then discuss theoretically the contribution of fluid viscosity and seismic radiation to energy dissipation. Finally, we obtain a crack geometry that successfully explains the characteristic ratio between the first two modes of the VLP seismic signals from the Fani Maoré submarine volcano in Mayotte. Our work not only reveals rich eigenmodes in complex-shaped cracks but also contributes to illuminating the subsurface plumbing system of active volcanoes. The developed model is readily applicable to crack wave resonances in other geological settings, such as glacier hydrology and hydrocarbon reservoirs.

We present a proof of concept for the probabilistic emulation of the Ring current-Atmosphere interactions Model with Self-Consistent magnetic field (RAM-SCB) particle flux. We extend the workflow developed by Licata and Mehta (2023) by applying it to the ring current and further developing its uncertainty quantification methodology. We introduce a novel approach for sampling over 20 years of solar and geomagnetic activity to identify 30 simulation periods, each one week long, to generate the training, validation, and test datasets. Large-scale physics-based simulation models for the ring current can be computationally expensive. This work aims at creating an emulator that is more efficient, capable of forecasting, and provides an estimate on the uncertainty of its predictions, all without requiring large computational resources. We demonstrate the emulation process on a subset of particle flux: a single energy channel of omnidirectional flux. A principal component analysis (PCA) is used for the dimensionality reduction into the reduced-space, and the dynamic modeling is performed with a recurrent neural network. A hierarchical ensemble of Long-Short Term Memory (LSTM) neural networks provides the statistics needed to produce a probabilistic output, resulting in a reduced-order probabilistic emulator (ROPE) that performs time-series forecasting of the ring current’s particle flux with an estimate on its uncertainty distribution. The resulting ROPE from this smaller subset of RAM-SCB particle flux provides dynamic predictions with errors less than 11% and calibration scores under 10%, demonstrating that this workflow can provide a probabilistic emulator with a robust and reliable uncertainty estimate when applied to the ring current.

The long-term net sink of carbon (C), nitrogen (N) and greenhouse gases (GHGs) in the northern permafrost region is projected to weaken or shift under climate change. But large uncertainties remain, even on present-day GHG budgets. We compare bottom-up (data-driven upscaling, process-based models) and top-down budgets (atmospheric inversion models) of the main GHGs (CO2, CH4, and N2O) and lateral fluxes of C and N across the region over 2000-2020. Bottom-up approaches estimate higher land to atmosphere fluxes for all GHGs compared to top-down atmospheric inversions. Both bottom-up and top-down approaches respectively show a net sink of CO2 in natural ecosystems (-31 (-667, 559) and -587 (-862, -312), respectively) but sources of CH4 (38 (23, 53) and 15 (11, 18) Tg CH4-C yr-1) and N2O (0.6 (0.03, 1.2) and 0.09 (-0.19, 0.37) Tg N2O-N yr-1) in natural ecosystems. Assuming equal weight to bottom-up and top-down budgets and including anthropogenic emissions, the combined GHG budget is a source of 147 (-492, 759) Tg CO2-Ceq yr-1 (GWP100). A net CO2 sink in boreal forests and wetlands is offset by CO2 emissions from inland waters and CH4 emissions from wetlands and inland waters, with a smaller additional warming from N2O emissions. Priorities for future research include representation of inland waters in process-based models and compilation of process-model ensembles for CH4 and N2O. Discrepancies between bottom-up and top-down methods call for analyses of how prior flux ensembles impact inversion budgets, more in-situ flux observations and improved resolution in upscaling.

Fully diffuse seismic wavefields are ideal for Ambient Noise Imaging (ANI) of subsurface structures. However, the lack of feasible methods to identify highly diffuse wave hampers applications of ANI for imaging including evaluation of seismic attenuation and temporal changes with high temporal resolution. Conventional ANI approaches require data normalization, which results in significant loss of amplitude and phase information. Here we propose a method to quantitatively evaluate the degree of diffuseness of seismic wavefields by analyzing their statistical characteristics of modal amplitudes in the frequency domain. Tests on synthetic waveform and real data show that the method can effectively distinguish between diffuse and non-diffuse waveforms. By identifying a 60-second-long diffuse coda of a local M 2.2 earthquake recorded by a dense nodal array on the San Jacinto Fault Zone, we successfully extract high-quality dispersion curve and Q-value without performing data normalization. Our proposed method can advance the imaging of subsurface velocity and attenuation structures and monitoring temporal changes for scientific studies and engineering applications.

It is estimated that 2 billion people will move to cities in the next 30 years, many of which possess high seismic risk, underscoring the importance of reliable hazard assessments. Current ground motion models for these assessments typically rely on an extensive catalogue of events to derive empirical Ground Motion Prediction Equations (GMPEs), which are often unavailable in developing countries. Considering the challenge, we choose an alternative method utilizing physics-based (PB) ground motion simulations, and develop a simplified decomposition of ground motion estimation by considering regional attenuation (\(\Delta\)) and local site amplification (\(A\)), thereby exploring how much of the observed variability can be explained solely by wave propagation effects. We deterministically evaluate these parameters in a virtual city named Tomorrowville, located in a 3D layered crustal velocity model containing sedimentary basins, using randomly oriented extended sources. Using these physics-based empirical parameters (\(\Delta\) and \(A\)), we evaluate the intensities, particularly Peak Ground Accelerations (PGA), of hypothetical future earthquakes. The results suggest that the estimation of PGA using the deterministic decomposition exhibits a robust spatial correlation with the PGA obtained from simulations within Tomorrowville. This method exposes an order of magnitude spatial variability in PGA within Tomorrowville, primarily associated with the near surface geology and largely independent of the seismic source. In conclusion, advances in PB simulations and improved crustal structure determination offer the potential to overcome the limitations of earthquake data availability to some extent, enabling prompt evaluation of ground motion intensities.