Geophysical Journal International

SummaryThe gravity-geologic method (GGM) is an approach that utilizes marine gravity anomalies and shipborne bathymetric data to invert seafloor topography by resolving short-wavelength gravity anomalies through the Bouguer plate approximation. Such an approximation ignores the nonlinear effects caused by surrounding seafloor topographical undulations that actually exist in short-wavelength gravity anomalies, and thus leaving the space for further modification of GGM. This study thoroughly derives the relationship between seafloor topography and gravity anomaly (GA), as well as the formula of GGM. Then, we propose a self-adaptive method to improve the accuracy of the inversion significantly: the enhanced gravity-geologic method (EGGM). The method employs the equivalent mass line method to approximate the nonlinear gravitational effects of the surrounding seafloor topography to correct the short-wavelength gravity anomalies. By introducing two optimal density contrast parameters, EGGM has been designed to effectively integrate the combined effects of various nonlinear factors to a certain extent. The accuracy of the seafloor topography models, produced with a spatial resolution of 1'×1', was evaluated over the study area (132 °E-136 °E, 36 °N-40 °N) located in the Sea of Japan. The results indicate that the accuracy of EGGM has a relative improvement of 13.73% compared to that of GGM in the overall study area, while the accuracy of both models is higher than that of the SIO_unadjusted model. The study further investigated the feasibility and stability of EGGM by examining the accuracy of both GGM and EGGM in various water depth ranges and areas with diverse terrain characteristics.

SummaryWe present a new 3-D radially anisotropic seismic velocity model EARA2024 of the crust and mantle beneath East Asia and the northwestern Pacific using adjoint full-waveform inversion tomography. We construct the EARA2024 model by iteratively minimizing the waveform similarity misfit between the synthetic and observed waveforms from 142 earthquakes recorded by about 2,000 broadband stations in East Asia. Compared to previous studies, this new model renders significantly improved images of the subducted oceanic plate in the upper mantle, mantle transition zone, and uppermost lower mantle along the Kuril, Japan, Izu-Bonin, and Ryukyu Trenches. Complex slab deformation and break-offs are observed at different depths. Moreover, our model provides new insights into the origins of intraplate volcanoes in East Asia, including the Changbaishan, Datong-Fengzhen, Tengchong, and Hainan volcanic fields.

SummarySeismic phase detection and classification using deep learning is so far poorly investigated for regional events since most studies focus on local events and short time windows as the input to the detection models. To evaluate deep learning on regional seismic records, we create a dataset of events in Northern Europe and the European Arctic. This dataset consists of about 151,000 three component event waveforms and corresponding phase arrival picks at stations in mainland Norway, Finland, and Svalbard. We train several state-of-the-art and one newly-developed deep learning model on this dataset to pick P and S wave arrivals. The new method modifies the popular PhaseNet model with new convolutional blocks including transformers. This yields more accurate predictions on the long input time windows associated with regional events. Evaluated on event records not used for training, our new method improves the performance of the current state-of-the-art methods when it comes to recall, precision and pick time residuals. Finally, we test our new model for continuous mode processing on four days of single-station data from the ARCES array. Results show that our new method outperforms the existing array detector at ARCES. This opens up new opportunities to improve automatic array processing with deep learning detectors.

SummaryA basaltic lava flow erupted from the Tajogaite volcano on December 4th, 2021, in La Palma (Canary Islands, Spain) was sampled to find out to what extent reliable and correct information on both intensity and direction of the Earth's magnetic field can be obtained from the paleomagnetic signal recorded in a lava flow which erupted under known conditions. Samples were taken every few centimetres across a flow up to a total of 27 oriented cores. Paleomagnetic experiments showed a strong viscous overprint in many samples. Nevertheless, the mean paleomagnetic direction obtained agrees well with the actual value from IGRF-13. Rock magnetic experiments were performed to obtain additional information about the quality and reliability of the results and the reasons for unsuccessful determinations. Analysis of mostly irreversible thermomagnetic curves showed that the carriers of remanence were magnetite and titanomagnetite of low and/or intermediate Curie-temperature. Hysteresis parameter ratios showed a pronounced variability across the flow. Analyses of frequency dependent susceptibility, IRM acquisition coercivity spectra and FORCs showed a noticeably presence of very low coercivity grains (multidomain and superparamagnetic-single domain boundary). Multimethod paleointensity experiments were performed with the Thellier-Coe, multispecimen and Tsunakawa-Shaw methods. Only three of 25 cores from the flow yielded successful Thellier-Coe determinations, in agreement with the expected field value of 38.7μT (IGRF-13). However, paleointensities of 60% of the specimens agree with the expected value performing an informal analysis without considering criteria thresholds. Four of six Tsunakawa-Shaw determinations performed on samples from the flow yielded correct results, but three multispecimen determinations providing apparently successful determinations largely underestimate the expected field intensity. Combination of three Thellier-Coe and four Tsunakawa-Shaw successful determinations yields a multimethod paleointensity result B = (36.9 ± 2.0) μT in good agreement with the expected field intensity.

SummaryFor the analysis of spectral induced polarization (SIP) measurements and for the description of frequency-dependent electrical relaxation responses, so-called Cole-Cole models (CCMs) are widely used. Typically, CCM formulations in terms of complex electrical conductivity or complex electrical resistivity are used in geophysical applications. The differences between these model descriptions, in particular between the respective time constants, and their conversion have been studied. A third variant of the model is formulated in terms of complex permittivity, commonly used in materials science. In general, all these model formulations can be used equivalently for fitting SIP data, which, however, results in differing values for some of the model parameters. For a meaningful comparison of CCM parameters of different samples or measurements, it is necessary that they are based on the same model formulation. In this work, the relationships between the Debye model (DM) and CCM parameters in the formulation for complex permittivity and complex conductivity are studied. A direct analytical conversion is possible for generalized DM formulations, both in single- and multi-term model formulations, resulting in relationships between the respective relaxation time distributions (RTDs). Such a direct conversion for CCM formulations is not possible. We however derived an approximate relationship between log -normal RTD and CCM formulations and respective permittivity and conductivity parameter values. Our study also highlights the significance of using consistent model formulations when experimental data are compared in terms of DM or CCM parameters, as parameters used to predict ice temperature are incorrect if the conductivity time constant is used to predict the temperature from interpolation of a permittivity time constant-temperature relationship.

SummaryThe Mw 7.8 Kaikoura earthquake, which occurred on November 13, 2016, ruptured a complex system of strike-slip and reverse faults in northeastern South Island, New Zealand. However, the postseismic afterslip behavior and its relationship to the coseismic slip remain incompletely understood. This study investigates the spatiotemporal characteristics of afterslip following the mainshock by using four years of position data from 58 continuous GPS (cGPS) stations, considering the viscoelastic relaxation. Meanwhile, this study considers the contributions of crustal and the interface faults when exploring the combined effect of afterslip and viscoelastic relaxation. Results reveal substantial coseismic deformation northeastern of the epicenter, and postseismic displacements exhibit a continuation of the northeastward evolution. The primary coseismic slip occurred along the Kekerengu and Jordan Thrust faults, while secondary slip was accommodated by the Humps fault and the shallow subduction interface. Two primary afterslip zones are identified: one extending downdip from the secondary coseismic slip areas, and the other adjacent to shallow primary coseismic slip areas near the seismogenic Needles and Hope faults. The afterslip distribution exhibits a spatially complementary pattern to the coseismic slip areas, suggesting that velocity-strengthening zones may have hindered coseismic rupture propagation. The total seismic moment released by afterslip is estimated at ∼2.51×1020 N·m (Mw ∼7.53), approximately 30% of the coseismic moment. Meanwhile, about 80% of the postseismic seismic moment is ascribed to the slip along the southern subduction interface, suggesting the subduction fault plays an important role during postseismic slip. Temporal evolution modeling highlights that roughly 55% of the total afterslip moment was released within the initial three months. Postseismic afterslip dominated during the first month following the earthquake, with a slip rate of ∼10 mm/day. This rate subsequently decreased to ∼5 mm/day over the following two to three months, indicating that the majority of postseismic afterslip occurred shortly after the mainshock. In contrast to the earlier afterslip stages, the latter stages show continued movement along the Needles fault and the subduction interface. Cumulative peak slips have reached 2 cm since mid-2018, with fault slip rates decreasing to approximately 0.6-1.0 mm/day. This indicates ongoing afterslip at shallow faults and the subduction interface, with a steady slip rate over time. Importantly, the cumulative Coulomb stress changes induced by both coseismic slip and afterslip have increased the earthquake hazards potential near the Wellington fault, a densely populated region warranting further investigation.

SummaryThe Epidemic-Type Aftershock Sequence (ETAS) model is currently the most powerful statistical seismicity model that reproduces the general characteristics of earthquake clustering in space and time. However, its application can be hampered by biased parameter estimations related to earthquake catalog deficiencies, particularly in regions where the spatial coverage of local recording networks is relatively poor. Here, we systematically investigate the possible influences of the effect introduced by data truncation through the choice of the cutoff magnitude (${m}_{cut})$ and missing events due to heterogeneity of the seismic network on ETAS parameter estimates along the East African Rift System (EARS). After dividing the region into six source zones based on rheological and mechanical behaviors, the ETAS model is fitted to the earthquakes within each zone using the Davidon-Fletcher-Powell optimization algorithm. The fits and variations in parameter estimates are compared for each zone to the others and the seismological implications are discussed. We found that some parameters vary as a function of ${m}_{cut}$ primarily driven by changes in catalog size. Additionally, a systematic regional dependency of ETAS parameters is found across source zones. Furthermore, a median heat flow value for each analyzed source zone in the EARS is calculated. In contrast to previous findings in other tectonic settings, the results reveal no significant correlations between the crustal heat flows and the ETAS parameters describing earthquake productivity (${K}_0$) and the relative efficiency of an earthquake with magnitude M to produce aftershocks ($\alpha $). Our findings have significant implications for understanding the mechanisms of earthquake interaction and, therefore, provide tight constraints on the model's parameters that may serve as a testbed for existing earthquake forecasting models in this region where the vulnerability of local buildings and structures exacerbate seismic risk.

SummaryTo what extent mechanical anisotropy is required to explain the dynamics of the lithosphere is an important yet unresolved question. If anisotropy affects stress and deformation, and hence processes such as fault loading, how can we quantify its role from observations? Here, we derive analytical solutions and build a theoretical framework to explore how a shear zone with linear anisotropic viscosity can lead to deviatoric stress heterogeneity, strain-rate enhancement, as well as non-coaxial principal stress and strain rate. We develop an open-source finite-element software based on FEniCS for more complicated scenarios in both 2-D and 3-D. Mechanics of shear zones with transversely isotropic and orthorhombic anisotropy subjected to misoriented shortening and simple shearing are explored. A simple regional example for potential non-coaxiality for the Leech River Schist above the Cascadia subduction zone is presented. Our findings and these tools may help to better understand, detect, and evaluate mechanical anisotropy in natural settings, with potential implications including the transfer of lithospheric stress and deformation through fault loading.

SummaryThe columnar-flow approximation allows the development of computationally efficient numerical models tailored to the study of the rapidly rotating dynamics of Earth’s fluid outer core. In this paper we extend a novel columnar-flow formulation, called Plesio-Geostrophy (PG) by including thermal effects and viscous boundary conditions. The effect of both no-slip and stress-free boundaries, the latter being a novelty for columnar-flow models, are included. We obtain a set of fully 2D evolution equations for fluid flows and temperature where no assumption is made regarding the geometry of the latter, except in the derivation of an approximate thermal diffusion operator. To test the new PG implementation, we calculated the critical parameters for onset of thermal convection in a spherical domain. We found that the PG model prediction is in better agreement with unapproximated, 3D calculations in rapidly rotating regimes, compared to another state-of-the-art columnar-flow model.

SummaryDownward flow of surface-derived water deep into the upper crust is investigated using two dimensional coupled hydrothermal numerical models. In the models, downward flow is driven by either topographic gradients or seismic pumping, while it is facilitated by large episodic variations in fault permeability, intended to mimic fracturing and healing on a fault over repeated seismic cycles. The models show that both forcing scenarios are equally capable of driving surface-derived fluid to the base of faults at 10 km depth in several tens of thousands of years under certain conditions. Downward flow of cold fluid occurs almost exclusively during and shortly after earthquakes, while during the remaining portion of the seismic cycle fluids remain relatively stationary while they undergo thermal relaxation (i.e., heating). Rapid downward flow is favoured by a large coseismic permeability, long permeability healing time scale, and large coseismic dilatancy or high topographic relief above the fault at the surface. However, downward fluid flow is completely inhibited if fluid pressures exceeds the hydrostatic gradient, even by modest amounts, which suggests that deep fluid infiltration is unlikely to occur in every region.

SummaryIntermediate-depth earthquakes, accommodating intra-slab deformation, typically occur within subduction zone settings at depths between 60-300 km. These events are in a unique position to inform us about the geodynamics of the subducting slab, specifically the geometry of the slab and the stress state of the host material. Improvements in the density and quality of recorded seismic data enhance our ability to determine precise locations of intermediate-depth earthquakes, in order to establish connections between event nucleation and the tectonic setting. Depth phases (near-source surface reflections, e.g. pP and sP) are crucial for the accurate determination of earthquake source depth using global seismic data. However, they suffer from poor signal-to-noise ratios in the P-wave coda. This reduces the ability to systematically measure differential travel times to the corresponding direct arrival, particularly for the frequent lower-magnitude seismicity which highlights considerable seismogenic regions of the subducted slabs. To address this limitation, we have developed an automated approach to group globally-distributed stations at teleseismic distances into ad-hoc arrays with apertures of 2.5○, before optimising and applying phase-weighted beamforming techniques to each array. Resultant vespagrams allow automated picking algorithms to determine differential arrival times between the depth phases (pP, sP) and their corresponding direct P arrival. These are subsequently used to invert for a new depth. These will allow new comparisons and insights into the governing controls on the distribution of earthquakes in subducted slabs. We demonstrate this method by relocating intermediate-depth events associated with northern Chile and the Peruvian flat slab regions of the subducting Nazca plate. The relocated Chilean catalogue contains comparable event depths to an established catalogue, calculated using a semi-automated global methodology, which serves to validate our new fully automatic methodology. Our new Peruvian catalogue indicates 3 broad zones of seismicity approximately between latitudes 1-7○S, 7-13○S and 13-19○S. These align with flat to steep slab dip transitions and the previously identified Pucallpa Nest. We also find a regionally deeper slab top than indicated by recent slab models, with intra-slab events concentrated at points where the slab bends, suggesting a link between slab flexure and intermediate-depth earthquake nucleation.

SummaryThe dam of Lampy (Black Mountain, Aude, France) is considered as one of the oldest dams in France. A geophysical survey is performed to better understand the pattern of groundwater flow downstream of this dam in the granitic substratum. Induced polarization is first used to image both electrical conductivity and normalized chargeability. 8 core samples of granite from this site are measured and analyzed in the laboratory. Their electrical conductivity and normalized chargeability are expressed as a function of the porosity and Cation Exchange Capacity (CEC). The field data and the petrophysical results are used to image the water content, the CEC, and the permeability distribution of the substratum. Then, self-potential is used as a complementary passive geophysical technique, which, in absence of metallic bodies, is directly sensitive to groundwater flow through the so-called streaming potential effect. Indeed, the excess of electrical charges in the vicinity of the solid grains, in the so-called double layer, is dragged by the ground water flow generating in turn an electrical (streaming) current and therefore an electrical field. A map of the resulting self-potential signals is done over the area covered by the induced polarization profiles. This map shows a large positive anomaly with an amplitude of ∼80 mV possibly associated with upwelling groundwater in an area where the soil is water-saturated. A groundwater flow simulation is performed to model this anomaly. This is done in two steps. A preliminary groundwater flow model is built using the permeability and water content distributions obtained from the induced polarization data. Then, this groundwater flow model is updated using the information contained in the self-potential data including the electrical conductivity distribution obtained through resistivity tomography. The algorithm for the inversion of the self-potential data is validated through a 2D numerical test. This analysis yields a groundwater flow model with the flow being focused through a high permeability zone. This study shows how three geoelectrical methods (self-potential, induced polarization and electrical resistivity) can be efficiently combined to image groundwater flow in the vicinity of a dam.

SummaryWe present a general framework for multiphysics joint inversion of any number of geophysical datasets. Its main feature is the use of the variable splitting approach: an auxiliary multiparameter model space is introduced in which minimization of the coupling and stabilizing functionals is carried out. The use of rediscretization and interpolation to map between this auxiliary space and the model spaces allows the coupled models to have completely different parameterizations. Joint inversion is decoupled into the individual inversion and the coupling-regularization subproblems, each of which can be solved by a different optimization algorithm. For each subproblem, the linking term controlling the distance between the model and the corresponding auxiliary variable takes the form of a quadratic regularization with a reference model. As a result, any existing inversion code supporting such regularization can be integrated without modifications into the developed framework. As a concrete example scheme, we consider an application of the framework to 3D joint inversion of magnetotelluric, seismic refraction and gravity data. We discuss different coupling functionals, mainly those corresponding to the more universal structural constraints: joint total variation, joint minimum support, cross-gradient, one-way cross-gradient and their combinations for a general multi-model case. The use of coupling based on explicit ”petrophysical” relationship between the properties is also considered. Performance of the developed framework is studied on three synthetic cases: a time-lapse joint inversion of full-tensor gravity gradiometry and seismic data, a joint inversion of magnetotelluric, seismic and gravity data and a joint inversion for electrical resistivity tomography and audio-magnetotellurics.

SummaryIn the past two decades or so, ambient noise tomography (ANT) has emerged as a powerful tool for investigating high-resolution crustal and upper-mantle structures. A crucial step in the ANT involves extracting phase velocities from cross-correlation functions (CCFs). However, obtaining precise phase velocities can be a formidable challenge, particularly when significant lateral velocity variations exist in shallow subsurface imaging that relies on short-period surface waves from ambient noise. To address this challenge, we propose an unwrapping correction method that enables the accurate extraction of short-period dispersion curves. Our method relies on the examination of the continuity of phase velocities extracted from CCFs between a common station and other neighboring stations along a linear array. We demonstrate the effectiveness of our approach by applying our method to both synthetic data and field data. Both applications suggest our unwrapping correction method can identify and correct unwrapping errors in phase velocity measurements, ensuring the extraction of accurate and reliable dispersion curves at short periods from ambient noise, which is essential for subsequent inversion for subsurface structures.

SummaryDetection and separation of the subtle postseismic deformation signals associated with moderate magnitude earthquakes from Interferometric Synthetic Aperture Radar (InSAR) time-series is often challenging. Singular Spectrum Analysis (SSA) is a statistical non‐parametric technique used to decompose and reconstruct signals from complex time-series data. We show that the SSA analysis effectively distinguished the postseismic signal associated with the 2019 Mw 6 Mirpur earthquake from periodic and noise components. The SSA derived postseismic deformation signal is smoother and fits better to an exponential model with a decay time of 34 days. The postseismic deformation is confined to the southeast of the rupture area and lasted for ∼90 days following the mainshock. Inversion of the postseismic deformation suggests an afterslip mechanism with a maximum slip of ∼0.07 m on the shallow, up-dip portions of the Main Himalayan Thrust. The 2019 Mirpur earthquake and afterslip together released less than 12 per cent of the accumulated strain energy since the 1555 Kashmir earthquake and implies continued seismic hazard in the future.

SummaryThe 2008 Mw7.9 Wenchuan earthquake ruptured the middle and northeastern segments of the Longmenshan Fault Zone (LMSFZ), and the 2013 Mw6.6 Lushan earthquake ruptured a 50km-long fault in the southwestern segment. Subsequently, an Mw5.8 earthquake occurred approximately 10 km distant from the Mw6.6 Lushan earthquake. Therefore, the potential risk for larger earthquakes (>Mw6.6)on the southwestern section must be considered. This study collects the latest seismological and GPS data to construct an integrated seismotectonic model for the two neighboring earthquake sequences. The model integrates the fault planes involved, the mainshock rupture processes, the mainshock-caused Coulomb stress perturbation, the aftershock distribution and the 3-D velocity structure of the source region, providing information for seismic risk evaluation. We find that three fault planes were involved, two of which were related to the mainshocks, and the third was generated by the aftershocks following the first mainshock. The mainshocks were caused by nearly pure thrust faulting on the two planes with dip angles of approximately 45° and almost opposite dipping directions, thereby forming a conjugate angle of around 90°. The third plane was located between the two mainshocks, approximately parallel to the second mainshock's fault plane. Each of the mainshocks primarily ruptured a single asperity, displaying simple time history. The Coulomb stress change of the first mainshock facilitated the generation of the second mainshock and the third fault plane, and the second mainshock increased the stress on the first mainshock's fault plane. The aftershocks were distributed within stratified materials by spatially varying interfaces and characterized by high Vp and Vs velocity and a low Vp/Vs ratio. The atypical dip angles of approximately 45° for thrust faults and the conjugate angle of approximately 90° are indicative of high stress state. The single asperity rupture implies simple stress accumulation. The mainshock-caused Coulomb stress change did not reduce the seismic risk in the source region. The varying interfaces are interpreted as a consequence of long-term horizontal compression. All of these characteristics suggest that the two earthquake sequences were generated by the breakage of three immature faults under strong compression by background stress, and the high stress state remains within the southwestern LMSFZ.

SummaryTime-varying gravity fields play a crucial role in understanding and analyzing geodynamic processes, particularly the migration of matter across the Earth's surface. However, the current limitations in spatiotemporal resolution hinder their accurate representation. In this context, the use of a giant constellation of low-orbit satellites holds great potential for accurately recovering time-varying gravity fields with high spatiotemporal resolution. Based on the orbital parameters of 5199 satellites in 123 different orbital planes in the first phase configuration of the Starlink constellation and the orbital parameters of the Bender constellation in the next generation gravity mission, we conducted a closed-loop simulation to analyze the recovery ability of time-varying gravity field in 9 days using the short-arc integral method. The errors of aliasing AOHIS signal (Atmosphere, Ocean, Hydrology, Ice, and Solid Earth), ocean tide models, orbit positions, inter-satellite range rates, and accelerometer observations were considered in the numerical simulation. Compared with the Bender constellation, the Starlink-like constellation can effectively decrease the aliasing errors in the spatial- and frequency-domain when the observation noise is not considered. The Starlink-like constellation can also effectively improve the reliability of low-degree coefficients (below degree 15) of retrieved time-varying gravity field models and present higher time resolution (within 9 days) for the full degree spherical harmonic solutions than the Bender constellation when the observation noise is considered. The aliasing effect on the low-degree part of the Bender constellation can be significantly decreased by combining the Starlink-like and Bender constellations, and the accuracy of the recovered time-varying gravity field within degree 30 can be improved by about 0.5 ∼ 1 order of magnitude. Our results can provide a technical reference for the design of future gravity satellite mission.

SummaryGrid point discretization of the model has a significant impact on the accuracy of finite-difference seismic waveform simulations. Discretizing the discontinuous velocity model using local point medium parameters can lead to artifact diffraction caused by the stair-step representation and inaccuracies in calculated waveforms due to interface errors, particularly evident when employing coarse grids. To accurately represent model interfaces and reduce interface errors in finite-difference calculations, various equivalent medium parametrization methods have been developed in recent years. Most of these methods require volume-integrated averaging calculations of the medium parameter values within grid cells. The simplest way to achieve this volume averaging is to apply numerical integration averaging to all grid cells. However, this approach demands considerable computational time. To address this computational challenge, we propose employing a set of auxiliary grids to identify which grid cells intersected by the welded interface and perform volume averaging only on these specific cells, thereby reducing unnecessary computational overhead. Additionally, we present a three-dimensional tilted transversely isotropic equivalent medium parameterization method, which effectively suppresses interface errors and artefact diffraction under the application of coarse grids. We also provide an approach for computing the normal direction of the interface, which is essential for the tilted transversely isotropic equivalent medium parameterization. Numerical tests validate the accuracy of the tilted transversely isotropic equivalent medium parameterization method and demonstrate the practicality of the implementation proposed in this paper for complex models.

SummaryUltra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted, and diffracted arrivals along the mantle side of the core-mantle boundary (CMB). Estimation of ULVZ geometrical (i.e., shape and size) and elastic (i.e., velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modeling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g., ScP) still use 1-D modeling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analyzed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP postcursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multi-dimensional Gaussian ULVZ if its length is 12o or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modeling can provide incorrect ULVZ parameters. Hence previous ULVZ modeling efforts using 1-D parameter estimation methods may be incorrect.

SummarySalt diapirs dominate the structure in many sedimentary basins and control the preservation and migration of hydrocarbon. The formation of salt diapirs generally falls into two endmember models: active (up-building) and passive (down-building) diapirism. In the active model, salt diapirs rise from salt buoyancy to pierce through the sedimentary overburden, whereas in the passive model, salt diapirs result from differential loading of sediments during deposition. These endmember models are mostly conceptual or kinematic, the mechanics of active and passive diapirism, and their relative roles and interactions in the formation of salt diapirs, remain uncertain. Here, we use two-dimensional high-resolution numerical models to investigate the primary factors and critical conditions for active and passive diapirism. Our results indicate that it is improper to use driving mechanisms to classify salt diapirs, because the buoyancy-driven active salt diapirism involves differential loading, while the passive diapirism requires salt buoyancy. The rise of salt diapirs is more sensitive to the effective viscosity of the overburden than to the salt viscosity. Stiff overburdens could prevent the rise of salt diapirs, but they could be pierced by salt diapirs if plastic yield of the overburden is allowed. During deposition, the coupled salt-sediment deformation, driven by both salt buoyancy and differential loading of sediments, can lead to various diapiric salt structures and minibasins. Regional tectonic stress generally promotes salt diapirism by enhancing strain weakening of salts and overburdens. We suggest that the classification of active and passive salt diapirism is an oversimplification in most cases. We propose a general model of the formation of salt diapirs that usually begins with dome initiation driven by salt buoyancy, followed by syndepositional down-building controlled by sedimentation and differential loading, and ends with canopy formation when sedimentation stops.