Vp Model Research Articles

Evidence for Magma‐Rich and Magma‐Poor Characteristics Across the Continent‐Ocean Transition Offshore Central Nova Scotia as Deduced From Vp/Vs Ratios Using 4‐Component Seismic Data

AbstractTo improve identification of crustal rock types within the continent‐ocean transition (COT) offshore Nova Scotia based on the standard approach of analyzing P‐wave velocities (Vp), we incorporate S‐wave velocity (Vs) modeling and determine Vp/Vs ratios. In this work, we construct detailed layered Vp and Vs models using four component wide‐angle ocean bottom seismometer data from profiles SMART‐2 and ‐3 across the central and southwestern Scotian margin, respectively. Along profile SMART‐3, the lower continental crust displays low Vp/Vs ratios (1.7), akin to felsic granulite. Vp/Vs ratios (1.7–1.9) within the COT suggest that syn‐rift volcanism and magmatism resulted in a basalt‐dominated upper crust overlying a thick layer (4–10 km) of laterally heterogeneous gabbroic underplate that jointly cover an estimated profile area of 1,230 . On Profile SMART‐2, we observe upper continental crust (Vp/Vs 1.7) with highly felsic composition within the continental and the COT domains. Serpentinized mantle (Vp/Vs 1.9) is interpreted within the seaward‐most segment of the transitional lower crust, alongside gabbroic underplate, based on its Vp/Vs ratio of 1.7–1.8 and high Vp of 6.9–7.4 km/s, notably thickening beneath the East Coast Magnetic Anomaly near the landward COT limit. Syn‐rift volcanism and magmatism SMART‐2 profile area is estimated at 513 . Our results portray the southwestern Scotian margin as magma‐rich, the northeastern as magma‐poor, and the central margin as a hybrid zone amalgamating characteristics from both. Regardless of margin type, the thinnest transitional crust is consistently observed toward its seaward termination leading into the full‐scale seafloor spreading.

Crustal deformation shown by high-resolution P-Wave velocity structures in the Ordos Block and northeastern Qinghai–Xizang Plateau

Summary The interior of the Ordos block, located northeast of the Qinghai–Xizang Plateau, is not entirely stable or homogeneous and may have been subjected to tectonic deformation. However, there has been no definite conclusion regarding the extent and mechanism of this deformation. Here, we obtained a high-resolution P-wave velocity (Vp) model for the northeastern Qinghai–Xizang (Tibetan) Plateau, Ordos block, and surrounding areas using travel time data involving over 1200 stations from a newly deployed dense temporary array and permanent seismic network. Our results showed that the northern part of Ordos has a relatively high Vp with minimal lateral change. In contrast, the Vp structure in the southern Ordos block varied in the lateral direction, with low-velocity anomalies at a depth of 15 km. The lower crust with Vp from 6.8 to 7.3 km/s is thicker in the west and gradually thins towards the east. The northern part of the Ordos block is relatively stable, whereas the southern part has undergone crustal deformation. This deformation may be related to the eastward compressive forces from the Longxi block, which could be associated with the change in the Haiyuan fault from thrust to strike-slip. In the Bayan Har block, a thick low-velocity anomaly exists in the middle and upper crusts, whereas the lower crust is relatively thin. The crust in this area likely experienced crustal shortening and delamination. Beneath the Longxi and Alxa blocks, a low-velocity layer appears in the middle crust, which may be related to ductile shear in the crustal brittle-ductile transition zone caused by plateau expansion. The range of this low-velocity layer indicates that the influence of the plateau expansion exceeded that of the Haiyuan–Tianjingshan fault zone.

Seismicity and Present‐Day Crustal Deformation in the Southern Puna Plateau

AbstractThe Southern Puna plateau in the central Andes has a complicated tectonic history that includes episodes of distributed shortening and extension, lithospheric delamination, uplift and Quaternary backarc volcanism. In this study, the upper crustal structure and present‐day deformation in this area is investigated using a new regional earthquake catalog derived with a deep‐learning‐based phase picker. Results show abundant strike‐slip seismicity at shallow depths in the eastern Southern Puna plateau that reveals active fault systems in the area and indicates N‐S extension/E‐W compression that changes orientation and relative magnitude from north to south. A broad zone of seismic quiescence in the western plateau may indicate a zone of upper crustal decoupling from large‐scale deformation. The region separating the western and eastern plateau exhibits a complex stress field that can be related to the boundary of east/west oriented middle‐to‐lower crustal flow in the main volcanic arc. Southeast of the plateau in the Sierras Pampeanas, crustal seismicity deepens and is dominated by conjugate reverse faulting structures associated with the direction of plate convergence. Vp and Vs seismic velocity models of the upper crust obtained through local earthquake tomography with the improved seismic catalog show low‐velocity anomalies near intermontane basins, except in the Antofagasta basin where a high‐velocity anomaly possibly represents shallow intrusive component of Quaternary basaltic volcanism. Below the Cerro Galan caldera, an upper crustal 10‐day long earthquake swarm is observed which may indicate local stress perturbations from fluids at the top of the crustal magmatic system that feeds this volcano.

Imaging Complex Structures of the Los Angeles Basin via Adjoint-State Travel-Time Tomography

ABSTRACT In this study, we present high-resolution seismic velocity models for the Los Angeles basin (LAB) and its adjacent area using adjoint-state travel-time tomography, fitting an extensive database of P- and S-wave travel times accumulated from 1980 to 2021. We select 151,193 first P-wave travel times and 149,997 first S-wave travel times from local earthquakes archived in the Southern California Earthquake Data Center to determine the velocity models, with earthquake locations updated at each iteration. With seismic stations spaced more than 3.5 km apart, our dataset has limited resolution in the uppermost 1–2 km. However, starting from three different initial models, our VP models, which are optimally imaged between 3 and 15 km, show similar velocity heterogeneity and provide a better fit to the observed first travel-time data compared to the Community Velocity Model-Harvard 15.1.0 and Community Velocity Model-Southern California Earthquake Center 4.26. Our models provide a detailed delineation of the subsurface structure beneath the LAB, revealing significant velocity variations across active faults, a 10-km-thick sequence of sedimentary rocks within the basin, and a distinct basin margin marked by transitions from low to high-velocities. In addition, these models highlight basement structures with elevated VP and VS located at depths of 9 to 12 km and beyond. Specifically, beneath the northeastern part of the basin, the models demonstrate improved accuracy and reliability in reflecting the linear relationship between VP and VS in mafic rocks. The accurate delineation of the basin’s structure provided by our models could also offer robust constraints for seismic response modeling and seismic hazard assessment in the region.

Assessing the value of combined use of distributed acoustic sensing (DAS) and multicomponent vertical seismic profiling (VSP) data with network-based full waveform inversion and uncertainty quantification: A case study in Alberta, Canada

Distributed acoustic sensing (DAS) technology, deployed in a vertical seismic profiling (VSP) experimental configuration, has emerged as a candidate for non-disruptive and low-cost seismic monitoring of CO2 geostorage and plume evolution. Full waveform inversion (FWI) has likewise received significant attention because it uses relatively complete physical models of wave propagation and because of its sample-by-sample incorporation of data information. Recent artificial neural network-based FWI algorithms (built with, for instance, recursive neural networks, or RNNs) have added to FWI a range of flexible and efficient tools for gradient computation and options for uncertainty assessment and initial model proxies. An important current research area for the use of DAS data is to better understand how they change our confidence levels in models selected through FWI. In particular, we seek to understand whether either DAS data or conventional geophone data alone are optimal for FWI model selection in the CO2 problem, and if not, to what degree they complement each other. The Snowflake 4D VSP dataset, which includes multi-offset and multi-azimuth broadband sources illuminating both fiberoptic cable and densely sampled accelerometer phones in the borehole, was acquired by our group to directly address these questions. In this study, we quantify uncertainty (UQ) by sampling the posterior model covariance matrix from the inverse Hessian matrix at the end of RNN-FWI runs on the Snowflake baseline data, invoking a velocity-density parameterization, and involving mixtures of accelerometer and DAS data. In this UQ context, the complementary effect of combining accelerometer and DAS data is evident in the Vp and Vs models. Our confidence in the approach is bolstered by its independent prediction of a region of known high uncertainty. In the overall pursuit of reliable and low-cost monitoring tools, this supports continued consideration of a multicomponent sensors supported by DAS approach.

Insights into the crustal and the magmatic feeding structure at the Payunia Volcanic Province highlighted by geophysical methods, in the retroarc of the Southern Central Andes

The Payunia Volcanic Province is a Quaternary volcanic plateau emplaced in the retroarc area in the northern Neuquén Mesozoic Basin, associated with a hydrocarbon system. At deeper levels, this basin is linked to different intrusive systems that developed in the retroarc region at different times, during the Jurassic, Cretaceous, Eocene, Miocene, and even the Quaternary which influenced the hydrocarbon system maturity. We analyzed the Moho structure through this retroarc region, as well as the crustal structure affected by different stages of regional extension. From continuous seismic noise data, we calculated the autocorrelograms to obtain the reflection response below each seismological station. This allowed imaging the surface of primary crustal reflectors and in a few stations the top of an asthenospheric anomaly (SWAP) found by magnetotelluric survey and in concordance with satellite magnetic data. The crustal reflectors were identified in all stations at a mean two-way travel time of about ∼8.5 s and ∼12.5 s using frequency bands of about 1.0–2.4 Hz. Therefore, this is the first geophysical research that estimates the depth of the magmatic system, hosted at the top of the lower crust and the Moho discontinuity. The deepest reflector, only recognized in 4 stations, was observed with a two-way travel time of 17.2 s to 19.6 s. We used a mean one-dimensional Vp model to obtain the corresponding reflector depths which constrain the two-dimensional forward gravity model that fits with the observed regional anomaly for the region. We finally established a relationship between the shallowest sublithospheric electrical conductivity anomalies determined in previous researches and the strong deep reflections observed in some of the seismological stations. This information may help to constrain geochemical and petrological models and re-evaluate the hydrocarbon system maturity of the northern Neuquén basin.

Crustal and uppermost mantle structures of the North American Midcontinent Rift revealed by joint full-waveform inversion of ambient-noise data and teleseismic P waves

The Midcontinent Rift (MCR), hosting several world-class ore deposits, is the fossil remnant of a massive Mesoproterozoic rifting event (1.1 Ga) that did not lead to the formation of an ocean basin. To better understand the lithospheric processes associated with the rifting stage and its subsequent failure, we developed a novel full-waveform joint inversion method using ambient noise data and teleseismic P waves for this seismically inactive region. We apply this approach to three years (2011-2013) of seismic recordings from the Superior Province Rifting EarthScope Experiment (SPREE) (~12 km average station spacing) and the USArray Transportable Array (~70 km average station spacing), and obtain a new 3D high-resolution Vs model down to 100 km depth, as well as Vp and density models down to 60 km depth. The model shows major velocity anomalies in agreement with previous seismic studies for the western arm of the MCR. In particular, we observe high density (2.8-3.0 g/cm3), Vp (6.3-6.5 km/s), and Vs (3.6-3.7 km/s) structures in the shallow upper crust within the rift, likely associated with volcanic rocks. Similar to a previously identified underplated layer, we also observe extensive normal-to-high Vs (3.8-4.2 km/s) along the whole rift axis and Vp (6.8-7.5 km/s) beneath the northern segment of the rift within the lower crust. However, the Vs and Vp values are lower than average for typical underplated materials. We suggest that this underplated layer may represent a combination of different intrusive rock types (e.g., gabbro, anorthosite) developed during magma differentiation processes, or contamination of the mafic magma by surrounding crustal material, or intrusions of sills.

Adjoint inversion of antipodal PKPab waveforms for P wave velocity anomaly at the base of the lower mantle

We perform an adjoint inversion by using antipodal PKPab phases to estimate the heterogeneous Vp structure at the base of the lowermost mantle. We have carefully examined antipodal stations with high S/N ratios during the past 30 years and selected 20 source-receiver pairs with the epicentral distances >179.0 degree and the Mw <7.0. We have used the spectral-element method on the Earth Simulator of JAMSTEC to calculate the synthetic seismograms for a heterogeneous mantle Vp model with the accuracy of period about 8 s (110 km wavelength at the CMB). We have set up the time window to retrieve the PKPab phase from the vertical component of both observed and computed seismograms to calculate the adjoint source to obtain the sensitivity kernels of Vp in the mantle. The computed Vp sensitivity kernel for each event shows the characteristic annulus pattern in the lowermost mantle, which covers a large area of the CMB. The twenty PKPab earthquake-station pairs in this antipodal study contribute the equivalent of about 3140 measurements at the CMB—compared with 1871 previously studied—and provide new data. Therefore, although the number of individual source-receiver pairs is not large, the summed sensitivity kernels of the PKPab phase for Vp structure at the base of the mantle may be sufficient to model heterogeneity of the Vp structure at the CMB. We summed each event kernel to set up a sensitivity kernel of Vp in the lowermost mantle and iterated the inversion to estimate a heterogeneous structure in D″. Although we have iterations that dominantly affect both South America and the South Pacific, the summary final model shows features within South of Africa, South Pacific, SE Australia, and Central & South America. Keeping the Vs model fixed, we map the CMB Vp heterogeneity measured by the parameter Rs,p = dlnVs/dlnVp and find qualitative, proximal correspondence with the degree-2 pattern of Large Low Velocity Shear Provinces observed in shear tomographic models: in the Pacific and Africa Rs,p > 2.0, whereas the surrounding edges of the edges of the Pacific show Rs,p < 2.0.

Characterization and Modelling of Particulate Composite with Flaky Aluminium Additives

The utilisation of flaky aluminium as an additive in the composite matrix has shown the ability to overcome the brittle nature encountered in epoxy resins. The additive increases the overall composite toughness without compromising its strength. The novelty of the present work is the demonstration of modelling the constitutive response of the particulate composite from the uniaxial tension tests conducted at three different displacement rates. Utilising two different mean-field homogenisation schemes (Dilute inclusion and Mori-Tanaka), the macro-scale properties were estimated and compared with the material constants obtained from experiments for this new particulate composite, which is proposed in the current study. The rate-independent response, at lower loading rates, is modelled by the elasto-plastic (EP) model, and the rate-dependent response, at higher loading rates, is modelled by elastic-viscoplastic (VP) model while capturing an inelastic deformation as seen in the fractographs. The EP and VP model material constants are then successfully obtained by optimisation employing the Levenberg–Marquardt algorithm and are eventually used in validating the experimental stress response.

Comparison of surface-wave techniques to estimate S- and P-wave velocity models from active seismic data

Abstract. The acquisition of seismic exploration data in remote locations presents several logistical and economic criticalities. The irregular distribution of sources and/or receivers facilitates seismic acquisition operations in these areas. A convenient approach is to deploy nodal receivers on a regular grid and to use sources only in accessible locations, creating an irregular source–receiver layout. It is essential to evaluate, adapt, and verify processing workflows, specifically for near-surface velocity model estimation using surface-wave analysis, when working with these types of datasets. In this study, we applied three surface-wave techniques (i.e., wavelength–depth (W/D) method, laterally constrained inversion (LCI), and surface-wave tomography (SWT)) to a large-scale 3D dataset obtained from a hard-rock site using the irregular source–receiver acquisition method. The methods were fine-tuned for the data obtained from hard-rock sites, which typically exhibit a low signal-to-noise ratio. The wavelength–depth method is a data transformation method that is based on a relationship between skin depth and surface-wave wavelength and provides both S- and P-wave velocity (Vs and Vp) models. We used Poisson's ratios estimated through the wavelength–depth method to constrain the laterally constrained inversion and surface-wave tomography and to retrieve both Vs and Vp also from these methods. The pseudo-3D Vs and Vp models were obtained down to 140 m depth over an area of approximately 900 × 1500 m2. The estimated models from the methods matched the geological information available for the site. A difference of less than 6 % was observed between the estimated Vs models from the three methods, whereas this value was 7.1 % for the retrieved Vp models. The methods were critically compared in terms of resolution and efficiency, which provides valuable insights into the potential of surface-wave analysis for estimating near-surface models at hard-rock sites.

Overpressure Estimation Based on Velocity Data Analysis in The Subduction Zone of Hyuga-nada at Western Nankai Trough, Japan

Subduction zone is one of the areas which has great potential of earthquake. Nankai trough in Japan is such an area with subduction zone where earthquake records are well archived, despite its complexity. Also, slow earthquakes have been detected in this zone over the past 20 years, so that there are many opportunities to understand the dynamics and deformation in such an active setting. To expand the knowledge about slow earthquake which might occur and trigger a bigger earthquake, this study wants to infer the relationship between overpressure, one of the states and properties of seismogenic zone, and tremors. To carry out this research, 4 velocity models of seismicity line taken in Hyuga-nada are used. And the recorded tremor data placed around the lines came from Yamashita. In correlating the overpressure and tremors, this research is started by converting the Vp model to porosity model using Wyllie equation to predict the pore pressure conditions in the area. Then, the model will be used for fitting the actual data. Assuming there is an exact location following the normal compaction curve as a function of effective stress, it can be used for estimating the overpressure. Overpressure is economized by comparing the observed porosity and the reference curve at the same depth below seafloor. Potential overpressure occurred may be associated with areas of low Vp. The estimated overpressure is in the range of 1.2 to 12 MPa in the depth range of 700 to 4000mbsf. This study also concludes that the overpressure zone overlaps with the tremor location, although the overpressure value estimated is also still uncertain due to the uncertainty of the density value used.

Velocity structure (1D) and earthquakes relocation in the flat-slab to normal plate transition zone in the Argentinean transarc

The Argentinean Andean foreland region between latitudes 31°S to 33.5°S presents an intense crustal and upper mantle seismic activity that was recorded by a local seismological network called CHARSME (Chile Argentina SeisMic Experiment), and was used to obtain new velocity determinations. In a first moment, from 4 P-wave velocity (VP) models proposed by other authors, the detail in the layers was increased, resulting in better models used as initial model in the investment. Finally, by carefully selecting a set of earthquakes, a simultaneous velocity inversion, hypocentre determination and station correction were performed using the VELEST code. In order to obtain better velocity models (VP and VS) and VP/VS ratio, different tests were carried out in all processes.The final VP model presented in this paper incorporates considerations made in other seismological studies. This model accurately determined velocities between 10 and 65 km depth. The upper mantle velocity (VP) was defined in 8.05 km/s. From the Wadatti diagram, which represents a line whose slope is related to the wave velocities and the Poisson's coefficient, a VP/VS value of 1.732 was obtained. Alternative VP/VS values were also considered to invert VP, VS and VP/VS simultaneously. The minimum detailed model of VP, VS and VP/VS decreased the RMS by 25% relative to the initial models, implying an improvement in the uncertainty of locations. The new and precise locations of crustal earthquakes allowed defining clusters within Cuyania, a tectonic terrain that collided to Gondwana in the Early Paleozoic. Although it is known that foreland seismicity is mainly concentrated in this terrain, this work shows that there are well-defined zones within Cuyania that are more seismically active. Earthquakes of intermediate depth (upper mantle) are strongly agglomerated in the flat-slab zone of the subducting Nazca Plate.The discontinuities present in our model suggest upper, middle and lower crustal structuring, as well as a marked contrast between the sediments and the underlying basement. It was possible to interpret an upper crust consisting of sediments and basement to a depth of 25 km, a middle crust between 25 and 45 km and a lower crust between 45 and 55 km. We also defined the Mohorovičić (Moho) discontinuity at 55 km depth. The crustal velocities are in agreement with previous seismological studies.

High‐resolution P‐ and S‐wavefield seismic investigations of a quick‐clay site in southwest of Sweden

AbstractSeismic investigations were performed at a site in the southwest of Sweden where major quick‐clay landslides have occurred in the past. Given the potential high risk of the area and the presence of medium infrastructures, the site posed a need for detailed investigations in a wide depth range and in high resolution. A high‐fold seismic survey was designed and conducted along two profiles using a 1–2 m receiver and shot spacing in order to retrieve both P‐ and S‐wavefield seismic images from vertical component data. The data were analysed by combining first‐break traveltime tomography and surface‐wave analysis as well as P‐ and S‐wavefield reflection seismic imaging. Using the first breaks, P‐wave velocity (VP) models were estimated, indicating the bedrock topography along the profiles and the sediment characteristics. The S‐wave velocity (Vs) models were estimated from the surface waves and indicated areas of low shear strength. Combined with VP and Vs models, this permits the estimation of VP/VS, a parameter that can indicate areas with high water content, significant for the detection of quick clays and possible liquefaction issues. The results are integrated with the P‐ and S‐wave reflection seismic images and compared with other geophysical investigations, such as magnetic and gravity data that were collected along the profiles.

Tomographic filtering of shear and compressional wave models reveals uncorrelated variations in the lowermost mantle

SUMMARY Seismic tomography models reveal differences in the geographic distribution and magnitude of P- and S-wave velocity variations (VP and VS, respectively) below ∼2200 km depth in the Earth’s mantle. In particular, large low shear velocity provinces (LLSVPs) beneath the Pacific and Africa exhibit a distinct low velocity population in the distribution of VS that does not stand out in VP models, carrying important implications for the origin of these features. However, it is possible that the absence of a distinct low velocity feature in VP models is an artefact of VP models having lower resolution compared to VS models owing to differences in coverage. Here, we use ‘tomographic filters’ computed from the singular value decomposition of the sensitivity matrices for a pair of VP and VS models in order to test whether such low velocity features are suppressed in VP models. Our ‘cross-filtered’ results show that resolution alone cannot explain the absence of a corresponding low VP population. We additionally apply the joint VP and VS tomographic filter technique to thermochemical mantle convection models to show that cases with distinct phase and/or composition may be differentiated from cases where only temperature varies. We then develop a new proxy for exploring uncorrelated VP and VS more broadly using the difference between the observed VP model and the filtered VS model input. Our results show that ‘large uncorrelated modulus provinces’ (LUMPs) extend beyond the boundaries of LLSVPs, and exhibit anomalies in both fast and slow regions.

New Insights Into the Rift‐To‐Drift Process of the Northern South China Sea Margin Constrained by a Three‐Dimensional Wide‐Angle Seismic Velocity Model

AbstractA three‐dimensional (3D) P‐wave seismic velocity (Vp) model of the crust in the northern South China Sea margin drilled by IODP Expeditions 367/368/368X has been obtained with first‐arrival travel‐time tomography using wide‐angle seismic data from a network of 49 OBSs and 11 air‐gun shot lines. The 3D Vp distribution constrains the extent, structure and nature of the continental, continent to ocean transition (COT), and oceanic domains. Continental crust laterally ranges in thickness from ∼8 to 20 km, a ∼20 km‐width COT contains no evidence of exhumed mantle, and crust with clear oceanic seismic structure ranges in thickness from ∼4.5 to 9 km. A high‐velocity (7.0–7.5 km/s) lower crust (HVLC) ranges in thickness from ∼1 to 9 km across the continental and COT domains, which is interpreted as a proxy of syn‐rift and syn‐breakup magma associated to underplating and/or intrusions. Continental crust thinning style is abrupter in the NE segment and gradual in the SW segment. Abrupter continental thinning exhibits thicker HVLC at stretching factor (β) &lt; ∼3, whereas gentler thinning associates to thinner HVLC at β &gt; ∼4. Opening of the NE segment thus occurred by comparatively increased magmatism, whereas tectonic extension was more important in the SW segment. The Vp distribution shows the changes in deformation and magmatism are abrupt along the strike of the margin, with the segments possibly bounded by a transfer fault system. No conventional model explains the structure and segmentation of tectonic and magmatic processes. Local inherited lithospheric heterogeneities during rifting may have modulated the contrasting opening styles.

Seismic imaging of the Java subduction zone: New insight into arc volcanism and seismogenesis

The Java subduction zone is located in the southern part of Southeast Asia, where the strong plate convergence results in clustered arc volcanoes and intense seismicity. To better understand the arc magmatism and seismogenesis in this region, we determine a high-resolution 3-D P-wave velocity (Vp) model of the crust and upper mantle down to 600 km depth beneath Java. The geometry of the subducting Australian slab is taken into account in the starting model to obtain a better tomographic result by inverting a large number of travel-time data of local and teleseismic events. Our results show lower intra-slab Vp anomalies at depths of ∼100–200 km than other parts of the slab, which may reflect dehydration embrittlement and a slab hole. Feeble low-Vp anomalies appear below the subducting Australian slab, which is probably due to the slab hole where subslab hot materials flow into the mantle wedge. This feature may be related to the occurrence of potassium-rich back-arc volcanoes and the lack of giant megathrust earthquakes (Mw ≥ 8.5) in Java. Our tomographic results also support the hot finger model for explaining the arc magmatism in Java.

2‐D Vp and Vs Models of the Indian Oceanic Crust Adjacent to the NinetyEast Ridge

AbstractUntil now, few offshore seismic studies have acquired simultaneously P‐ and S‐ wave data to derive in detail the seismic structure of the oceanic crust. We present 2‐D Vp and Vs models using wide‐angle seismic data at the Indian basin adjacent to the NinetyEast Ridge. Here, an outcrop basement located at the middle of the seismic line presents uppermost crustal Poisson's ratios (ν) of 0.28–0.29 (Vp ∼ 4.2 km/s and Vs ∼ 2.3 km/s). At the flanks of the outcrop basement, the sediment cover is 200–300 m thick and ν values are similar (0.28–0.3), but Vp and Vs values are higher (4.5–4.8 and 2.4–2.6 km/s, respectively). We interpret the relatively lower Vp and Vs around the basement outcrop in terms of hydrothermal alteration, while at the flanks of the basement outcrop, hydrothermal alteration has most likely ceased by sedimentation and compaction processes. Across the seismic layer 2, the Vp–Vs trend is linear and follows a ν value of 0.28–0.29, however, at the seismic layer 2/3 transition, the Vp–Vs trend abruptly changes following a ν value of 0.25–0.26. These reduced observed ν values at the layer 2/3 transition are lower than those reported by laboratory measurements for gabbro (ν ∼ 0.293) and are interpreted in terms of epidotization at the dike‐gabbro contact and/or crack‐change properties around the lower part of the intrusive sheeted dike section.

Understanding Subsurface Fracture Evolution Dynamics Using Time‐Lapse Full Waveform Inversion of Continuous Active‐Source Seismic Monitoring Data

AbstractPredicting the behavior, geometry, and flow properties of subsurface fractures remains a challenging problem. Seismic models that can characterize fractures usually suffer from low spatiotemporal resolution. Here, we develop a correlative double‐difference time‐lapse full waveform inversion of continuous active source seismic monitoring data for determining high‐spatiotemporal‐resolution time‐lapse Vp models of in‐situ fracture evolution at a shallow contamination site in Wyoming, USA. Assisted by rock physics modeling, we find that (a) rapidly increasing pore pressure initializes and grows the fracture, increasing the porosity slightly (from ∼13.7% to ∼14.6%) in the tight clay formation, thus decreasing Vp (∼50 m/s); (b) the fluid injection continues decreasing Vp, likely through the introduction of gas bubbles in the injectate; and (c) final Vp reductions reach over ∼150 m/s due to a posited ∼4.5% gas saturation. Our results demonstrate that high‐resolution Vp changes are indicative of mechanical and fluid changes within the fracture zone during hydrofracturing.

Crustal Deformation in the Sierra Nevada and Walker Lane Region Inferred From P‐Wave Azimuthal Anisotropy

AbstractCrustal deformation in the Sierra Nevada and Walker Lane is mainly investigated by geologic and geodetic constraints which suffer from a lack of depth information. In this study, we construct a new depth‐dependent azimuthally anisotropic P‐wave velocity (Vp) model of this area to investigate regional deformation regimes in the upper and middle crust. The model is built based on adjoint‐state traveltime tomography of ∼650,000 local direct P arrivals collected from 1967 to 2021. The average Vp structure agrees well with previous tomographic models with refined velocity features benefiting from the ray‐free adjoint‐state tomography and more arrival time data. The azimuthal anisotropy of Vp reveals distinct differences between the rigid Sierra Nevada block and the Walker Lane shear zone: (a) The western Sierra Nevada shows weak anisotropy (&lt;2%) and NW‐SE oriented fast velocity directions that are parallel to the Pacific‐North American plate boundary, mainly reflecting preserved paleofabrics developed in past subduction processes. (b) In the shallow Walker Lane (&lt;4 km), the orientation of fast‐velocity directions is NNE controlled by regional compressive stress, and changes to NNW at greater depths in the north under shear deformation regimes. (c) In the central Walker Lane, strong anisotropy with ENE‐oriented P fast axes is imaged at 6–16 km from the area south of Lake Tahoe (2%–4%) to the Long Valley volcanic area (4%–8%) because of clockwise crustal block rotations and crustal fluids. The proposed anisotropic Vp model provides new insight into how past and present‐day deformation is accommodated in this tectonic complex plate boundary region.

Mantle Tomography of Central‐Eastern USA: Influence of Inversion Volume Size

AbstractTeleseismic tomography is a powerful tool to study the 3‐D structure of the upper mantle beneath a local seismic network, but the robustness of its images has been challenged by the increasing size of the study volume in recent studies. This is because teleseismic tomography uses relative travel‐time residuals (RTTRs) that are contaminated more by the whole‐mantle heterogeneity for a larger study region. In this work, we correct the RTTRs of teleseismic events for the mantle heterogeneities outside the study volume using two global tomographic models. Our results show that the whole‐mantle correction is necessary when a study region is wider than ∼10°, and the maximum outside residual reaches 1.2 s for a study region of ∼35° wide. After the whole‐mantle correction, those teleseismic events with a large ray spread angle are still useful for tomographic imaging. Applying this approach to teleseismic travel‐time data recorded by the USArray, we obtain a high‐resolution 3‐D P‐wave velocity (Vp) model beneath the central and eastern United States. It shows three low‐velocity anomalies beneath the New Madrid Seismic Zone, the East Tennessee Seismic Zone, and the South Carolina Seismic Zone, suggesting that lithospheric weakness induced by fluids may play an important role in reactivating the intraplate seismic zones. The passage of the Bermuda hotspot and remnants of the subducted Farallon slab in the lower mantle might be responsible for the fluid release and lithospheric weakening, which reactivated ancient rifts and suture zones in the crust and facilitated the formation of the intraplate seismic zones.