Geophysical Journal International

SummaryCrashing ocean waves, or surf, have previously been identified as persistent generators of coherent infrasound signals from 0.5 to 20 Hz. Here, we demonstrate that infrasonic and seismic (seismo-acoustic) signals from surf are composed of repetitive transient events which can be detected and characterized using template matching. Using data collected from a series of field experiments designed to study seismo-acoustic surf signals in Santa Barbara, California, we show that source regions of these events can be constrained primarily to just offshore of a local coastal headland using a reverse-time-migration implementation on a small spatial scale (<5 km2). Our data include one continuously running infrasound sensor (September 2022–July 2023) to examine temporal signal evolution, complemented by several short-duration campaigns involving various infrasound arrays, co-located seismometers, and video recordings. Throughout varied oceanographic and atmospheric conditions, we detect up to tens of thousands of independent surf repeaters per day over the course of a year. The amplitudes of detected infrasound signals are correlated with offshore significant wave height and local wind speed. We identify coincident arrivals of seismic and infrasound signals with similar spectral characteristics, suggesting a linked source mechanism locally producing both the seismic and acoustic transient signals. Source regions estimated from array- and network-based methods correspond to the surf zone as seen in video footage, and the directions of selected transient signals align with the location of a rocky reef shelf nearshore. This work showcases the ability to extract near-real-time information about the coastal sea state from seismic and acoustic signal features.

SummaryCurrent geodetic velocities show that over half (up to 10 mm/yr) of Arabia-Eurasia shortening in the west is accommodated within a relatively narrow zone across the Kura basin of Azerbaijan, in which the most prominent active structure is the Kura fold-and-thrust belt, bordering the southern margin of the Greater Caucasus. The GNSS velocities furthermore suggest equivalent amounts of north-south right-lateral shear across the eastern Kura basin along the West Caspian fault zone that is accommodating relative motion between the Kura basin and the South Caspian basin. Although destructive historical earthquakes are known to have occurred, their spread is restricted geographically and their moment release accounts for only half of the accumulated deformation. These observations can be explained by incompleteness of the historical record, that the faults fail in rare larger earthquakes, or that they slip aseismically. To distinguish between these hypotheses we produce an InSAR velocity field using Sentinel-1 SAR data to image active tectonic deformation within the Kura basin of Azerbaijan. Tectonic signals are superimposed on those relating to non-tectonic processes, including widespread mud volcano inflation that highlights the important role of fluid flow within the basin sediments. We show aseismic creep occurs on two parallel faults of the West Caspian fault zone, and infer this also on the Kura Fold and Thrust Belt from sharp gradients in velocity indicating active fold growth. Recent paleoseismic studies of the faults imaged here indicate discrete slip events, and we speculate that the creep may be episodic, perhaps triggered by deeper earthquake events or by periods of enhanced fluid mobilisation. Together, the right-lateral and left-lateral faults appear to accommodate a large-scale expulsion of the Absheron region towards the South Caspian basin, perhaps driven by gravitational potential energy contrasts.

SummaryWhile global sea level rise is a major concern for ocean-connected coastal regions, inland seas such as the Black Sea exhibit water level changes primarily governed by regional hydrological and climatic factors. Understanding the drivers of water level variability in the Black Sea is essential due to its sensitivity to river inflow, evaporation, and limited connection to the global ocean. This study, for the first time, integrates satellite altimetry and satellite gravity data from 2003 to 2023 to analyze the long-term and seasonal variations in the Black Sea water levels, as well as local sea level variability and its driving factors. The results indicate that the sea level in the Black Sea experiences a positive trend of 1.04 ± 0.39 mm/yr. This comes, however, with a negative trend on a seasonal scale during autumn (–1.14 ± 0.27 mm/yr), which contrasts with the rising trends observed in other seasons. We found that the loading deformation induced by global mass redistribution contributes to 39 per cent of the Black Sea level trend, leading to an overestimation of actual climate-induced sea level change by 0.41 mm/yr. We further found that the reduction of precipitation and river inflow from surrounding basins leads to an increase in the salinity of the Black Sea, driving the decline in steric sea level. On the other hand, it has also increased the water mass of the Black Sea, compensating for the steric sea level drop.

SummaryDetecting marsquakes is crucial for understanding the interior structure of Mars. However, the detection and analysis of such seismic events is challenging due to the strong background noise and low signal-to-noise ratios (SNR) of marsquake seismograms. Moreover, the volume of the seismic data needed to create an efficient method to detect marsquakes, and the required amplitudes of such events is limited by the low energy of the seismic. Here, we develop a new deep learning-based detection algorithm, MarsConvNet (MANet), which can fully exploit the latent information of seismic sequences by upsampling and downsampling for detecting marsquakes in the continuous seismic records. Our approach is to construct a deep encoder structure and nonlinear mapping relationship between seismic signals and arrival times of P and S waves by training on the STanford EArthquake Dataset (STEAD) from Earth. Application to the continuous waveform data from the InSight seismic data from 2019 to 2022 shows that the algorithm can detect marsquakes with weak amplitude in addition to reproducing all events detected manually by the Marsquake Service (MQS). Using our method, we detected 67 previously undetected marsquakes.

SummaryA new approach for the parallel forward modelling of transient electromagnetic (TEM) fields is presented. It is based on a family of uniform-in-time rational approximants to the matrix exponential that share a common denominator independent of the evaluation time points. The partial fraction decomposition of this family is exploited to devise a fast solver with high parallel efficiency. The number of shifted linear systems that need to be solved in parallel does not depend on the number of required time channels nor the spatial discretization. We also argue that similar parallel efficiency gains can be expected when solving the inverse TEM problem.

AbstractDesert strata consist of dune and interdune deposits, and they have different magnetic properties. Contrasting magnetic properties of these two types of strata is a foundation for extracting paleoclimate information from dune-interdune strata in these eco-fragile regions, but few have done so. Here we compared magnetic properties between dune and interdune strata from the late Miocene desert deposits in the northwestern Tarim Basin. We found that interdune strata displayed enhanced magnetic properties mainly due to content increase of stable single-domain (SSD) and small pseudo single-domain magnetite (PSD) within chlorite layers. But magnetic enhancement in interdune strata does not correspond with content increase of superparamagnetic (SP) ferrimagnetic minerals. Our high-resolution transmission electron microscope results suggest pedogenic SSD and small PSD magnetite form within chlorite layers, and we propose weathering of chlorites resulted in high concentration of Fe (II) and Fe (III) ions within layers, facilitating the formation of SSD and small PSD magnetite. By contrast, dry climate in the Tarim Basin results in micropore spaces between mineral particles having limited concentration of Fe (II) and Fe (III) ions, inhibiting SP magnetite formation. This pattern is different from Chinese loess-paleosols sequence, where magnetic enhancement is accompanied with content increase of both SP and SSD or small PSD ferrimagnetic mineral content. Therefore, magnetic enhancement pattern differences (whether magnetic enhancement is accompanied with SP magnetite formation) may inform climate conditions. To infer environmental variations in arid regions using magnetic parameters, reliance on proxies that indicate the relative content of ferrimagnetic grains or content of magnetic grains with stable remanence is essential.

SUMMARYSeismology has advanced significantly with ambient noise interferometry, enhancing the extraction of surface waves for detailed Vs imaging. However, direct Vp measurements remain critical for geological and engineering applications. Guided P waves offer potential for Vp insights, yet their detection from ambient noise is challenging, especially in urban environments. This study explores the feasibility of using underground urban seismic noise, specifically tunnel traffic, for guided P wave extraction and subsurface imaging. We introduce an adaptive workflow for preprocessing and enhancing guided P wave signals, applied to data from the Zijingang Tunnel beneath the campus of the Zhejiang University. Our refined data processing workflow significantly improves the quality of retrieved empirical Green’s functions of guided P waves. The joint inversion of surface waves and guided P waves provides high-resolution Vs and Vp profiles, revealing detailed subsurface structures. We also examine the impacts of source depth and type on guided P wave retrieval. We observed distinct differences in urban ambient noise characteristics between daytime and nighttime, suggesting that daytime traffic noise, with higher frequency content, is more suitable for high-frequency guided P wave retrieval. Our results demonstrate the effectiveness of using underground urban seismic noise for guided P wave extraction, highlighting the potential applications of urban ambient noise imaging in geotechnical engineering, urban planning, and seismic hazard assessment.

SUMMARYAirborne natural source electromagnetics (EM) provide spatially densely gridded, frequency-dependent inter-site transfer function estimates referring airborne vertical and horizontal magnetic field data to the horizontal components of a ground site. Common processing schemes for airborne natural source EM data are conceptually based on approaches developed for ground-based magnetotelluric sounding curve estimation and yield statistically independent transfer function estimates for selected times windows and separately for each component of the airborne recording. However, the magnetic field is spatially smooth and, moreover, its components are mutually dependent in space. Here, we propose to exploit spatial potential field relations directly in the processing workflow to refine transfer functions. The self-consistent response function estimator is applied after a first statistical estimation of transfer functions and supports the determination of a spatially smooth and self-consistent set of vertical and horizontal magnetic transfer functions that reflects physical constraints. Self-consistency is enforced by fitting sources distributed within an equivalent sheet to a preliminary set of transfer function estimates determined on a statistical basis. Smoothness is controlled by the depth to the equivalent sources. Once estimated, the distribution of vertical and horizontal magnetic transfer functions can be predicted from the equivalent sheet at any point in the air half-space. Additionally, the scheme allows for the identification of anomalous parts of the horizontal fields at the ground site. The procedure is demonstrated on a large-scale field data set from Gobabis (Namibia). We observe that transfer functions derived by using the equivalent source layer interpolator improve processing results when compared to purely statistically determined transfer function averages.

AbstractThis study investigates the mantle transition zone (MTZ) beneath Central and Eastern Europe using a 3D Common Conversion Point migration of P-to-S receiver functions derived from a dense regional seismic network. The analysis focuses on the major seismic discontinuities at ∼410 km, ∼520 km, and ∼660 km depth to assess their depth variations, continuity, and implications for past and ongoing geodynamic processes.Our results reveal significant spatial variations in the thickness and topography of the MTZ. In the Western Alps and central Pannonian Basin, the MTZ is thickened up to ∼280 km, deviating from the global average of ∼250 km. This thickening is attributed to the presence of stagnant slab material in the transition zone, suggesting a long-lasting influence of past subduction, particularly of the Adria Plate and Vrancea slab. In the Carpathians and Dinarides, the 410 km discontinuity is uplifted to depths as shallow as ∼400 km, while the 660 km discontinuity reaches depths of ∼670–680 km in regions affected by subducted lithosphere, further supporting the presence of cold slab remnants.Additionally, the 520 km discontinuity—often intermittent or absent in global studies—is clearly imaged in many parts of the region, and found at variable depths ranging between ∼510 and ∼540 km. These depth anomalies suggest the presence of compositional heterogeneities and thermal variations within the MTZ, possibly linked to subducted oceanic crust or recycled lithospheric material.Evidence for mantle upwellings is also observed, particularly beneath the Pannonian Basin, where low-velocity anomalies near the 410 km discontinuity are consistent with small-scale plumes or thermal anomalies. These may be associated with post-subduction processes or intraplate volcanism. Importantly, the Alpine slab itself is not clearly detected in the transition zone, indicating that it may have already sunk below the MTZ or is not well-coupled to the upper mantle structure imaged by receiver functions.By providing new constraints on the structure of the upper mantle and its transition zone, this study refines existing models of regional tectonic evolution. Our findings emphasize the interplay between surface tectonics and deep mantle dynamics and demonstrate that the observed MTZ features preserve a strong geodynamic imprint of both past subduction and intraplate processes across the Alpine-Carpathian-Pannonian-Dinarides region.

SUMMARYThe temporal variations in seismic wave velocity provide critical insights into the sources and physical mechanisms underlying diverse geophysical processes. While traditional approaches rely on measuring coda wave traveltime shifts to estimate velocity changes, the increasing availability of dense seismic networks has shifted attention toward ballistic waves for seismic velocity monitoring. Current methodologies for measuring ballistic wave time shifts predominantly employ the wavelet-transform technique, which, despite its proven reliability for coda wave analysis, introduces nonnegligible biases in ballistic wave monitoring due to the spectral leakage effect. To address this limitation, we propose a novel frequency-domain approach that estimates time shifts at each frequency, leveraging the characteristics of invariant phase shifts of ballistic waves along lag time. This method offers enhanced computational efficiency and simplicity in phase shift measurements compared to the time-frequency domain analysis. The phase velocity change is subsequently determined through a linear regression of phase time shifts along the offset. Synthetic tests validate the superior stability and accuracy of our method in estimating ballistic wave phase velocity changes. We further apply this approach to extract surface wave relative phase velocity changes from field data. Our results bring a robust and efficient method for measuring relative phase velocity changes in ballistic wave seismic monitoring.

AbstractSpectral electrical impedance tomography (sEIT) has attracted increasing interest in hydrogeology, biogeosciences, agriculture, and environmental studies. However, broadband sEIT measurements, particularly at frequencies above 50 Hz, have long been challenging due to electromagnetic (EM) coupling effects. Recent advances in instrumentation, data correction, and filtering have improved sEIT measurements at higher frequencies, yet many of these developments rely on a customized system with distributed amplifiers. Extending these advancements to more universally applicable methods is necessary, as sEIT measurements are often acquired using systems with centralized multiplexers. This study aims to bridge this gap by developing model-based data correction methods to mitigate EM coupling effects in sEIT measurements acquired with such a set-up. For this, the differences in EM coupling effects between measurements with a centralized multiplexer and distributed amplifiers were discussed, and the required correction methods in case of a centralized multiplexer were developed. The effectiveness of the developed corrections was tested using sEIT measurements acquired with a centralized multiplexer. A dataset obtained using distributed amplifiers and corrected using previously developed approaches served as a reference. Finally, inversion results of all datasets were compared. It was shown that cable capacitance dominates the capacitive coupling in the sEIT measurements acquired with a centralized multiplexer when coaxial cables are used. Improvements were observed after each correction step using the developed methods. It was concluded that broadband sEIT imaging results can be obtained using measurements with a centralized multiplexer and coaxial cables using the proposed data correction and filtering methods.

SummaryThe dynamic Stern layer (DSL) petrophysical model can be used to interpret field induced polarization data and can be applied to both magmatic (volcanic and igneous) rocks and sediments. Thanks to it, field-scale tomograms of conductivity and normalized chargeability can be transformed into tomograms of porosity, Cation Exchange Capacity (CEC), and temperature. Furthermore, kilometer-scale galvanometric induced-polarization surveys are nowadays doable thanks to the recent development of independent stations measuring the primary and secondary electrical fields. This approach reduces capacitive and inductive coupling effects inherent to systems based on long cables and allow for deeper investigations. We apply here this combined methodology (novel equipment and revised petrophysical model) to a geothermal prospect located at Mashyuza, Republic of Rwanda, in Eastern Africa. At this site, the rifting activity led to the occurrence of an extensional regime favoring the occurrence of a rising thermal plume at the intersection between two faults. The existence of this plume is expressed at the ground surface by the presence of a hot spring at a temperature of ∼52°C (a well nearby provides a temperature of ∼65°C). A time-domain large scale induced polarization survey is performed. The current source signal is produced by a VIP-5000 squared signal injector and injection current values range from 1 A to 4 A, with stacking of 1s on-off signals ranging from 100 to 300 stacks to improve the signal-to-noise ratio. The size of the 3D array is ∼1.5 km and allows a tomography down to a depth of ∼300 meters. The data are inverted with the deterministic least-square technique, penalizing the roughness of the resulting tomograms. The conductivity and normalized chargeability tomograms are combined to get the temperature, porosity, and CEC distributions. The temperature distribution is consistent with the temperature of the hot spring and well. The results are interpreted in terms of ground water flow pattern and dilution of the mineralized thermal water with the fresher surface meteoric water. The survey images a rising plume of warm water from a depth of at least 300 m along intersecting fracture systems.

SummaryTianshan Mountains in Central Asia are one of the largest and most active orogenic belts in the world, characterized by complex structures and strong seismic activity. In this paper, we use recently updated GNSS data to self-consistently estimate the slip rates of major faults in Tianshan region via the elastic block model. Our results indicate that crustal deformation in Tianshan region is predominantly manifested as crustal shortening, regulated by foreland thrust belts and intermontane basin boundary faults. The shortening rate decreases from 14.2 ± 3.4 mm/yr in the west to approximately 3 mm/yr in the east. By estimating the seismic moment accumulation rates of the major seismic belts and comparing them with the historical earthquake catalog, we identify six seismic belts with significant seismic moment deficits. This indicates a potential risk for earthquakes exceeding magnitude 7, including the Kash fault, Keping fault, the Maidan fault zone, and the North Tianshan seismic belt. The Nalati seismic belt exhibits a relatively small seismic moment deficit, indicating the potential of earthquakes in the magnitude range of 6 to 7. In contrast, Qiulitage fault, the West Tianshan seismic belt and the Manas fold-and-thrust belt show a moment surplus, suggesting a low likelihood of strong earthquakes occurring in the near future. This study provides critical data and theoretical support for the prediction and risk assessment of seismic activity in Tianshan region.

SummaryWe used the multiple-frequency seismic tomography method to image the upper mantle beneath Northeast Brazil by processing P-wave broadband seismograms in six frequency passbands. The data comprised 87 896 relative traveltime residuals for P and PKIKP phases, simultaneously inverted to obtain 3D models of P-velocity anomalies. We conducted resolution tests using checkerboard patterns with horizontal dimensions of 312×312 km and 390×390 km. For the 312×312 km structures, we observed good horizontal recovery beneath the areas of the Borborema province and northern São Francisco craton at the depths of 136 and 226 km, with the areas of the São Luís craton and Parnaíba basin also presenting recovery, albeit with reduced amplitudes. For the 390×390 km structures we observed good horizontal and amplitude recovery throughout the entire study area. Our model was unable to recover the sharp vertical transitions between the anomalies. We present the preferred model for Parnaíba basin and Borborema province. The model shows a fragmented basement for the Parnaíba basin, with two strong high-velocity anomalies consistent with the Parnaíba and Granja blocks, and another slight high-velocity anomaly north of the basin, consistent with the São Luís craton. To the west of the basin, the Parnaíba block appears separate from another high-velocity anomaly associated with the Amazonian craton. A strong high-velocity anomaly south of the Borborema province is interpreted as the northern portion of the São Francisco craton. The São Francisco craton anomaly presents strong high-velocity anomalies, interpreted as a thickening of certain portions of the craton, separated by a weaker positive anomaly, interpreted as the Paramirim Aulacogen. The Borborema province is characterized by a low-velocity anomaly. The central and northeastern portions of this anomaly presented even lower velocities, which was interpreted as lateral flow in the asthenosphere, originating from the passage of a plume to the north of the province. A low-velocity anomaly located west of the Borborema province strikes roughly NE-SW and separates the São Francisco craton from the Parnaíba block and the Amazonian craton. It is interpreted as the Transbrasiliano Lineament. To test the capability of our data to resolve the limits of large-scale structures, we created four synthetic models simulating the presence of different cratonic nuclei. The models show good horizontal recovery, with the fourth model, based on our findings, presenting the best correlation between the real and recovered models. Seismicity in the study region is mainly correlated to low-velocity anomalies.

SummaryThe reliable classification of seismic events is crucial to precise seismic cataloging and robust hazard evaluation. Recent advances in deep learning have achieved great success in seismic event identification, leveraging their exceptional ability to automatically extract and recognize features. However, existing deep learning approaches to seismic classification rely exclusively on deterministic models, which cannot quantify epistemic uncertainty, preventing the estimation of prediction confidence that is critical for reliability evaluation. To address this issue, in this paper, we develop two uncertainty-aware deep learning models for earthquake (EQ) vs. explosion (EP) classification using the DiTing 2.0 artificial intelligence training dataset: a Bayesian convolutional neural network (BCNN) and a dropout-based CNN (DropCNN). We also implement a conventional deterministic CNN as a baseline model for comparative analysis. The experimental results demonstrate that both the BCNN and DropCNN can achieve a classification accuracy comparable to the conventional CNN, while providing additional uncertainty metrics for estimation confidence of prediction. Crucially, their uncertainty scores increase markedly in terms of encountering misclassifications or out-of-distribution samples compared to correct classifications, enabling automatic rejection of unreliable predictions based on the uncertainty threshold setting, triggering human verification or alternative discrimination methods. We then apply the trained models to analyze suspicious explosion events in the DiTing 2.0 dataset. The BCNN and DropCNN results exhibits strong agreement, consistently identifying 79 EP and two EQ events and flagging the remaining samples as uncertain classifications needing further verification. Our findings demonstrate that deep learning methods incorporating uncertainty estimation not only maintain a high accuracy in seismic event discrimination but also provide uncertainty estimation. This capability significantly enhances the model’s reliability and decision-making value in practical applications.

SummaryThe central portion of the 2019 ± 2 Ma Vredefort (South Africa) impact structure comprises a 40-50 km diameter central uplift of Archean basement rocks surrounded by a 15-20 km wide collar of late Archaean to early Proterozoic Witwatersrand Supergroup sedimentary and volcanic rocks. The collar is characterized by a ring of strongly negative (←5 500 nT) aeromagnetic anomalies surrounding much of the structure where the strata dip steeply to overturned. To better understand the origin of this magnetic feature, we undertook a ground survey along 20 transects (340 km) in the Vredefort structure using a three-axis fluxgate magnetometer mounted on a mountain bicycle. Upward continuation of our profiles to 150 m matches the aeromagnetic data in shape and amplitude. From the bicycle measurements, we pinpointed the rocks responsible for the extremely negative anomalies. Field observations and microfabric analyses of the rocks from six outcrops substantiated that the magnetic signal correlates with 10-100 m thick metamorphosed banded iron formations (BIFs) at the base of the supergroup as the main producer of the anomalies. Paleomagnetic samples collected from the rocks at the surface that produce the most intense anomalies (up to -22 000 nT) have extremely high natural remanent magnetization intensities (up to > 1000 A·m−1) likely arising from lightning strikes. Stepwise demagnetization and rock magnetic experiments establish a new protocol to distinguish samples that escaped remagnetization from lightning and possess the established 2.02 Ga paleodirection at Vredefort. From a suite of thermoremanent magnetization (TRM) experiments, the best estimate for the paleofield intensity at the time of impact was 52 μT, corresponding to an average remanence of 32.5 A·m−1. The results of the TRM experiments together with the paleodirection enabled us to successfully model the prominent negative anomalies in the metasediments only when accounting for the post-impact orientation of the BIFs. We interpret the strongly negative magnetic anomalies in the collar region as being formed directly after crater exhumation and uplift of the rocks. This interpretation implies that Bushveld-related metamorphism at 2.06 Ga created the up to mm-sized magnetite and garnet crystals in the BIFs, which resided at temperatures higher than the Curie temperature of magnetite (580°C) until the impact rapidly brought the BIFs close to the surface, where magnetite cooled to acquire a thermal remanence in the 2.02 Ga field.

SummaryMagnetic fabric analysis of dikes is a powerful technique when assessing magma transfer processes. This study presents an integrated analysis combining magnetic susceptibility and anisotropy of magnetic susceptibility (AMS), magnetic mineralogy, geochemistry, and new ⁴⁰Ar/³⁹Ar dating of dikes intruding formations ranging from the Lower Cretaceous to the Miocene on the island of Maio, in the Cabo Verde archipelago. We show that the dikes, dated at ≈ 9.2 Ma, intruding the younger Miocene Casas Velhas formation, display a Ti-rich titanomagnetite composition, higher whole-rock TiO2 content and very high magnetic anisotropy. They are clearly distinguished from the dikes, ranging in age from ≈ 9.3 to 11.3 Ma, intruding older formations, which show a predominantly Ti-poor titanomagnetite composition with multiple magnetic phases, lower whole-rock TiO2 concentration, higher range of magnetic susceptibilities and very low anisotropy. Magnetic fabric is predominantly normal with no significant imbrication relative to the dike margins. Numerical analysis of fabric shows a dominant coaxiality between the magnetic lineation and the preferred orientation of opaques and phenocrystals suggesting that magnetic lineation is, therefore, the proxy of the magmatic flow axis orientation. Based on the orientation of the magnetic fabric, we infer that magmatic flow within the studied dikes is predominantly vertical. The differences observed between the younger dikes and all other dikes may be related to magma sourced from distinct magma chambers. One, probably shallow, underneath the Casas Velhas fm in the southwest of the island, which would explain the very high values of magnetic anisotropy and the inferred vertical flow, and another located in a central position in the island, responsible for the dikes intruding the older formations. The location of such magma reservoirs and the dikes ages suggest a hypothetical migration with age of the magmatic sources that fed the dikes from the central part of the island to the southwest region. The magnetic and mineralogical heterogeneities of the dikes intruding older Lower Cretaceous formations may also be a result of a wider age range of the intrusions.

SummaryIn Distributed Acoustic Sensing (DAS), a fiber-optic cable is used as a distributed seismic sensor, with channels representing successive short sections of the fiber, spaced at defined intervals along the 1D fiber axis. Typically, the positions of these channels are assumed to be a line projection along the cable's position. In reality, a fiber-optic cable contains many fibers that are not perfectly straight and are thus longer than the cable itself. Consequently, the real channel positions may not correspond to a simple interpolation along the cable axis. Moreover, the precise cable coordinates are often sensitive information and may not be provided to the end user who uses the cable for sensing applications. On land, a tap test is usually carried out before the start of a DAS acquisition to determine the exact channel locations. DAS with marine horizontal cables has recently been used for various offshore applications, including seismic imaging. To avoid errors in the seismic image, a precise receiver location is required. In this paper, we propose a traveltime-based inversion workflow to determine a more accurate channel position on the seafloor. Moreover, we show that we can resolve an unknown time shift between the acquisition and the recording system, in addition to the fiber position.

SummaryTransform faults are one of the major tectonic plate boundaries offsetting the global mid-oceanic ridge system. The topographic features within these transform faults provide crucial evidence for tectono-magmatic processes and crustal accretion in transform fault zones. These interesting features include median ridges, which are major bathymetric anomalies found within both slow-slipping and fast-slipping transform faults, often associated with exposures of ultramafic rocks on the seafloor. To explain the origin of median ridges, previous studies have invoked multiple processes such as serpentinite diapirism, thermal uplift at ridge-transform intersections, or transpressive uplift induced by global plate reorganization, without any knowledge of the seismic structure. Here, we present results from 2D travel time tomography of downward-continued multi-channel seismic data along and across an ∼80 km long median ridge that lies within the eastern end of the slow-slipping (∼3.4 cm/yr) Chain transform fault in the equatorial Atlantic Ocean. The data were acquired during the 2018 ILAB-SPARC survey using a 6-km long streamer. Our high-resolution P-wave velocity model of the median ridge shows distinct high and low velocities ranging from 2.5 to 5 km/s within 500 m below the seafloor, on either side of the presently active strike-slip fault trace that cuts through the ridge. The low velocity on the eastern side of the ridge could be due to the presence of highly fractured basalt (with porosity in the range of 28 to 36 per cent) due to transform fault motion, whereas the high velocity on the western flank could be due to the presence of gabbro or highly serpentinised peridotite. The basaltic origin of the median ridge is supported by the observation of a seismic triplication event, which we call the T-event. The depth at which the T-event maps is shallow (200–500 m below seafloor) in high-velocity regions and deeper (600–1400 m) in low-velocity regions. We also find that the currently active strike-slip fault has been active since at least 0.26 Ma and has sliced the ridge. We image low-velocity pockets at the northern and southern limits of the median ridge that could represent the expression of the currently less active strike-slip faults.

SummaryThe northeastern Tibetan Plateau is bounded by the left-lateral Altyn Tagh and Haiyuan faults. How crustal motion along these fault systems transitions to crustal shortening and uplift is key for deciphering the geodynamic link between the escape tectonics and the growth of the Tibetan Plateau. Here, we use the PS-InSAR observations, combined with GNSS and leveling data, to obtain a high-resolution 3D model of the present-day crustal motion in the northeastern Tibetan Plateau. The resolved deformation field covers the entire northeastern Tibetan Plateau with a spatial resolution of approximately 0.01° × 0.01 °. Our analysis of slip rates and strain partitioning reveals that crustal motion along the Altyn Tagh fault gradually diminishes eastward and is absorbed by thrusting and uplift in the Qilianshan orogenic belt within the plateau. A similar tectonic transition occurs between the Haiyuan fault and the Liupanshan orogen on the eastern margin of the plateau. Some of the eastward crustal motion is accommodated by the younger Xiangshan-Tianjingshan fault system to the north of the Haiyuan fault, indicating the ongoing northward expansion of the Tibetan Plateau. Our results align with geological evidence of crustal deformation in the past few million years, highlighting the continuing tectonic transition from eastward crustal motion along the left-lateral strike-slip faults to the growth of the Tibetan Plateau.