Low-velocity Anomalies Research Articles

Advances in seismic imaging of magma and crystal mush

Seismic imaging methods have provided detailed three-dimensional constraints on the physical properties of magmatic systems leading to invaluable insight into the storage, differentiation and dynamics of magma. These constraints have been crucial to the development of our modern understanding of magmatic systems. However, there are still outstanding knowledge gaps resulting from the challenges inherent in seismic imaging of volcanoes. These challenges stem from the complex physics of wave propagation across highly heterogeneous low-velocity anomalies associated with magma reservoirs. Ray-based seismic imaging methods such as travel-time and surface-wave tomography lead to under-recovery of such velocity anomalies and to under-estimation of melt fractions. This review aims to help the volcanologist to fully utilize the insights gained from seismic imaging and account for the resolution limits. We summarize the advantages and limitations of the most common imaging methods and propose best practices for their implementation and the quantitative interpretation of low-velocity anomalies. We constructed and analysed a database of 277 seismic imaging studies at 78 arc, hotspot and continental rift volcanoes. Each study is accompanied by information about the seismic source, part of the wavefield used, imaging method, any detected low-velocity zones, and estimated melt fraction. Thirty nine studies attempted to estimate melt fractions at 22 different volcanoes. Only five studies have found evidence of melt storage at melt fractions above the critical porosity that separates crystal mush from mobile magma. The median reported melt fraction is 13% suggesting that magma storage is dominated by low-melt fraction crystal mush. However, due to the limits of seismic resolution, the seismological evidence does not rule out the presence of small (<10 km3) and medium-sized (<100 km3) high-melt fraction magma chambers at many of the studied volcanoes. The combination of multiple tomographic imaging methods and the wider adoption of methods that use more of the seismic wavefield than the first arriving travel-times, promise to overcome some of the limitations of seismic tomography and provide more reliable constraints on melt fractions. Wider adoption of these new methods and advances in data collection are needed to enable a revolution in imaging magma reservoirs.

Seismic imaging of the crust and uppermost mantle structure in the Qinling orogenic belt and its surroundings: Geodynamic implications

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

Waveform inversion of large data sets for radially anisotropic Earth structure

SUMMARY The amount of high-quality seismic data is expanding rapidly, and there is a need for algorithms that take advantage of classical methods to achieve high efficiency using widely available computing power. In this study, we develop a novel waveform inversion method to retrieve radially anisotropic Earth models that can be used to investigate deformation and flow in the mantle. Our method is comprised of two parts: (1) extraction and fitting of the fundamental mode and (2) fitting of the full synthetic waveform. The waveform inversion method results in path average model constraints with uniquely determined independent uncertainties. We demonstrate through synthetic testing that the method is able to retrieve radially anisotropic perturbations down to the mantle transition zone, and leakage effects due to ignoring P-wave anisotropy are minimal. We apply the method to ∼16 000 waveforms generated by earthquakes occurring in the East Sea (Sea of Japan) region, and we demonstrate that the subsequent linear inversion of radially anisotropic path constraints produces models that are similar to those resulting from full waveform adjoint tomography methods. We validate our model by predicting waveforms for earthquakes not included in our inversion, and we show that our method is able to extract structural information. Our results indicate low-velocity anomalies and weak radial anisotropy in NE Japan, which may be due to competing influences from ascending fluids and/or melts and horizontal flow in the lower crust and upper mantle. In the southern East Sea, we image low velocities and relatively high radial anisotropy, which may reflect high temperatures, shallow dehydration and olivine LPO in the upper mantle.

Structure and dynamics of the Tonga subduction zone: New insight from P-wave anisotropic tomography

The Tonga-Lau-Fiji region is important to study plate-plume and subduction-ridge interactions, but its deep mantle structure is still not very clear. Here we present high-resolution tomography of 3-D P-wave azimuthal anisotropy down to 400 km depth of the Tonga subduction zone derived from arrival-time data of local earthquakes recorded at seafloor and land seismometers. The subducting Tong slab is imaged as high-velocity anomalies at depths of 100-400 km, whereas large-scale low-velocity anomalies down to 400 km depth are revealed in the mantle wedge beneath the backarc basin and volcanic arc. Trench-parallel anisotropy beneath the Lau Basin extends southwards to ∼140 km depth at ∼20.5°S, representing the extent of both southward flow of the Samoan plume and toroidal flow by the slab rollback. At depths of 140-400 km, the Lau Basin and Fiji Plateau mainly exhibit plate-parallel fast-velocity directions (FVDs) north of ∼20.5°S, indicating strong corner flow in the mantle wedge driven by the slab subduction and dehydration. The Tonga slab exhibits trench-parallel FVDs at depths of <200 km, reflecting fossil fabric formed during the plate spreading stage, whereas, at greater depths, the slab mainly exhibits trench-normal FVDs, which may reflect complicated deformations within the slab. These results suggest that the Samoan plume has a significant impact on the Tonga-Lau-Fiji region, leading to variations in the scale and depth extent of mantle flows.

Sound velocity anomalies of limestone at high pressure and implications for the mantle wedge

ABSTRACT Limestone mainly consisting of CaCO3 is one of the most abundant carbonates on Earth's surface. The sound velocities of limestone at high pressure were determined at room temperature using ultrasonic interferometry in a multi-anvil apparatus. Softening and discontinuities in compressional (P) and shear (S) wave velocities have been observed at around 1.45 GPa due to the phase transition of CaCO3-I to CaCO3-II, and the coexistence of CaCO3-IIIb and CaCO3-III stay up to ∼5 GPa. Limestone under CaCO3-I and CaCO3-III phase has much lower velocities and higher V P/V S ratio than most crustal and mantle minerals and the PREM model. Phase transitions from CaCO3-I to CaCO3-II cause the abrupt reduction of V P, V S, and V P/V S. The low velocities and high/low V P/V S ratio are well consistent with the seismology observed in the mantle wedge. The result suggests that the subduction of limestone into Earth's interior would cause low-velocity anomalies in mantle wedges.

Anatomy of large earthquakes in North China

• Most of the large events occurred in high-V or close to the boundary between low-V and high-V zones. • Prominent low-V and high-σ anomalies in the middle and lower crust reflect crustal fluids . • Crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge. • Crustal fluids affected the nucleation and generation of intracontinental large earthquakes. We present detailed 3-D isotropic P and S wave velocity (Vp, Vs) images and Vp azimuthal anisotropy in source areas of 26 large crustal earthquakes (M5.6–7.8) that occurred in North China during 1966 to 2021. Most of the large earthquakes are located in high-velocity (high-V) zones or close to the boundary between low-velocity (low-V) and high-V anomalies. Prominent low-V and high Poisson’s ratio (high-σ) anomalies are revealed in the middle and lower crust under almost all the source areas, which may reflect crustal fluids. The low-V and high-σ anomalies may cause local thinning and weakening of the brittle seismogenic layer above them. The crustal fluids may originate from hot and wet upwelling flows in the big mantle wedge (BMW) beneath East Asia due to corner flows in the BMW and deep dehydration reactions of the stagnant Pacific slab in the mantle transition zone.

Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).

Plume-lithosphere interaction beneath southwestern Africa – Insights from multi-mode Rayleigh wave tomography

A stationary mantle plume heats the overlying lithosphere from below, generating melts and causing lithospheric erosion. When the lithosphere moves away from the plume, it can be re-established by melt residues that have been since attached, partly due to cooling, at the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia used to overlie the Tristan da Cunha mantle plume in the Early Cretaceous and thus provides an ideal place to examine plume-lithosphere interaction. Here we determine the upper mantle shear wave velocity structure of southern Africa down to 400 km depth by waveform inversion of multi-mode Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume. Thick lithosphere with high shear wave velocities is observed beneath the Congo and Kalahari Cratons, extending down to 200 km depth with the largest thickness beneath the Limpopo Belt. Along the landfall of the Walvis Ridge and Damara Belt, our model reveals a thick lithosphere down to 100–200 km depths. The thick lithosphere seems to constitute the Congo Craton, but the thick Congo cratonic lithosphere is extended farther south than observed in most of the previous models. We propose that the lithosphere along the landward extension of the Walvis Ridge was affected by magmatic processes related to the plume, but has been partially reconstructed since then by melt depletion and successive cooling. A high velocity anomaly beneath the northwest coast of Namibia down to a depth of ∼80 km coincides with distribution of the Etendeka basalt at the surface and the seaward-dipping reflectors at the continental margin. At sub-lithospheric depths, a low-velocity anomaly (at 100–200 km depth) and a high-velocity anomaly (at 250–350 km depth) can be detected. The origin of these anomalies could be related to an ongoing edge-driven convection process.

The deep background of large-scale, Mesozoic Cu-Au-W metallogenesis in northeastern South China: Constraints from Yingshan-Changshan wide-angle seismic reflection/refraction data

To investigate the geodynamic processes of Mesozoic large-scale mineralization in South China, we deployed a 350-km-long, wide-angle seismic reflection/refraction sounding profile between Yingshan in Hubei and Changshan in Zhejiang. This profile traverses the Cu-Au metallogenic belt in the middle and lower reaches of the Yangtze River (YMB), the Jiangnan W-polymetal metallogenic belt (JNMB), and the Qinhang Cu-polymetal metallogenic belt (QHMB). Our imaging results reveal various interesting velocity features along the profile. (1) The velocity structure is characterized by vertical layering and horizontal blocking; (2) the YMB is marked by high velocity and high Vp/Vs ratios in general with a significantly uplifted Moho interface and a thin crust of ∼31 km, and the lower crust contains high-velocity anomalies and has the characteristics of a crust-mantle transition zone; (3) the JNMB is bounded by the Jiangnan fault and Jingdezhen-Huangshan fault and has low-velocity anomalies and low Vp/Vs ratios; and (4) the QHMB is characterized by high-velocity anomalies and high Vp/Vs ratios. The high-velocity anomalies in the YMB and QHMB represent relatively Cu-Au-rich mafic juvenile lower crust. The formation of this kind of crust is considered to be related to mantle-derived magma underplating or residues of Neoproterozoic oceanic crustal materials, and it also provided sources for large-scale Cu-Au mineralization in the Mesozoic. The JNMB has features similar to those of ancient crusts enriched in W-Sn, the partial melting of which played a leading role in the formation of the superlarge W deposits in this belt. Considering these results and other regional geological data, we propose that a large-scale oblique upwelling of the asthenosphere along the collisional belt of the Yangtze and Cathaysia blocks during the Mesozoic was the deep driving mechanism for the explosive mineralization of Cu, Au, and W in northeastern South China. The boundaries of the blocks or terrains and discontinuities of the lithosphere were the main channels for deep heat and magmas and therefore controlled the spatial distribution of the metallogenic belt.

3D S-wave velocity structure of the Ningdu basin in Jiangxi province inferred from ambient noise tomography with dense array

The Ningdu basin, located in southern Jiangxi province of southwest China, is one of the Mesozoic basin groups which has exploration prospects for geothermal energy. A study on the detailed velocity structure of the Ningdu basin can provide important information for geothermal resource exploration. In this study, we deployed a dense seismic array in the Ningdu basin to investigate the 3D velocity structure and discuss implications for geothermal exploration and geological evolution. Based on the dense seismic array including 35 short-period (5 s-100 ​Hz) seismometers with an average interstation distance of ∼5 ​km, Rayleigh surface wave dispersion curves were extracted from the continuous ambient noise data for surface wave tomographic inversion. Group velocity tomography was conducted and the 3D S-wave velocity structure was inverted by the neighborhood algorithm. The results revealed obvious low-velocity anomalies in the center of the basin, consistent with the low-velocity Cretaceous sedimentary rocks. The basement and basin-controlling fault can also be depicted by the S-wave velocity anomalies. The obvious seismic interface is about 2 ​km depth in the basin center and decreases to 700 ​m depth near the basin boundary, suggesting spatial thickness variations of the Cretaceous sediment. The fault features of the S-wave velocity profile coincide with the geological cognition of the western boundary basin-controlling fault, which may provide possible upwelling channels for geothermal fluid. This study suggests that seismic tomography with a dense array is an effective method and can play an important role in the detailed investigations of sedimentary basins.

Imaging sub-caldera structure with local seismicity, Okataina Volcanic Centre, Taupo Volcanic Zone, using double-difference seismic tomography

We examine sub-caldera structure and seismicity in and around the Okataina Volcanic Centre (OVC), Taupo Volcanic Zone, New Zealand, using seismic data collected over 4+ years with a temporary array of broadband and short-period seismometers, supplementing data from the permanent New Zealand seismometer network. We derive a new 3-D image of P-wave seismic velocity for the upper crust in the region, using double-difference seismic tomography and utilising waveform cross-correlations. We subsequently relocate 6989 earthquakes which occurred in the region over the 2010–2021 time period, using the 3D velocity model. The seismicity distribution shows spatial clusters west of Lake Rotomahana, as well as beneath Haroharo and the Makatiti Dome, inside the Okataina caldera. Beneath Makatiti Dome 90% of the events are shallower than 7.7 +/− 0.5 km. Outside of the Okataina caldera event relocations highlight short (∼3–4 km long) streaks of activity in the Ngakuru graben, part of the active Taupo Rift southwest of Okataina caldera. Inside the OVC the relocated seismicity beneath Makatiti appears closely associated with low (∼10%) P-wave velocity anomalies, which we resolve in the ∼5-to-8-km depth range beneath the Okataina caldera, and which are likely related to partial melt and/or fluid-volatile pathways. Moment tensor analyses for two larger-magnitude events (ML4.5 and ML4.9) near Haroharo indicate normal faulting, with NNE-SSW fault strike, but with positive CLVD and positive isotropic components when allowing for a full moment tensor, consistent with a magmatic environment with degassing and/or fluid migration.

Deep hydration and lithospheric thinning at oceanic transform plate boundaries.

Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. Here we present ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending down to ~60 km depth below sea level. We performed 3D thermal modelling that suggests the anomaly is likely to be due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere-asthenosphere boundary, causing significant thinning of the lithosphere at the transform fault. The presence of a thinned lithosphere at transform faults could explain observations of volcanism, thickened crust and intra-transform spreading centres at transform faults. It also suggests that migration and mixing of water-induced melt with the high temperature melt may occur beneath the ridge axis.

Upper crustal structure beneath the Qin-Hang and Wuyishan metallogenic belts in Southeast China as revealed by a joint active and passive seismic experiment

SUMMARY Southeast China is one of the most important polymetallic metallogenic provinces in the world. However, distinct differences in the type and origin of the mineral deposits are present between the major metallogenic belts in this province. To deepen our knowledge of this regional metallogenic setting, investigate the factors controlling its metallogenic differences and explore the feasibility of active and passive seismic imaging, we performed a multibatch seismic experiment using active and passive sources with the help of 437 short-period seismographs deployed across the Qin-Hang and Wuyishan metallogenic belts in Southeast China. We resolved the S-wave velocity structure beneath the profile using ambient noise tomography and derived a Vp/Vs model by combining with a published P-wave model. A good correlation between the obtained seismic velocity models and geological observations is observed. The Proterozoic volcanic-sedimentary rock series and the Fuzhou-Yongfeng sedimentary basin are identified by low P- and S-wave velocities, high Vp/Vs ratios and low S-wave velocity perturbations. Within the core of the Wuyishan Metallogenic Belt, several narrow, elongated and low seismic velocity anomalies are considered to be signs of the nearby fault zones, and these anomalies provide reference information on the extension and pattern of these faults at depth. Combined with the geological and mineralogical evidence, we propose that theses faults function as ore-guiding structures and facilitate the formation of regionally representative mineral species and metal assemblages.

Velocity and azimuthal anisotropy structures beneath the Dianzhong Block and its vicinity, SE Tibetan Plateau, revealed by eikonal equation-based traveltime tomography

The Dianzhong Block (DZB) located on the southeastern margin of the Tibetan Plateau is an important tectonic zone characterized by the intense flow of crustal material. Although it is widely accepted that materials originating from the Tibetan Plateau migrate southeastward in the DZB and its vicinity, the flow mechanism and subsurface structures remain controversial. Here, we image the subsurface velocity and azimuthal anisotropy structures beneath the DZB and its surroundings by using eikonal equation-based tomography to study the crustal flow and deformation mechanisms. In this study, we collect high-quality seismic data from 290 broadband seismic stations and manually pick a total of 31,981 first arrivals. Our tomographic results display prominent velocity anomalies and complex azimuthal anisotropy patterns. In the upper crust, our results reveal predominant high-velocity anomalies, which are related to the Emeishan large igneous province (ELIP) and the stable Simao Block. We find strong low-velocity anomalies beneath the DZB in the middle–lower crust. The low-velocity anomalies extend southward from the Xiaojinhe Fault (XJHF) to the Red River Fault (RRF) and then rotate southeastward to the east of the Xiaojiang Fault (XJF). In addition, the inverted fast velocity directions generally follow the strike of the low-velocity anomalies. Based on these findings, we consider the existence of middle–lower crustal flow inside the DZB. Three distinct features of azimuthal anisotropy in the middle-upper crust are found around the XJF reflecting complex deformation, which may indicate different factors controlling the deformation of the XJF from the north to the south. In the lower crust and uppermost mantle, the azimuthal anisotropy beneath the RRF is relatively complex, exhibiting two patterns: one perpendicular and another parallel to the fault.

Nucleation mechanism of the 2021 Mw 7.4 Maduo earthquake, NE Tibetan Plateau: Insights from seismic tomography and numerical modeling

The 2021 Mw 7.4 Maduo earthquake ruptured the Jiangcuo fault, a secondary fault within the Bayan Har block, NE Tibetan Plateau, which attests that large earthquakes can occur on faults with very low slip rates. In this study, we use data from local seismic stations to explore the 3D velocity structure beneath the source region of the Maduo earthquake. Our results indicate that there is a distinct low-velocity anomaly in the mid-lower crust, and the thickness of this low-velocity zone decreases sharply to the east. The Maduo earthquake occurred in the transition zone between the high- and low-velocity regions. Based on our local velocity model and our knowledge of the regional crustal structures, we constructed numerical models to investigate the stress and strain energy distributions in our study area. As indicated by the stress concentration, strain energy accumulation, and low strain rate in the shallow upper crust around the source region, we conclude that the seismic velocity heterogeneity and the weak ductile mid-lower crust provide a unique tectonic environment that facilitated the nucleation of the Maduo earthquake. This study helps to understand the mechanisms of the Maduo earthquake, and more importantly reconciles the seeming contradiction between large earthquakes and low slip and strain rates.

Eikonal surface wave tomography of central and eastern China

SUMMARY Eikonal tomography has become a key approach to image lithospheric structures with surface waves recorded by dense regional arrays. Its main advantage is that phase velocities can be determined directly from phase measurements without resolving a tomographic inverse problem. Here, we apply a new smoothing spline eikonal tomography approach to a selection of 40 large (Mw larger than 6.5) teleseismic events recorded by the permanent seismic stations of the CEArray. We first apply a time–frequency filter to isolate the fundamental mode of Rayleigh waves and cross-correlate the cleaned and isolated surface wave records to measure precise relative traveltimes. The phase measurements are then unwrapped and corrected for cycle skipping. Finally, we derive phase-velocity maps from the gradient of the reconstructed traveltime fields, using the eikonal equation. We obtain finely resolved phase-velocity maps from 25 to 150 s period that are inverted to obtain a 3-D shear wave velocity model which is in good agreement with previous tomographic studies. The lithospheric architecture emerging from the phase-velocity maps and 3-D S-wave velocities correlates with surface geology and major tectonic provinces. In particular, the architecture of the narrow rift systems, the South China Craton, and the North China Craton (NCC) are revealed with unprecedented details. The South China Craton is characterized by very high velocities beneath the Sichuan Basin. The NCC shows more complex structures with two high-velocity anomalies beneath the Ordos Basin and the southeastern NCC and low-velocity anomalies beneath the Cenozoic rift systems.

Seismic Tomography of the Trans-North China Orogen and Its Dynamic Implications

We present a high-resolution S wave velocity model of the Trans-North China Orogen (TNCO) and its adjacent areas derived from S wave traveltime residuals of teleseismic waveforms recorded by the dense ChinArray seismic network. Our model reveals S wave velocity anomalies beneath the study area, suggesting the strong lateral heterogeneities of the upper mantle structures. The upper mantle of the Datong volcano is dominated by the significant low velocity anomalies. In addition to the low-velocity zone rooted in the mantle transition zone beneath the Datong volcano region, two low velocity layers extend northwestward to the Hetao graben and southwestward to the southern TNCO within depths of 100–300 km, respectively. Based on these low-velocity anomalies, we provide new insights into the origin of the Datong volcano, which may be related to the asthenospheric upwelling originated from the water-rich mantle transition zone and horizontal mantle flows driven by the extrusion northeastward of the Tibetan plateau. A clear velocity contrast in the upper mantle above 300 km depth beneath the Ordos block and the TNCO implies that the Ordos block still preserves thick cratonic root while the lithosphere of the TNCO is modified. We propose that the asthenospheric upwelling and northeastward push of the Tibetan plateau account for the lithospheric deformation of the TNCO and its adjacent areas.

Combining Dense Seismic Arrays and Broadband Data to Image the Subsurface Velocity Structure in Geothermally Active South‐Central Utah

AbstractSouth‐central Utah is characterized by Quaternary volcanism, current geothermal activity, and unusually high surface heat flux across the region. Additionally, there are three operating geothermal power plants in this region, known as the Sevier thermal area. This setting is very similar to that found in the Coso geothermal area. However, the source and lateral extent of subsurface heat in Utah is poorly understood. We use temporally separated geophone arrays combined with regional broadband data to perform ambient noise seismic tomography to study the subsurface shear velocity structure in the region. We find good recovery of ocean‐seism excited Rayleigh wave signals between the periods of 5 and 10 s. For each period, we measure the Rayleigh wave phase velocity and ellipticity across the region using the beamforming methodology and horizontal to vertical (H/V) amplitude ratio. Finally, we combine spline‐based 1D shear velocity models inverted using Rayleigh wave phase velocity and H/V measurements on a 2D, beamforming discretized grid to construct a final 3D model. The final 3D model has many similarities to imaged velocity models at the Coso geothermal area. We find a strong correlation between low velocity anomalies and high surface heat flux and geothermal activity. Additionally, we find a laterally continuous low‐velocity anomaly between 5 and 15 km depth, which may represent elevated temperature of hundreds of degrees Celsius, partial melt, and/or the presence of water, and a common heat source across the region which likely originates from the mantle.

Upper- to mid-crustal seismic attenuation structure above the mantle wedge in East Anatolia, Turkey: Imaging crustal scale segmentation and differentiation

Multi-frequency P- and S-wave attenuation tomography models of Lake Van area (East Anatolia) have been obtained by estimating coda-normalized wave spectra of 3027 local earthquakes (2.0 < Mw < 7.1). The 6998 waveforms sampled from surface to a depth of 25 km, and were recorded from 2004 to 2020 at seven broadband, three-component digital seismic stations operated by Kandilli Observatory and Earthquake Research Institute (KOERI). We adopted a two-point ray-bending method to trace rays in a 3-D velocity model. We applied the coda normalization (CN) method to P- and S-wave data sets. We inverted the spectral data with a multiple resolution seismic attenuation (MuRAT) approach to obtain final tomographic models. On average, high (low) attenuation corresponds to low (high) velocity anomalies. The P- and S-wave attenuation contrasts delimit four well-known geological zones. High frequency-short wavelength attenuation contrasts constrain the 5-km-deep zone of interaction between magma and sediments within the Lake Van basin. Low frequency-long wavelength attenuation anomalies mark the central section of Lake Van between depths of 10 km and 20 km. This zone coincides with a rigid stable shear zone overlying a possible weak-ductile lower crust, interpreted as a detachment. Both low (5–15 km depth) and high attenuation (>20 km) anomalies mark the area of maximum seismic energy release during the Van event. Their contrast highlights the maximum seismogenetic depth above weak-warm, unstable materials. High attenuation in the SE-part of the Lake Van area coincides with a large hydrothermal and/or magmatic folding-overthrusting, interpreted as a suture-metamorphic complex between depths of 10–15 km. Other minor high-attenuation zones deeper than 5 km focus on complex shear zones in the area damaged by the Van event. The paired attenuation structure of the Lake Van area appears linked to multiple tectonic processes of crust-magma interaction better constrains subsurface segmentation structures and differential deformation types at upper-middle crustal depths.

Evidence for a lithospheric step and pervasive lithospheric thinning beneath southern New England, northeastern USA

AbstractIn this study, we use data from the SEISConn seismic experiment to calculate Sp receiver functions in order to characterize the geometry of upper-mantle structure beneath southern New England (northeastern United States). We image robust negative-velocity-gradient discontinuities beneath southern New England that we interpret as corresponding to the lithosphere-asthenosphere boundary (LAB) and identify a well-defined step of 15 km in LAB depth at a longitude of 73°W, which we interpret to be the boundary between Laurentian and Appalachian lithosphere, although the offset may be larger if the putative LAB phase is reinterpreted to be a mid-lithospheric discontinuity. We infer that the lithosphere throughout the region is substantially thinner than elsewhere in the continental interior, consistent with regional tomographic studies and previously published Sp receiver function results. The presence of thinned lithosphere suggests that the low-velocity Northern Appalachian Anomaly (NAA) in the upper mantle may extend as far south as coastal Connecticut. The presence of regionally thinned lithosphere and a step in lithospheric thickness suggests that inherited structure may be preserved in present-day lithosphere, even in the presence of more recent dynamic processes associated with the NAA.