High-velocity Anomaly Research Articles

Ambient seismic noise monitoring and imaging at the Theistareykir geothermal field (Iceland)

In autumn 2017 a network of 14 broadband seismic stations was deployed at the Theistareykir high temperature geothermal field (NE Iceland). This experiment was conducted as part of the current efforts to characterize the field's main structures, and possible short and long term stress variations due to the ongoing fluid injection and extraction operations which started in autumn 2017. In this work, we use two years of continuous seismic records (October 2017–October 2019) to compute a 3D shear wave velocity model of the geothermal field and to detect possible crustal stress changes related to the injection and production activities. From phase cross-correlations of the vertical component recordings, we measure the Rayleigh wave group velocity dispersion curves to obtain 2D group velocity maps between 1 and 5 s. Subsequently, we use a neighborhood algorithm to retrieve the 3D shear wave velocity model of Theistareykir. Mainly, two sets of elongated high and low velocity anomalies can be observed oriented in a NW/WNW direction, parallel to the lineaments of the active Tjörnes fracture zone. Velocity reductions west of Ketilfjall and at Baerjafjall could indicate the location of upflow zones of the magmatic reservoir or hydrothermal system. We analyzed the temporal evolution of phase and amplitude of phase auto-correlations using the stretching technique and discuss their behavior in relation to the geothermal field operations. We notice a slightly stronger long-term velocity decrease in the reservoir region compared to outer regions. This could be related to the mass depletion in that area (higher fluid extraction compared to the water reinjection). In summary, our findings show how a monitoring network can be set up to enable a detailed imaging and monitoring of reservoir behavior in general.

Strong influence of tomographic models on geoid prediction: Case studies from Indian Ocean and Ross Sea geoids

The geoid over the north Indian Ocean has a significantly large negative amplitude, even if the excess flattening of the Earth beyond its equilibrium shape is considered. The various mechanisms proposed for this geoid anomaly vary and acceptable geoid predictions are obtained for specific tomographic models only. In this study, we identified model-independent features in the mantle beneath the Indian Ocean and Ross Sea region by analyzing eight recent global tomographic models. We standardized each of the models and applied cluster analysis to regionalize geophysically significant features in various depth ranges. These regionalizations are compared grid-by-grid to construct vote-maps. They highlight the anomalous features consistent across models and their approximate dimensions. Low velocity anomalies of dVS ∼-1.1% in the ∼400–680 km depth range are consistent in almost all the models beneath the Indian Ocean and Ross Sea. High velocity anomalies of dVS≥1% at depths below 1600 km, incoherent in dimension and orientation, are also observed. High velocity anomalies are most likely subducted slabs while low velocity anomalies could be partial melts generated by hydration of mantle. Additionally, a consistent low velocity structure is seen throughout the mantle beneath the southwestern Indian Ocean and east Africa. It is mostly likely a plume rising from the African LLSVP. It connects to the probable partial melts beneath the Indian Ocean via a remnant trail. Forward modelling of the geoid using vote-maps reveals that the E-W extent of the Indian Ocean Geoid Low is precisely reproduced by the consistent low velocity anomalies in the upper mantle. However, the N-S extent, most likely dependent on lower mantle anomalies, is not expressed since the dimension, orientation and dVS characteristics of high velocity structures are inconsistent in different models. The inter-model agreement is insufficient to identify structure(s) seen across models that can explain the geoid low over the Ross Sea.

The shallow three-dimensional structure of Volcán de Colima revealed by ambient seismic noise tomography

Volcán de Colima is one of the most active volcanoes in North America. Even so, it has a poorly constrained upper crustal structure. Here, we present the highest-resolution three-dimensional shear-wave velocity tomography to date of the volcano.We measured group velocity dispersion curves of Rayleigh and Love waves extracted from ambient seismic noise recorded on the Colima Volcanic Complex and regionalised them into 2-D velocity maps. Those were then locally inverted using a neighborhood algorithm to obtain accurate shear-velocity models down to 4 km below sea level. The resulting three-dimensional shear wave velocity model gives us a new insight into the internal structure of the volcano to specifically better understand the shallow magma storage and magmatic plumbing system and, in turn, how it can be placed in a wider geotectonic context.Our results highlight a network of deeply rooted NE-SW low velocity zones oriented along a local fault system. The southward orientation of this low-velocity anomaly also roughly aligns with the north-south trend of the volcanoes that compose the quaternary Colima Volcanic Complex and could be associated with the gradual trenchward shift of the magmatic front of the volcanic complex dating from the formation of the Cántaro Volcano. The low-velocity anomaly overlaps a negative radial anisotropy indicating that magma follows vertically oriented structures, such as interfingered dikes or faults and cracks with a substantial vertical component.The low velocity anomaly under Volcán de Colima contrasts with a distinct high-velocity anomaly under Nevado de Colima that highlights the difference between the former active system, filled with solidified dikes and sills, and the current one, associated with a network of fluid-filled dikes.

Seismic acoustic full waveform inversion based on the steepest descent method and simple linear regression analysis

Full waveform inversion (FWI) is a high-resolution imaging tool in seismic exploration. The steepest descent (SD) method is the simplest local optimization algorithm to update the P-wave velocity model in acoustic FWI, but it takes more iterations than other algorithms due to its inefficient search direction. In this study, we provide a SDSLR method, which can significantly shorten the convergence process of the SD method by using the simple linear regression (SLR) analysis. The SLR analysis has two important effects in this method: compute a SLR model to approximately estimate the recent variation trend of P-wave velocity model and directly predict the inversion result of the SD method of the next iteration. Based on the prediction of the SLR analysis, the SDSLR method can obtain the final inversion result of the traditional SD method with fewer iterations. Three numerical examples have tested the performance of the SDSLR method. Results show that the SDSLR method has great potential in constructing high-velocity anomalies. The computational efficiency of the SDSLR method is similar to that of the SD and L-BFGS methods, while the convergence rate and accuracy of the SDSLR method are clearly better than those of the SD and L-BFGS methods.

Cenozoic delamination of the southwestern Yangtze craton owing to densification during subduction and collision

Abstract It is widely thought that oceanic subduction can trigger cratonic keel delamination, but the southwestern Yangtze craton (SYC; southwestern China) lost its lower keel during Cenozoic continental collision. The upper mantle beneath the thinned SYC contains its incompletely delaminated keel, which has high-velocity seismic anomalies. Combining geophysical observations with the geochemistry of Eocene mafic potassic lavas derived from the SYC mantle at different depths, we suggest that the deep (~130 km) delaminated lithosphere was more fertile and dense, with low-forsterite (Fo; molar 100 × Mg/[Mg + Fe] = 91.3) and high-δ18O (5.9‰) olivine, than the shallow (~55 km) intact lithosphere (Fo = 94.2; δ18O = 5.2‰), although both were rehydrated and oxidized. The deep keel underwent strong refertilization and densification owing to the addition of Fe-rich basaltic melts during earlier oceanic subduction. Subduction-driven refertilization and subsequent collision-driven cooling caused cratonic keel delamination due to compositional and thermal densification rather than hydration- or oxidation-induced rheological weakening. Our study provides an example of Cenozoic cratonic keel delamination in a collisional orogen and highlights the key roles of compositional and thermal densification in delamination during subduction and collision.

P-Wave Velocity Structure of the Lower Crust and Uppermost Mantle beneath the Sichuan–Yunnan (China) Region

Abstract We conduct a tomographic inversion for the 3D P-wave velocity structure in the lower crust and uppermost mantle of the Sichuan–Yunnan region in the southeastern margin of the Tibetan Plateau. A total of 43,450 reliable arrival times of P waves are picked from over 300,000 regional seismic records using an automatic algorithm based on deep learning. A two-stage iterative inversion process in which events are relocated, leading to a significant reduction in travel-time residuals, is adopted. A statistical resolution matrix analysis suggests that our model has an optimal spatial resolution length of ∼0.4° in the lower crust and ∼0.2° in the uppermost mantle. Our 3D model shows that both the lower crust and uppermost mantle in the region are characterized by strong lateral heterogeneities. The unusually low velocities in the lower crust may indicate the existence of lower crustal flow, whereas the high-velocity anomalies in the uppermost mantle in and around the Sichuan–Yunnan Rhombic block (SYRB) may be an important factor in preventing the ductile materials in the lower crust from moving eastward. Our model also indicates a coupling between the surface deformation and the material flow in the lower crust. Finally, the lower crustal flow may influence the materials in the shallow part of the uppermost mantle beneath the SYRB, and the crust–mantle transition zone beneath the Songpan–Ganzi block may be influenced by weak materials from both the uppermost mantle and the lower crust.

Imaging the crust and uppermost mantle structure of Portugal (West Iberia) with seismic ambient noise

SUMMARYWe present a new high-resolution 3-D shear wave velocity (Vs) model of the crust and uppermost mantle beneath Portugal, inferred from ambient seismic noise tomography. We use broad-band seismic data from a dense temporary deployment covering the entire Portuguese mainland between 2010 and 2012 in the scope of the WILAS project. Vertical component data are processed using phase correlation and phase weighted stack to obtain empirical Green functions (EGFs) for 2016 station pairs. Further, we use a random sampling and subset stacking strategy to measure robust Rayleigh-wave group velocities in the period range 7–30 s and associated uncertainties. The tomographic inversion is performed in two steps: First, we determine group-velocity lateral variations for each period. Next, we invert them at each grid point using a new trans-dimensional inversion scheme to obtain the 3-D shear wave velocity model. The final 3-D model extends from the upper crust (5 km) down to the uppermost mantle (60 km) and has a lateral resolution of ∼50 km. In the upper and middle crusts, the Vs anomaly pattern matches the tectonic units of the Variscan Massif and Alpine basins. The transition between the Lusitanian Basin and the Ossa Morena Zone is marked by a contrast between moderate- and high-velocity anomalies, in addition to two arched earthquake lineations. Some faults, namely, the Manteigas–Vilariça–Bragança fault and the Porto–Tomar–Ferreira do Alentejo fault, have a clear signature from the upper crust down to the uppermost mantle (60 km). Our 3-D shear wave velocity model offers new insights into the continuation of the main tectonic units at depth and contributes to better understanding the seismicity of Portugal.

Mapping the mantle transition zone beneath the Indian Ocean geoid low from Ps receiver functions

The most pronounced high-amplitude (−40 to −106 m) geoid deficit with respect to the Earth's reference shape straddles the Indian Ocean. The existing hypotheses suggest a possible linkage of low density and low-velocity anomalies in the mid-and upper mantle and high-velocity anomalies near the base of the mantle to the source behind the lowest geoid. We obtained P to S wave radial receiver functions to investigate these mantle anomalies from an extensive array of Ocean Bottom Seismometers (OBSs) positioned in the centre of the Indian Ocean Geoid Low (IOGL). Our modelling suggests that the mean depths for d410 and d660 range from 386.0 ± 13.6 to 459.7 ± 2.9 km and 643.1 ± 7.4 to 710.2 ± 5.4 km with an average of 432.6 km and 680.2 km, respectively. The average thickness of the transition zone varies from 199.6 to 289.8 km. New results confirm an extensive ~800 km wide depression at d410 and d660 towards the centre of the geoid. The depression potentially implies a rather hot mantle material, in which majorite garnet to perovskite transition may become dominant at 660 km depth. The excess temperature calculated for d410 and d660 topography ranges from 139.5 to 557.5 K and 206 to >1000 K, respectively. The shear velocity anomalies derived from the excess temperature range from −0.89 to −3.52 (%) at d410, and at dd60 range from −1.13 to −7.87 (%). The velocity anomalies are highly underestimated at d660 and could be explained by hydrous mantle upwelling at the lower mantle transition zone. The present study outcomes offer new avenues towards exploring a mantle upwelling in the Indian Ocean and its linkage with geoid undulations.

Subducting slabs, Hainan plume and intraplate volcanism in SE Asia: Insight from P-wave mantle tomography

We present a high-resolution 3-D model of P wave tomography of the crust and mantle down to 1600 km depth under Southeast Asia (SE Asia), which is determined by inverting more than 1.3 million P wave arrival time data of local, regional and teleseismic events recorded at 1241 seismic stations deployed in SE Asia and mainland China. Strong and weak smoothing inversions are conducted to reveal different characteristic scales of velocity anomalies in the mantle. Both seismic and aseismic portions of the subducted plates in different parts of the circular subduction system in SE Asia are clearly revealed as high-velocity bodies in our tomographic images. The intraplate volcanoes in Borneo are underlain by significant low-velocity anomalies in the upper mantle, whereas a clear high-velocity anomaly is visible in the mantle transition zone and the lower mantle. We deem that hot and wet upwelling flows in the big mantle wedge above the stagnant slab in the mantle transition zone may have caused the intraplate volcanism in Borneo. A large low-velocity anomaly consisting of a flat head and a twisted tail extending to the bottom of our model is imaged beneath the southeastern Asian basalt province. The low-velocity anomaly possibly represents a hot zone whose shape may have been modified by mantle wind associated with a plate rotation. The hot zone has a lower-mantle origin and probably contains some small mantle upwellings inside, with the Hainan plume being a major trunk.

Three dimensional P-wave velocity structure underneath the southeastern margin of the Tibetan Plateau and the deep tectonic significance

The deep velocity structure underneath the southeastern margin of the Tibetan Plateau is the key to understand the tectonic evolution of the Tibetan Plateau. In this study, 145,254 relative travel-time residuals were measured from the records of 1360 teleseismic events using the waveform correlation method, we determined the three-dimensional (3-D) P-wave velocity structures in the upper mantle using a large-scale dense seismic array. The western boundary of the Yangtze Craton extends from the Longmenshan fault zone in the north to the Red-River fault in the south, our imaging results show that in a depth range of 50–150 km, obvious differences in physical properties of the medium exist across the Longmenshan fault zone. The Red-River fault and the Xiaojiang fault are the western and eastern boundaries of Chuan-Dian diamond block, respectively. Low-velocity anomalies are visible from the north of the Red-River fault to the east of the Xiaojiang fault and their intersection region. The high-velocity anomaly below the Sichuan Basin is thinning westward, with the thickness of 350 km below eastern Sichuan Basin. We obtained a low-velocity anomaly extending 400 km below the Tengchong volcanic field. A high-velocity anomaly, extending down to the mantle transition zone, is thought to be related to the subduction of the Indian slab. Our results also reveal an obvious low-velocity anomaly exists to the south of 26°N, we speculate that the flow of the upper mantle material beneath the Tibetan Plateau affects the velocity structure in this region.

P‐wave Velocity Structure of Crustal and Upper Mantle beneath South China and its Tectonic Implications

AbstractThe 3D P‐wave velocity structure beneath the South China Block was determined by applying arrival times from 269 teleseismic events recorded by 240 seismic stations within the study region. Our tomographic results reveal the deep structural characteristics of major tectonic units and ore concentration areas. There are distinct high velocity anomalies beneath the ancient Yangtze and Cathaysia blocks, with the lithosphere of the Cathaysia Block being thinner than the Yangtze Block; the Jiangnan orogenic belt, located in the combined zone of two blocks, is a high and low velocity anomaly conversion zone; the famous metallogenic belts of Edongnan, the Youjiang Basin and the Cathaysia Block are obviously low velocity areas with different metallogenic mechanisms. The deep ore‐forming material source in the Edongnan metallogenic belt is different from that of the Cathaysia Block. The low velocity anomaly under the Cathaysia Block related to mineralization results from the upwelling of mantle material, caused by the joint action of the Paleo‐Tethys tectonic domain, the Paleo‐Pacific tectonic domain and the Hainan mantle plume migration and erosion, which has been occurring from northeast to southwest since 80 Ma. The low‐temperature mineralization mechanism of Youjiang Basin should be considered not only in terms of the influence of the Emeishan mantle plume in the west and the Paleo‐Tethys tectonic domain in the south, but also in the context of the influence of the upwelling of asthenospheric material from the Paleo‐Pacific tectonic domain in the east.

Mapping the edge of subducted slabs in the lower mantle beneath southern Asia

SUMMARYWe investigate the presence of seismic structures in the Earth's mantle by searching for seismic signals that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle. We focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, we measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering. We backtrace the signals as P-to-P and S-to-P waves and extend the methodology to P-to-S waves. There seems to be a low number of reflection points in the regions beneath Eurasia in our study. Investigating possible causes, we find that the focal mechanism influences the presence of out-of-plane reflected waves. However, the potential coverage with our data set is large and should allow detection, but there may potentially be few seismically visible structures in the region. Most of our backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. We also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly. These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle.

Crustal and upper mantle structure of northern Algeria inferred from a 3-D inversion of teleseismic tomography

The northern Algeria forms the southern edge of the Western Mediterranean Sea that formed within the zone of convergence between the African and Eurasian plates. It represents a key area in the Mediterranean formation where controversial kinematic and geodynamical models are proposed. The current structure of northern Algeria is the result of the geodynamic evolution that occurred in the western Mediterranean. Several tomographic studies revealed the presence of velocity anomalies beneath the south of the western Mediterranean region. Namely, these anomalies suggest the existence of abnormally cold material in the asthenosphere, interpreted as the trace of subducted plunging panels (slab). Unfortunately, the lack of suitable data remains the main limit confronted by most tomographic studies in the northern Algeria, which remains to date one of the poorly studied regions in the western Mediterranean. During the recent decades, the seismological network in northern Algeria has significantly developed, giving rise to a significant increase in the amount of information given by the network database. This development motivated us to investigate the crust and the upper mantle structure beneath the northern Algeria using a non-linear teleseismic tomography method. An overall of 29 stations distributed along the northern part of Algeria have been used to collect 11018 P-waves from 2847 earthquakes recorded over ten years. The proposed model suggests the presence of high-velocity anomalies along of the northern Algeria. We interpreted these abnormal anomalies as a slab teared into two panels. A central segment bending along the Atlasic massif of Hodna and plunging under the Lesser Kabylia. The second segment at the North-East, shifted to the north plunging towards the NE. A series of Tellian high-velocity anomalies axes mark the limit between the African and the Eurasian plate. The south eastern region covered by our model (Aurès massif) is characterized by a low-velocity anomaly potentially linked to the remnant of the Mesozoic rifting.

Adjoint Tomography of Northeast Japan Revealed by Common-Source Double-Difference Travel-Time Data

AbstractWe present the wave equation-based adjoint tomography of northeastern Japan using common-source double-difference travel-time data. More than 30,000 high-quality first P-wave arrivals from 117 local earthquakes recorded by 713 seismic stations are included in the inversion, generating more than 350,000 double-difference travel times of first P-wave arrivals. In comparison with commonly used travel-time data, the common-source double-difference travel times are insensitive to source parameters and source-side structures but place enhanced constraints on receiver-side structures. As a result, these robust double-difference travel times generate a reliable VP model down to a depth of 120 km. Our inversion results clearly show crustal low-velocity anomalies beneath the volcanic arc. The subducting Pacific slab is illuminated as a landward-dipping high-velocity anomaly, and the arc magmatism in the mantle wedge is shown as low-velocity anomalies. Thanks to the newly operated Seafloor Observation Network (S-net), prominent high-velocity bodies are detected in the fore-arc mantle wedge at depths of 30–60 km. These abnormal high-VP bodies indicate a low degree of serpentinization in the fore-arc mantle wedge at low temperatures, correlating well with the previous finding that the uppermost mantle in the Kuril and Tohoku fore-arc is cold and dry. Our inversion results reveal reliable tectonic features in the subduction zone beneath northeastern Japan, suggesting that wave equation-based common-source double-difference travel-time adjoint tomography is an effective and robust method to illuminate detailed structures of the crust and uppermost mantle.

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 > 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.

Constraints on fluids in the continental crust from laboratory-based electrical conductivity measurements of plagioclase

Aqueous fluids are regarded as the crucial origin of high conductivity and low velocity anomalies observed by magnetotelluric and seismic surveys in the middle to lower continental crust. Integration of field geophysical methods with experimental conductivity data is essential to accurately constrain the fluid fraction and salinity. Studies of fluid content and salinity in the crust from the experimental or theoretically calculated conductivity of fluid–minerals systems and saline-bearing fluid have demonstrated that crustal constituent minerals play a prominent role in the conductivity of fluid–mineral systems. We performed systematic measurements on electrical conductivity of dry and hydrous plagioclase solid solutions at high temperature and pressure. A positive dependence of electrical conductivity on Na contents is observed in dry samples, and discontinuous variation in conductivity is associated with the degree of Al/Si order–disorder. For hydrous samples, trace amounts of water considerably enhance the electrical conductivity. The dominant charge carriers in dry and hydrous plagioclase are Na ions and hydrogen, respectively, both of which migrate via the Frenkel mechanism. If there are no free fluids present in minerals, the crustal major constituent minerals containing structure water cannot account for the high-conductivity anomalies in the middle to lower crust based on our model of the rock conductivity–depth profiles in the continental crust. The calculated bulk conductivities of fluid–rock systems further suggest that at least 1 vol% of fluid with seawater salinity (3.5 wt%) is necessary in the middle to lower crust to interpret the high conductivity anomalies.

Velocity Anomalies Around the Mantle Transition Zone Beneath the Qiangtang Terrane, Central Tibetan Plateau From Triplicated P Waveforms

AbstractThe Qiangtang terrane exhibits many unique features compared with surrounding areas in the Tibetan plateau. It has been largely remodeled by the northward downwelling of the Indian lithosphere and the vertical upwelling of the Eurasian asthenosphere. Here, we investigate the P‐wave velocity around the mantle transition zone (MTZ) beneath the Qiangtang terrane using high‐frequency triplicated waveforms recorded by the China Digital Seismic Network. We found a low‐velocity layer (LVL) atop the MTZ and a high‐velocity anomaly (HVA) in the lower part of the MTZ. They are characterized by thickness of 36 km with velocity drop of −2.0% and 155 km with velocity increment of 1.2%, respectively. The 410 and 660 km discontinuities are depressed by different depths of 12 and 30 km, suggesting a thickened MTZ. In combination with previous multi‐disciplinary observations, we infer that the LVL is a hydrous partial melt affected by dehydration and high temperature, and the HVA is due to water and low temperature conditions. The downwelling cold continental lithosphere may have accumulated in the lower MTZ and induced the hydrous hot MTZ materials to upwelling until the shallow depths of the upper mantle beneath the Qiangtang terrane.

Ambient noise tomography of local shallow structure of the southern segment of Tanlu fault zone at Suqian, eastern China

Comparable to San Andreas fault in western United States, the Tanlu fault zone (TLFZ) is a large-scale continental tectonic belt in eastern China and has played an important role controlling the regional tectonics and earthquake activity in eastern China. It is known there exists an earthquake gap in the southern segment around Suqian, Jiangsu province, where no earthquakes greater than M5.5 occurred. Fine fault zone structure could help to understand the fault activity and the potential earthquake hazards. However, only regional scale velocity model is available in this region, making it difficult to characterize detailed fault structures. In this study, we obtained local Vs model in the upper crust in this region by ambient noise tomography with a dense seismic array. For the Suqian segment, Cretaceous brittle normal faulting controlled the development of the eastern and western graben as well as the central horst between them. The graben-boundary faults are known simply as F1-F4 from east to west, and the NNE-striking fault F5 is between F1 and F2. The eastern boundary fault F1 and western boundary fault F4 separated by about 30 km are associated with intermittent high velocity anomalies in the shallow crust and are inactive. In comparison, faults F2, F5 and F3 are mainly associated with low velocity anomalies and are relatively active. These high velocity anomalies are very likely caused by the upwelling magma migrated along faults F1 and F4 in the crust, which on freezing could weld the fault zone and make it less active. Because faults F1 and F4 are hindered to move, the deformation could be transferred to nearby faults F2, F3 and F5. Our study sheds light on what controls fault activities along the TLFZ by obtaining local velocity structure in the Suqian segment.

Dynamic model of the upper mantle beneath the northeastern Tibetan Plateau - constraints from the 410 km and 660 km discontinuities

We determined the topography of the upper mantle 410 km and 660 km discontinuities (‘410’ and ‘660’) beneath the northeastern Tibetan Plateau and surrounding areas using a dense broadband seismic array. We binned the 95,474 teleseismic receiver functions into 0.5 × 0.5° grids; then the P410s and P660s arrival times were selected. Based on receiver function migration using three different regional tomographic models, the average ‘410’ and ‘660’ depths obtained are 413 ± 1 km and 672 ± 2 km, and the mantle transition zone (MTZ) thickness is ∼ 260 km. Combining these data with the velocity anomalies in the tomographic models, we find that the depressed ‘660’ beneath the northeastern Tibetan Plateau may be affected by cold anomalies delaminated from the thickened lithosphere. The depressed ‘410’ beneath the same area may be mainly affected by upwelling hot materials, which resulted from the delamination process and mixed with the eastward escape flow from the Tibetan Plateau. The slight uplift of ‘410’ is consistent with the extension of the high-velocity anomaly beneath the middle Ordos block. The thinning MTZ and depressed ‘410’ suggest that there are two branches of upwelling hot mantle beneath the northern and southern Ordos blocks. Considering the remote effects of both the subducted Indian Plate and Pacific Plate, we conclude that the dynamic model beneath the northeastern Tibetan Plateau and western North China craton (NCC) is controlled by two different large-scale subduction systems.

High-resolution Vs tomography of South China by joint inversion of body wave and surface wave data

To take advantage of complementary strengths of earthquake body wave data and surface wave data from both earthquake and ambient noise sources, we have jointly used them to determine a high-resolution 3D Vs model of the lithosphere in South China by the joint inversion algorithm of Zhang et al. (2014). For body wave data, we assembled P- and S-wave arrival times for 40,028 earthquakes during the period of 2008/10–2018/06 recorded by 676 stations in South China. For surface wave dispersion dataset, it includes Rayleigh wave phase velocity maps at periods of 8–70 s and group velocity maps at periods of 8–50 s from Shen et al. (2016). Due to the complementary strengths of the two data types, the resolution of the jointly inverted Vs model has improved compared to the separately inverted models. The joint inversion model can also fit both data types at a similar level compared to separate inversions. In comparison with previous models, the new velocity model better delineates the sedimentary basins in the shallow crust and high velocity root in the lithosphere beneath the Sichuan basin, as well as better resolves the velocity variations between western and eastern Yangtze craton, thin lithosphere beneath eastern Yangtze craton and Cathaysia block, and helps us to better understand the mechanism of the lithosphere thinning. From the correlation between high velocity anomalies in the middle crust and the distribution of Neoproterozoic basalt samples, we propose the amalgamation suture between the Yangtze craton and Cathaysia block is along the eastern segment of the Jiangshan-Shaoxing fault and the west margin of the Jiangnan orogen volcanic rocks. Our new joint inversion model is useful for constructing a community velocity model and for better understanding the complex tectonics of the South China block.