Uppermost Crust Research Articles

Physically adjusted ground motion prediction equations for induced seismicity at Preston New Road, UK

Predicting ground motions due to induced seismicity is a challenging task owing to the scarcity of data and heterogeneity of the uppermost crust. Dealing with this requires a thorough understanding of the underlying physics and consideration of inter-site variability. The most common ground motion model used in practice is the parametric ground motion prediction equation (GMPE), of which hundreds exist in the literature. However, relatively few are developed with a focus on induced seismicity. Developing GMPEs that are specific to an appropriate magnitude-distance range (R<30\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$R < 30$$\\end{document} km; 2≤M≤6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$2 \\le M \\le 6$$\\end{document}) is important for induced seismicity applications. This paper proposes a framework for the development of physically-based GMPEs to provide more accurate and reliable estimates of the potential induced-seismicity ground motion hazard, allowing for better risk assessment and management strategies. To demonstrate this approach, a new set of GMPEs for the 2018-2019 induced seismicity sequence at the Preston New Road (PNR) shale gas site near Blackpool, United Kingdom, is presented. The physically-based GMPE was developed based on a pseudo-finite-fault stochastic ground motion simulation, calibrated with parameters derived from the spectral analysis of weak-motion records from induced seismic events. An optimization-based calibration technique using the area metric (AM) was subsequently performed to calibrate optimal parameters for simulating ground motion at the PNR site. Finally, using a suite of forward simulations for events with 1≤M≤6\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1 \\le M \\le 6$$\\end{document} recorded at distances up to 30 km, combined with empirical data, a location-specific GMPE was derived through adjustment of an existing model.

Downward continued ocean bottom seismometer data show continued hydrothermal evolution of mature oceanic upper crust

Abstract Heat flow measurements indicate hydrothermal activity in oceanic crust continues at least for 65 m.y. after formation. Hydrothermal activity progressively fills cracks and pores with alteration products, which is expected to lead to a trend of increasing seismic velocities with age. Compilations of seismic-P-wave velocity models inverted from ocean bottom seismometer (OBS) data have failed to detect such an aging trend beyond crustal ages of ca. 10 Ma. However, in these models, the velocities of the uppermost crust, where fluid flow would be most concentrated, are poorly resolved. This is because as the oceanic crust matures, the first crustal arrivals on OBS records (which best resolve upper crustal velocities using tomographic inversion), become hidden in the coda of the water wave. This may lead to the masking of any aging trend in the seismic velocities. For the first time, we show how including downward continuation (DC) in the analysis of OBS data collected across 65 Ma seafloor significantly improves measurements of the P-wave velocities of the upper crust. Our new analysis reveals a highly heterogeneous upper crust, with ridge-parallel P-wave velocity variations of 25%, implying local porosity values that are up to double that of global averages. Our new results, combined with other most recent advanced seismic analyses, reveal that seismic velocities indeed evolve with age up to at least 70 Ma, confirming that hydrothermal activity continues in mature oceanic crust.

The subsurface anatomy of a mid-upper crustal magmatic intrusion zone beneath the Boku volcanic complex, Main Ethiopian Rift inferred from gravity data

Strain accommodation in the Main Ethiopian Rift has been localized since the Quaternary in axial magmatic segments that contain magma intrusion, volcanic complexes, and fault zones. However, the crustal structure and magmatic plumbing features of the individual volcanic complexes within these magmatic segments are poorly constrained. In this study, gravity data from the Global Gravity Model plus2013 was used to interpret the crustal structure and subsurface volcanic network at and near the Boku Volcanic Complex (Boku VC). Two-dimensional gravity models and an upward continuation map analysis of the upper crust reveal a gravity maximum that is interpreted as mafic intrusion at depths between 5 and 10 km beneath the Boku VC. A circular gravity maximum on the upward continued and residual gravity anomaly maps over the Boku VC and adjacent segments suggest the shallow plumbing systems beneath the segments are discrete, but that they merge into the deeper crust. The gravity models suggest that below 5 km beneath the center of magmatic segments nearly all the extension over the last 2 My can be accounted for by magmatic intrusion. Our models require faults in the uppermost crust which likely contribute to extension and may serve locally as conduits for the conveying melts or hydrothermal fluids. Our gravity analysis supports petrological studies that indicate a two-level magmatic plumbing system beneath the Wonji fault belts in which a melt supply from the upper mantle moves to mid-crust and then to shallow upper crust where the magma fractionates into more siliceous magma within smaller magma chambers.

Multi-stage ultrahigh temperature metamorphism in the lower crust of the Kaapvaal craton recorded by U–Pb ages of garnet

U–Pb ages were determined by split-stream LA-SF/MC-ICPMS in garnets from UHT granulite xenoliths (Star mine, South Africa; 124 Ma). They give a considerable age range of 400 million years with well-defined maximas at 3.09, 3.01 and 2.75 Ga. The oldest peak overlaps with the changeover from tonalites to K-granites at 3.14–3.04 Ga and with zircon ages of the mid-crustal granulites of the Vredefort dome (3.1 Ga) in the wake of the 3.2 Ga collision of three terrains that compose the Witwatersrand block. Subduction (or sagduction) of the uppermost crust in an ultrahot orogen setting brought shales and greenstones to the lower crust. Ultrahigh temperature (UHT) conditions are the result of high mantle potential temperatures and self- heating by the radioactive inventory of the subducted lithologies. Metamorphism, anatexis to very high degrees and melt extraction left UHT granulites as residue. Rejuvenation of UHT conditions was brought about by Dominion Group magmatism between 3.0 and 2.95 Ga. Magmatic uprise caused intense shearing in the lower crust followed by recrystallisation of the shear zones to generate the younger garnet age group. Ventersdorp flood basalt volcanism caused similar processes at around 2.72 Ga and generated the third garnet age group. Zircon gives U–Pb ages mainly around 2.72 Ga (both literature and our own data) i.e. zircon adjusted or newly crystallized at the youngest UHT event. Only few zircon grains retained older ages up to 2.94 Ga. Still unconstrained, but very high closure temperatures (≥ 1100 °C) for the U–Pb system in garnet keep the memory of the oldest ages in UHT granulites. Such ages can only be reset by recrystallization. This way, garnet records a prolonged high-temperature history of the lower crust of the Kaapvaal craton.

Mylonite, cataclasite, and gouge: Reconstruction of mechanical heterogeneity along a low-angle normal fault: Death valley, USA

The spectrum of slip behavior in crustal faults generates various rock types that can inform the mechanics of earthquake genesis. However, a single fault exposure may contain evidence of slip at various depths and temperatures due to progressive fault rock formation and overprinting during exhumation. Here, we unravel the spatiotemporal evolution of mechanical transitions along the Boundary Canyon detachment, a low-angle normal fault northeast of Death Valley, USA. Field, microstructural, and geochemical characterizations of fault rocks are compared to existing laboratory experiments and combined with a thermo-kinematic model of fault evolution. Together, these constrain the depths of mechanical transitions along the fault and reveal the evolution of earthquake nucleation zone thickness. Fault exposures from different initial paleodepths passed through the mechanical transitions during footwall exhumation, resulting in overprinting of mylonite by cataclasite and ubiquitous late formation of foliated, illite-rich gouge within the uppermost crust. We present evidence of coseismic low-angle normal fault slip (e.g., injection veins of cataclasite, laminar and grain-inertial fluidization). Coseismic slip likely nucleated at strength contrasts within the fault zone (i.e., contacts between quartzite breccia and calc-mylonite; quartz ribbons and mylonite matrix; breccia and clay gouge) at ≈ 5–9.5 km depth. Observations including mutually overprinting cataclasite/ultramylonite and exposures of pulverized gouge support that dynamic rupture propagated down-dip through the brittle-ductile transition zone (≈10–11 km depth) and up-dip through velocity-strengthening fault patches (≈0–5 km depth). Rapid fault exhumation increased the geotherm, leading to upward advection of the brittle-ductile transition and shallowing/thinning of the earthquake nucleation zone. This process may explain the rarity of large magnitude earthquakes on low-angle normal faults.

Complex rupture dynamics of the extremely shallow August 2020 M5.1 Sparta, North Carolina earthquake

On August 9, 2020, an Mw 5.1 earthquake ruptured the uppermost crust near the town of Sparta, North Carolina, creating the first co-seismic faulting surface rupture documented in the Eastern United States. Combining deep learning and matched filter earthquake detection, with differential-travel times relocation, we obtain a catalog of 1761 earthquakes, about 5.8 times the number of events listed in the standard USGS/NEIC catalog. The relocated seismicity revealed a complex fault structure with distinct planar alignments, supported by a moment tensor inversion with significant non-double-couple component. The Sparta mainshock with a centroid depth of 1.3 km is interpreted to have nucleated near the intersection of two main fault strands. The mainshock likely ruptured a blind strike-slip fault and a reverse fault associated with the identified surface rupture, both possibly part of a flower structure-like diffuse fault zone. Our observations highlight a complex behavior of extremely shallow earthquakes in stable continental regions.

Upper crustal seismic velocity structure of the Hayward fault zone, San Francisco Bay, California, USA: Results from the 2016 East Bay Seismic Experiment (EBSI-16)

We developed Vp, Vs, Vp/Vs ratio, and Poisson’s ratio models of the uppermost crust (&amp;lt;4 km depth) from the eastern San Francisco (SF) Bay (California, USA) to near the Calaveras fault along a 15-km-long, linear profile. Upper crustal velocities are highly variable beneath, west, and well east of the Hayward fault. We observe eight notable features, from west to east: (1) Near San Francisco Bay, there is an ∼2-km-wide structure with high Vp/Vs ratios (up to 5) and Poisson’s ratios (up to 0.48) extending from the surface to the base of our model, which we suggest the structure is a near-vertical fault that lies along a straight-line projection between the Silver Creek fault to the south and the Point Richmond fault to the north. The structure may be part of an ∼90-km-long fault along the eastern SF Bay. (2) The western East Bay Plain, the lower lying area between the bay and the hills, includes up to 800 m of low-velocity sediments (Vp ∼1600−3000 m/s, Vs ∼500 m/s to ∼1000 m/s), underlain by higher velocity basement rocks (Vp ∼3000−5800 m/s; Vs ∼1000−1500 m/s). (3) Between ∼1 km and 3 km east of the Bay shoreline, sediments thin in a series of steps (likely faults) toward the Hayward fault. (4) Between ∼3 km west and ∼1 km east of the Hayward fault (at the East Chabot fault) at depths greater than 1 km, basement Vp (up to 6000 m/s) and Vs (up to 2800 m/s) are high, and Vp/Vs ratios (&amp;lt;2) and Poisson’s ratios (&amp;lt;0.3) are low, suggesting crystalline rocks. Furthermore, a near-vertical zone of low Vp/Vs ratios and Poisson’s ratios is between near-surface traces of the Hayward and East Chabot faults, likely corresponding to the San Leandro Gabbro of Ponce et al. (2003). (5) Eastward of the East Chabot fault in the upper 1.5 km, basement Vp (∼3000 m/s to ∼4200 m/s) and Vs (∼1200−2000 m/s) are lower than those west of the fault. (6) In the eastern Hayward/Oakland Hills, there are zones of laterally varying, high- and low-velocity (Vp ∼2500−3000 m/s) Jurassic−Cretaceous and Tertiary sediments in the shallow subsurface that likely extend much deeper than imaged. (7) Seismic energy that propagates westward from sources east of the Hayward fault (HF) appear weaker than energy that propagates eastward from sources west of the HF, suggesting that the HF acts as a partial barrier to shallow seismic energy propagation into the more populated eastern SF Bay area. (8) Unlike many fault zones, it appears that the active trace of the Hayward fault (in our study area) is not cored by a prominent, low-velocity zone relative to rocks to the east and west of the active trace. However, the active trace does mark a prominent change from relatively higher velocities to the west and lower velocities to the east.

Artificial intelligent identification of apatite fission tracks based on machine learning

Over the past half century, apatite fission track (AFT) thermochronometry has been widely used in the studies of thermal histories of Earth’s uppermost crust. The acquired thermal histories in turn can be used to quantify many geologic processes such as erosion, sedimentary burial, and tectonic deformation. However, the current practice of acquiring AFT data has major limitations due to the use of traditional microscopes by human operators, which is slow and error-prone. This study uses the local binary pattern feature based on the OpenCV cascade classifier and the faster region-based convolutional neural network model based on the TensorFlow Object Detection API, these two methods offer a means for the rapid identification and measurement of apatite fission tracks, leading to significant improvements in the efficiency and accuracy of track counting. We employed a training dataset consisting of 50 spontaneous fission track images and 65 Durango standard samples as training data for both techniques. Subsequently, the performance of these methods was evaluated using additional 10 spontaneous fission track images and 15 Durango standard samples, which resulted in higher Precision, Recall, and F1-Score values. Through these illustrative examples, we have effectively demonstrated the higher accuracy of these newly developed methods in identifying apatite fission tracks. This suggests their potential for widespread applications in future apatite fission track research.

Seismic attenuation tomography of Eastern Europe from ambient seismic noise analysis

SUMMARY The Eastern-Europe region (EER), is a complex geotectonic area that captures part of the Alpine-Himalayan Orogen, the subduction of multiple NeoTethys Branches and part of the East European Craton. It is one of the most exciting geological areas in Europe due to a diversity of tectonic processes acting within it: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes EER an excellent natural laboratory to study the behaviour of the lithosphere–asthenosphere system in a heterogeneous tectonic setting. To investigate the lateral heterogeneity and physical properties of the crust in the EER, we use ambient seismic noise data recorded by the vertical components of broad-band stations that have been operational between 1999 and 2020 in Eastern Europe and surrounding regions. We used this significant amount of data and the latest processing techniques of the ambient seismic noise field based on the continuous wavelet transform to compute cross-correlations between various station pairs, turning every available seismic station into a virtual source. The coda of the interstation cross-correlograms were used to determine coda quality factors (Qc) of Rayleigh waves in four different period ranges (3.0–5.0, 5.0–10.0, 10.0–20.0 and 20.0–30.0 s) and to invert them in the 2-D space, constructing the highest resolution attenuation tomography of the region. Our results reveal high attenuation features throughout the northeast Pannonian region, the Bohemian Massif, the East Carpathians and the Moesian Platform. Nevertheless, our findings do not emphasize a close correlation between the depth of sedimentary basins and attenuation features identified at longer periods. In addition, Qc variations are larger at short periods, indicating higher heterogeneity in the uppermost crust of Eastern Europe. Our findings demonstrate the higher efficiency of noise correlation approaches relative to earthquake data analyses investigating Qc at low frequencies.

Magmatic–Hydrothermal Transport of Metals at Arc Plutonic Roots: Insights from the Ildeus Mafic–Ultramafic Complex, Stanovoy Suture Zone (Russian Far East)

The Ildeus mafic–ultramafic complex represents plutonic roots of a Triassic magmatic arc tectonically emplaced into the thickened uppermost crust beneath the Mesozoic Stanovoy collided margin. The mafic–ultramafic complex cumulates host Ni-Co-Cu-Pt-Ag-Au sulfide-native metal-alloy mineralization produced through magmatic differentiation of subduction-related primary mafic melt. This melt was sourced in the metal-rich sub-arc mantle wedge hybridized by reduced high-temperature H-S-Cl fluids and slab/sediment-derived siliceous melts carrying significant amounts of Pt, W, Au, Ag, Cu and Zn. Plutonic rocks experienced a pervasive later-stage metasomatic upgrade of the primary sulfide–native metal–alloy assemblage in the presence of oxidized hydrothermal fluid enriched in sulfate and chlorine. The new metasomatic assemblage formed in a shallow epithermal environment in the collided crust includes native gold, Ag-Au, Cu-Ag and Cu-Ag-Au alloys, heazlewoodite, digenite, chalcocite, cassiterite, galena, sphalerite, acanthite, composite Cu-Zn-Pb-Fe sulfides, Sb-As-Se sulfosalts and Pb-Ag tellurides. A two-stage model for magmatic–hydrothermal transport of some siderophile (W, Pt, Au) and chalcophile (Cu, Zn, Ag) metals in subduction–collision environments is proposed.

Seismic Efficiency and Seismic Moment for Small Craters on Mars Formed in the Layered Uppermost Crust

AbstractSeismic activity generated by impacts depends on impact conditions and properties of the impact site. Here, we combined mapping of the regolith thickness with numerical impact simulations to better estimate the seismic efficiency and seismic moment generated in small impact events in the uppermost crust on Mars. We used mapping of crater morphology to determine the regolith thickness that craters formed in. We found that local regolith thickness in the late Amazonian units is between 4 and 9 m. Combined with previous estimates for the NASA InSight landing site, we composed a more realistic uppermost crust analog and implemented it in numerical impact simulations. We estimated the seismic efficiency and seismic moment for small craters on Mars impacting a non‐porous or fractured bedrock overlaid by 5, 10, or 15 m thick regolith. Seismic energy showed more dependence on target properties. Three orders of magnitude more energy were produced in stronger targets. The seismic moment does not depend on target properties, and we confirm that seismic moment is almost proportional to impact momentum. The resulting seismic moment is in agreement up to a factor of 4 between different target types. We improved the scaling relationships developed from numerical simulations used in seismic moment approximations by constraining its dependence on more realistic target properties.

Systematic Comparison of InSAR and Seismic Source Models for Moderate‐Size Earthquakes in Western China: Implication to the Seismogenic Capacity of the Shallow Crust

AbstractEarthquake source parameters are important for understanding earthquake physics and crustal fault properties. However, strong trade‐offs between parameters (e.g., depth and origin time) and a lack of accurate velocity models and near‐field seismic stations could cause large uncertainties of these parameters in seismic catalogs, particularly for shallow events. To further improve the resolution of earthquake source parameters, we use Interferometric Synthetic Aperture Radar (InSAR) images to derive source solutions of 33 moderate‐size (Mw 4.1–6.6) earthquakes that occurred at shallow depths (&lt;20 km) from November 2014 to July 2020 in western China. After evaluating the uncertainties of the InSAR solutions, we systematically compare the location, centroid depth, focal mechanism and magnitude from InSAR models with that from seismic catalogs. We find that all seismic catalogs generally report deeper (4–10 km) hypocenters or centroid depths. The uncertainties of moment tensor solutions are partially related to the percentage of the non‐double‐couple components in the seismic catalogs. The InSAR solutions indicate that considerable seismic moments (i.e., ∼ Nm) were released in the uppermost crust (i.e., &lt;5 km) in a period of ∼6 years, which is not resolvable in the seismic catalogs. The smooth seismic moment distribution along depth indicates a gradual change of the frictional properties from the surface to the middle crust. As most of the studied earthquakes are located on secondary and/or unmapped faults, these findings imply a considerable portion of velocity‐weakening friction in the uppermost crust along immature, secondary fault systems, which should be included in the seismic hazard evaluation.

Seismic evidence of pop-up tectonics beneath the Shillong Plateau area of Northeast India

Our detailed 3-D seismic tomographic assimilation using high-quality phase arrival time data recorded by the local seismographic network demonstrated that heterogeneities in the crustal faults have contributed significantly to the pop-up tectonics beneath the Shillong Plateau, characterized by high-V and low-σ. The major seismogenic faults, namely, the north-dipping Dapsi thrust in association with Dauki fault in the south and south dipping Brahmaputra fault in the north, located either side of the Shillong Plateau that acted as the causative factors for the pop-up, which attributed to the lithostatic (high-V, low-σ) and sedimentary (low-V, high-σ) load, respectively. Seismicity is found confined to a depth ≤ 60 km. Uneven distribution of structural heterogeneities in the upper crust is responsible for earthquake genesis of varying strengths. It is intriguing to note that high-velocity anomalies and low-ϭ in the uppermost crust, interpreted as the Shillong Plateau that acted as a geometric asperity and the juxtaposition of high-V and low-V became the source zone of the 1897 Shillong earthquake (Ms 8.7) as a novel observation for the region. Structural heterogeneities are distinctly distributed between low-V, high-σ and high-V, low-σ in the lower crust plays a major role for future intense seismogenesis due to differential strain accumulation.

The Arkalochori Mw = 5.9 Earthquake of 27 September 2021 Inside the Heraklion Basin: A Shallow, Blind Rupture Event Highlighting the Orthogonal Extension of Central Crete

A strong, shallow earthquake occurred near Heraklion (Crete, Greece) on 27 September 2021. The earthquake produced significant ground deformation in the vicinity of Arkalochori village but without any evidence for surface ruptures of primary origin. We used geodetic (InSAR and GNSS) data to map motions of the Earth’s surface that occurred during and shortly after the earthquake. A 14 cm subsidence of the GNSS station ARKL and a maximum of 19 cm distance from the SAR satellite were recorded. The measured surface displacements were used to constrain the rupture geometry and slip distribution at depth. Our best-fitting inversion model suggests that the rupture occurred on a 13 km-long planar normal fault striking N195° E dipping 55° to the northwest, with major slip occurring to the east and updip of the hypocentre. The fault tip is located 1.2 km beneath the surface. The maximum coseismic slip occurred in the uppermost crust, in the depth interval of 4–6 km. A decrease in the fault offsets toward the Earth’s surface is likely caused by an increased frictional resistance of the shallow layers to rapid coseismic slip. Satellite observations made in the first month after the earthquake detected no post-seismic deformation (i.e., below one fringe or 2.8 cm). The seismic fault may be identified with the Avli (Lagouta) segment of the NNE-SSW striking, west-dipping, 23 km-long neotectonic Kastelli Fault Zone (KFZ). Part of the rupture occurred along the Kastelli segment, indicating a fault segment linkage and a history of overlapping ruptures along KFZ. Based on geological data and footwall topography we estimate an average slip rate between 0.17–0.26 mm/yr for the KFZ. The Arkalochori earthquake is a paradigm example for the on-going extension of Heraklion basin (central Crete) in the WNW-ESE direction, which is almost orthogonal to the E-W Messara graben and other active faults along the south coast of Crete.

Poisson's Ratio Structure Beneath the Nazca Ridge

AbstractThe Nazca Ridge (NR) was formed near the interaction of a hotspot mantle plume and an active spreading center. We use active‐source wide‐angle seismic data to obtain 2‐D Vp and Vs tomographic models, and hence the Poisson's ratio (ν) structure beneath the NR. Results show a ∼2 km thick seismic layer 2A with ν values of 0.25–0.32 in the uppermost crust interpreted as pillow basalts with a low degree of fracturing and/or hydrothermal alteration. The 2A/B boundary layer presents ν values of 0.27–0.29 consistent with pillow basalts/sheeted dykes units. A ∼3 km layer 2B overlies a ∼10 km layer 3 with ν values of 0.24–0.3 at the 2/3 boundary layer. The lowermost layer 3 presents ν values of 0.28 ± 0.02 suggesting an increase in Mg content (≥10% wt). The NR crust (∼15 km thick) requires an increment of the asthenospheric mantle potential temperature in ∼100°C formed by passive adiabatic decompression melting.

Along‐strike variation in the shear wave crustal structure of the NE Himalayan and Indo‐Burmese arc: Evidence based on surface wave dispersion analysis

Surface wave dispersion data is used to infer the shear wave velocity structure variation in the north‐east Himalaya and Indo‐Burmese arc regions. We have used 25 earthquakes data from four groups with a magnitude range of 5.0–6.7, epicentral distance range 368–800 km, and focal depth less than 50 km. Ray paths from the earthquake location to the seismic station are transversely passing different geotectonic units of the Himalayas, Indo‐Gangetic plains, and Indo‐Burma collision zones. The weighted average dispersion curve and the path averaged shear wave velocity models are computed for the four groups located at different azimuths around Shillong seismic station. Non‐linear least‐square inversion is performed to obtain the shear wave velocity structure of the crust and uppermost mantle from joint inversion of Rayleigh and Love waves group velocities. Subsequently, Rayleigh and Love waves group velocities are inverted separately to obtain vertical and horizontal components of S‐wave velocity. Next, using these components anisotropy coefficient values at different depths for each path are also estimated. A high variation of dispersion curves and the shear wave velocity models from one group to another indicates that the region is geotectonically very complex. An approximately 80 km‐thick zone beneath the study region has shear wave velocity as low as 1.7 km/s in the uppermost crust in the southern part and ~4.7 km in the uppermost mantle beneath the Eastern Himalayan Syntaxis (EHS). Inferred velocity is also lower than that of PREM and AK135 global models with a much thicker crust beneath the study region. Radial anisotropy varies even within the northern part from the Indo‐Eurasian collision zone to EHS and northern to southern Indo‐Burma ranges. Anisotropy is comparatively stronger in the deeper part below ~40 km for the three paths, except for the EHS, where the result is contrary.

Modelling S-Wave Velocity Structure Beneath the Central Main Ethiopian Rift Using Receiver Functions

We applied the receiver function (RF) technique on high-quality teleseismic earthquake data recorded by the RiftVolc broadband network from February 2016 to October 2017. We calculate RFs at 17 stations, which are inverted to estimate Vs, and Vp/Vs structure beneath the Central Main Ethiopian Rift and the Eastern plateau. The observed slow S-wave velocity (Vs) in the uppermost crust (&amp;lt;6 km depth) is interpreted as sedimentary and/or volcanic layers. Beneath the rift valley, crustal Vs is heterogeneous both laterally and with depth. In particular, slow Vs (∼2–3 km/s) is localised beneath volcanic centres in the upper-mid crust but ubiquitously slow in the lower crust with Vs as low as ∼3.5 km/s common. The slow lower crust is associated with high Vp/Vs ratios of ∼1.9–2.0. The Vs and Vp are consistent with the observed seismic velocities, and interpreted the presence of the small fraction (&amp;lt;5%) of partial melt from previous seismic imaging studies of the lower crust. In addition, the velocity contrast is small between the lower crust and upper mantle. The results suggest that partial melt in the lower crust beneath magmatically active rifts might be more widespread than previously thought and an important component of the magma plumbing system. In contrast, Vs is far more homogeneous and faster beneath the Eastern Plateau, with a distinct velocity contrast between the crust and upper mantle suggesting less crustal deformation than what is observed beneath the central rift zone.

The crustal and uppermost mantle dynamics of the Tengchong–Baoshan region revealed by P-wave velocity and azimuthal anisotropic tomography

SUMMARY The Tengchong–Baoshan region (TBR) is located at the margin of the southeastern Tibetan Plateau (SETP) and is subject to the complex interaction among multiple tectonic blocks. Therefore, the TBR is approximate for investigating the mechanism responsible for the interaction among the SETP and its surrounding blocks. Based on eikonal equation-based seismic tomography, we obtain high-resolution P-wave velocity and azimuthal anisotropy at depths of 0–40 km beneath the TBR. We discover a large magma reservoir (with horizontal scale of ∼50 km and vertical scale of ∼15 km) in the middle-lower crust beneath the Tengchong volcanic zone, which is directly fed by hot materials from the uppermost mantle. We also detect a shallow magma chamber in the uppermost crust above the large reservoir. These magma reservoirs at different depths constitute a multiscale magma system. Furthermore, we observe a low-velocity belt beneath the Weixi–Qiaohou–Weishan and Nantinghe faults and postulate that it corresponds to lower crustal flow and hot mantle upwelling. Additionally, we infer that two M &amp;gt; 7.0 earthquakes that have occurred in Baoshan block nucleated in the transition zone between low- and high-velocity anomalies. Based on the azimuthal anisotropy tomography results, we consider that incoherent deformation in the crust causes the uneven accumulation of stress, which facilitated the occurrence of these two events. The 2021 Ms 6.4 Yangbi earthquake also occurred in the transition zone between low- and high-velocity anomalies. Based on the P-wave velocity tomography results, we suggest that hot material decreases the fault strength and leads to the instability of the fault.

Global-scale emergence of continental crust during the Mesoarchean–early Neoarchean

Abstract The timing of the emergence of subaerial landmasses is equivocally constrained as post-Archean and continues to be a much-debated issue. In this study, we document exceptionally 18O-depleted (δ18O &amp;lt; 4.7‰) Mesoarchean to early Neoarchean magmatism in India that shows a similarity with the coeval low-δ18O magmas reported from Australia, South America, and northern China. Such global-scale low-δ18O magmatism would require high-temperature meteoric water–rock interaction in the uppermost crust synchronous with magma generation, necessitating the emergence of a substantial volume of the continental crust. The timing of this low-δ18O magmatism coincides with the development of extensive, subaerial large igneous provinces, a downward shift in δ18O and Δ17O values in pelitic rocks, the rise of normalized 87Sr/86Sr in seawater, and an intermittent upsurge in the proportion of atmospheric oxygen. We propose that the emergence of substantial volumes of continental crust initiated at ca. 3.2 Ga and peaked at 2.8–2.6 Ga, facilitating the generation of globally distributed low-δ18O magmas, and this event can be linked to the first appearance of atmospheric oxygen.

Strong Seasonal Variations of Seismic Velocity in Eastern Margin of Tibetan Plateau and Sichuan Basin From Ambient Noise Interferometry

AbstractAmbient noise interferometry between seismic stations makes it possible to monitor temporal changes of the Earth's media continuously. Here, we examined seasonal changes of Rayleigh‐wave empirical Greens functions at 2–10‐s period from 8 years of continuous data of 200 seismic station pairs in Sichuan, China. We used a wavelet decomposition to extract the seasonal signals. We found that most station pairs show seasonal variations with velocity perturbations of up to 0.8%. The seasonality varies in strength, amplitude, phase, periods, and location, but the phase is remarkably consistent, with the largest changes mostly in summer or winter months. We believe that the cause of the seasonality is likely the elastic loading due to regional precipitation. The effect is greater in the tectonically active Tibetan Plateau margin than in the stable Sichuan basin and in the deeper crust than in the uppermost crust. The effect is also strongly influenced by local crustal structure. Our results indicate that studies of tectonic crustal changes need to consider non‐tectonic effects.