Seismic Tomography Models Research Articles

Inferring the relationship between core-mantle heat flux and seismic tomography from mantle convection simulations

The heat flux pattern at Earth’s core-mantle boundary (CMB) imposes a heterogeneous boundary condition on core dynamics that may profoundly affect the geodynamo. Owing to the expected temperature dependence of seismic velocities, this pattern is classically approximated as proportional to the lowermost layer of seismic tomography models for the global mantle. Two biases however undermine such a simple linear relationship: 1) other contributions than thermal (compositional and mineralogical) influence seismic velocities and 2) the radial average is inherent to tomographic models whereas the local thermal state at the CMB is relevant for the heat flux. We analyze here simulations of thermochemical mantle convection where, owing to their spatial characteristics, specific mantle components are readily identified: hot thermochemical piles (TCPs), “normal” mantle (NM) and, when post-peroskite (pPv) is included, a cold region where this phase is present. Synthetic seismic velocities (i.e. from the mantle simulations) are then computed based on thermal, compositional and mineralogical sensitivities. A formalism to infer the CMB heat flux from these seismic shear velocity anomalies is derived. In this formalism, within each mantle population (i.e. TCPs, NM or pPv) the CMB heat flux vs. seismic anomalies follows a unique fitting function. The transition from one mantle population to another is marked by a jump in the seismic anomaly, i.e. a range of seismic anomalies in between two mantle populations corresponds to a similar CMB heat flux. Applying our formalism to the seismic anomalies from the mantle convection simulations provides far superior fits than the commonly used linear fits. The results highlight reduced negative heat flux anomalies beneath large low shear velocity provinces (LLSVPs), while positive heat flux anomalies are enhanced, both with respect to the classical linear interpretation.

Seismic Evidence for Metamorphic Densification of the Lower Continental Crust in Eastern North America

AbstractThe composition of the lower continental crust, as well as its formation, growth, and evolution, remains a fundamental subject to be understood. In this study, we carry out a comparative and integrative analysis of seismic tomographic models, teleseismic receiver function results, and Airy isostasy in order to investigate the properties of the lower continental crust in eastern North America. We extract the depths for Vs of 4.0 km/s, 4.2 km/s, and 4.5 km/s from three selected tomographic models and calculate the differences between the Vs depth contours and the Moho depth defined by receiver functions. We then calculate the Airy isostatic Moho depth and its misfit with the receiver‐function‐defined Moho. Our analysis reveals three key features: (a) the deepening of the Vs depth contours and the strong negative Airy misfit within the U.S. Grenville Province; (b) a seismically faster‐than‐average and compositionally denser‐than‐average lowermost crust in the eastern North American Craton and the Grenville Province; and (c) the thickest, seismically fastest, and densest lowermost crust beneath the southern Grenville Front, the southern Grenville‐Appalachian boundary, and the U.S.‐Canada national border. We suggest that the lower crust of the craton and the Grenville Province has densified through garnet‐forming metamorphic reactions during and after orogenesis, contributing to the widely distributed fast‐velocity layer. The lower crust beneath the tectonic boundaries could have experienced more extensive garnet growth during orogenesis and emplacement of mafic magma. This study provides new constraints on the seismic and compositional properties of the lower crust in eastern North America.

3D seismic tomography models of the Baikal Rift zone and surrounding areas based on regional seismological data

3D seismic tomography models of the Baikal Rift zone and surrounding areas based on regional seismological data

Tectonic Structure of Northern Sumatra Region Based on Seismic Tomography of P and S Wave Velocity

The tectonic setting of Sumatra Island is strongly influenced by the oblique subduction of the Indo-Australian Plate, which subducts the Eurasian Plate at a speed of 52–60 mm/year. The movement of these plates resulted in the Northern Sumatra region having seismic sources from tectonic and volcanic activity. The data used in this study is in the form of seismic wave travel-time recorded by numerous seismic stations in the research area from January 2012 to December 2020. The data comes from 5,003 earthquakes recorded by the BMKG seismic network. The inversion is a simultaneous inversion between seismic velocity models (Vp and Vs) and hypocenter parameters by applying a double-difference seismic tomography algorithm. Tomogram results in parts of Aceh (Singkil and Subulussalam) and North Sumatra (Pakpak Bharat and Dairi) at a depth of 0 km show negative perturbations in Vp and Vs values and high Vp/Vs values. The anomaly is most likely related to cracks in fluid-saturated rocks. The tomograms in the south of Lake Toba at depths of 30 km and 40 km have high Vp and Vs perturbation values and low Vp/Vs values. This anomaly indicates a magma supply line that is no longer active or has cooled for a long time. Based on the seismic tomography modeling results, the subducted Indo-Australian Plate to the Eurasian Plate is visible in the study area.

3-D density structure of the upper-mantle in the eastern Mediterranean Sea and surrounding region using gravity inversion constrained by seismic velocity model

A 3D density structure of the lithosphere and upper mantle beneath the eastern Mediterranean Sea (EMS) and its adjacent region was constructed based on gravity anomaly inversion constrained by a seismic tomography model. Gravity effects of terrain and crust were removed from the observed gravity field (EIGEN-6C4) to obtain the residual mantle gravity anomaly (RMGA). The density distribution of the lithosphere and upper mantle was investigated. The 3D inversion process was constrained by an initial density model projected from the shear-wave velocity model (SL2013sv). The results show some characteristics of the density distribution in the lithosphere and in the upper mantle that could be related to the tectonic importance of the Mediterranean Sea and the surrounding region. A low-density zone dominates the lithosphere beneath the Sea except for the area around the Arabia Shield and North Anatolian Fault belt. A thinner, high-density layer beneath the southwest of the Sea may be related to the older oceanic lithospheric fragments. The high-density anomalies appear at depths below 280 km beneath the Sea and the Turkish Aegean Sea Plate. However, the low-density anomalies appear on the upper mantle under the trenches of the southwestern part of the Mediterranean Sea, the eastern part of the Aegean Sea, the Red Sea, the Black Sea, and the middle of the Arabia shield. The deep structure under the Eratosthenes seamount in the Mediterranean Sea is the source of the intensity and genesis of tectonic activity. Furthermore, the convergence region of two low-density anomaly zones (Africa-Anatolia) may be interpreted as a significant tectonic unit (Eratosthenes seamount) caused by the arrival of the relatively thick and buoyant Eratosthenes block to its present location south of Cyprus in Holocene time based on the density model interpretation beneath the Mediterranean Sea during the Late Cretaceous and early Tertiary period.

A Set of Geophysical Fields for Modeling of the Lithosphere Structure and Dynamics in the Russian Arctic Zone

This paper presents a set of various geological and geophysical data for the Arctic zone, including some detailed models for the eastern part of the Russian Arctic zone. This hard-to-access territory has a complex geological structure, which is poorly studied by direct geophysical methods. Therefore, these data can be used in an integrative analysis for different purposes. These are the gravity field, heat flow, and various seismic tomography models. The gravity field data include several reductions calculated during our preceding studies, which are more appropriate for the study of the Earth’s interiors than the initial free air anomalies. Specifically, these are the Bouguer, isostatic, and decompensative gravity anomalies. A surface heat flow map included in the dataset is based on a joint inversion of multiple geophysical data constrained by the observations from the International Heat Flow Commission catalog. Available seismic tomography models were analyzed to select the best one for further investigation. We provide the models for the sedimentary cover and the Moho depth, which are significantly improved compared to the existing ones. The database provides a basis for qualitative and quantitative analysis of the region.

Mid-mantle seismic scatterers beneath the Samoan hotspot

We analyze teleseismic array data of deep and intermediate-depth earthquakes in the Fiji-Tonga-Kermadec region recorded at seismic networks at the west coast of US and Alaska with the aim of mapping seismic scatterers in the lower mantle and in the upper mantle transition zone beneath the Samoan hotspot. Short period and broadband waveform data are array-processed in order to detect coherent but anomalous phases in the P coda up to nearly 120 s after direct P waves and locate where they arrive from. Updating the data set reported in our previous study (Kaneshima and Helffrich, 2010; Kaneshima, 2018; Kaneshima, 2019), we examine nine scatterers near the hotspot, seven in the lower mantle and two in the transition zone, with the main focus on three lower mantle scatterers nearest to the hotspot at 1000 to 1200 km depths. We observe S-to-P waves generated from the three lower mantle scatterers for earthquakes with a wide range of hypocenters from 17°S to 24°S, while hardly any signs of corresponding P-to-P scattering waves are observed. The pulse shapes of the S-to-P waves from the three scatterers are simple and their pulse widths do not significantly differ from those of direct P waves when the signal quality is high enough. The geometry of the scatterers suggested by these observations is a steeply dipping sheet-like structure with the thickness less than several kilometers. The material consisting of the sheet needs to have a Vs value at least several percent lower than the surrounding mantle, as required by the amplitudes of S-to-P scattering waves relative to P, which is typically several percent. The low Vs anomaly of basaltic rocks relative to pyrolite under the shallow lower mantle conditions may be consistent with the observations (Wang et al., 2020). Seismic tomography models suggest that the nine scatterers tend to be located around the edge of a plume that ascends from the Pacific LLSVP to the Samoan hotspot. Numerical simulations of ascending plumes from a thermo-chemical pile at the bottom of the mantle show that the plumes tend to rise preferentially in the vicinity of the edge of the pile. Other simulations also show that blobs of recycled former oceanic crust subducting to the bottom of mantle are entrained into a plume and stretched vertically to a significant degree, consistently with the images suggested by geochemical studies of OIB. The scattering observations therefore may elucidate the structure of entrained basaltic blobs on their way of returning back to the surface hotspot.

Ghost-arc geochemical anomaly at a spreading ridge caused by supersized flat subduction

The Southern Atlantic-Southwest Indian ridges (SASWIR) host mid-ocean ridge basalts with a residual subduction-related geochemical fingerprint (i.e., a ghost-arc signature) of unclear origin. Here, we show through an analysis of plate kinematic reconstructions and seismic tomography models that the SASWIR subduction-modified mantle source formed in the Jurassic close to the Georgia Islands slab (GI) and remained near-stationary in the mantle reference frame. In this analysis, the GI lies far inboard the Jurassic Patagonian-Antarctic Peninsula active margin. This was formerly attributed to a large-scale flat subduction event in the Late Triassic-Early Jurassic. We propose that during this flat slab stage, the subduction-modified mantle areas beneath the Mesozoic active margin and surrounding sutures zones may have been bulldozed inland by >2280 km. After the demise of the flat slab, this mantle anomaly remained near-stationary and was sampled by the Karoo mantle plume 183 Million years (Myr) ago and again since 55 Myr ago by the SASWIR. We refer to this process as asthenospheric anomaly telescoping. This study provides a hitherto unrecognized geodynamic effect of flat subduction, the viability of which we support through numerical modeling.

Connections between arc volcanoes in Central Kamchatka and the subducting slab inferred from local earthquake seismic tomography

The area of Central Kamchatka limited by latitudes of 52.5 and 54 degrees includes six active volcanoes (Avacha, Koryaksky, Zhupanovsky, Mutnovsky, Gorely and Opala), as well as a number of dormant and extinct stratovolcanoes, monogenic cones and large calderas. Furthermore, it contains the Malko-Petropavlovsk fracture zone (MPZ), which marks the boundary between two distinct subduction regimes to the south and to the north. We present a new seismic tomography model for this area, which was constructed based on the joint use of data of the Kamchatkan permanent seismic stations and a temporary network installed in the region in 2019–2020. A series of synthetic tests have demonstrated fair resolution of the derived seismic velocity structures in the crust and in the mantle wedge down to ∼150 km. The distributions of the P and S wave velocities, and especially the Vp/Vs ratio, clearly highlight the connection between the volcanic centers in Central Kamchatka and the subducting slab. At depths below 40 km depth, we observe two large low-velocity anomalies centered below Zhupanovsky and Mutnovsky volcanoes and covering all other volcanoes in the area. In the vertical sections, the corresponding anomalies of high Vp/Vs ratio have mushroom shapes with the heads spreading along the bottom of the crust, which probably represent the underplating of magma material that feeds the volcanoes of the groups. The tomography results also reveal some important tectonic features, such as a V-shaped fault system in the Avacha Graben, which is the part of the MPZ.

A comparative study of seismic tomography models of Southwest China

The margin of the Tibetan Plateau of Southwest China is one of the most seismically active regions of China and is the location of the China Seismic Experimental Site (CSES). Many studies have developed seismic velocity models of Southwest China, but few have compared and evaluated these models which is important for further model improvement. Thus, we compared six published seismic shear-wave velocity models of Southwest China on absolute velocity and velocity perturbation patterns. The models are derived from different types of data (e.g., surface waves from ambient noise and earthquakes, body-wave travel times, receiver functions) and inversion methods. We interpolated the models into a uniform horizontal grid (0.5° × 0.5°) and vertically sampled them at 5, 10, 20, 30, 40, and 60 km depths. We found significant differences between the six models. Then, we selected three of them that showed greater consistency for further comparison. Our further comparisons revealed systematic biases between models in absolute velocity that may be related to different data types. The perturbation pattern of the model is especially divergent in the shallow part, but more consistent in the deep part. We conducted synthetic and inversion tests to explore possible causes and our results imply that systematic differences between the data, differences in methods, and other factors may directly affect the model. Therefore, the Southwest China velocity model still has considerable room for improvement, and the impact of inconsistency between different data types on the model needs further research. Finally, we proposed a new reference shear-wave velocity model of Southwest China (SwCM-S1.0) based on the three selected models with high consistency. We believe that this model is a better representation of more robust features of the models that are based on different data sets.

Long‐Distance Asthenospheric Transport of Plume‐Influenced Mantle From Afar to Anatolia

AbstractThe origin of widespread volcanism far from plate boundaries and mantle plumes remains a fundamental unsolved question. An example of this puzzle is the Anatolian region, where abundant intraplate volcanism has occurred since 10 Ma, but a nearby underlying plume structure in the deep mantle is lacking. We employed a combination of seismic and geochemical data to link intraplate volcanism in Anatolia to a trail of magmatic centers leading back to East Africa and its mantle plume, consistent with northward asthenospheric transport over a ∼2,500 km distance. Joint modeling of seismic imaging and petrological data indicates that the east Anatolian mantle potential temperature is higher than the ambient mantle (∼1,420°C). Based on multiple seismic tomography models, the Anatolian upper mantle is likely connected to East Africa by an asthenospheric channel with low seismic velocities. Along the channel, isotopic signatures among volcanoes are consistent with a common mantle source, and petrological data demonstrate similar elevated mantle temperatures, consistent with little cooling in the channel during the long‐distance transport. Horizontal asthenospheric pressure gradients originating from mantle plume upwelling beneath East Africa provide a mechanism for high lateral transport rates that match the relatively constant mantle potential temperatures along the channel. Rapid long‐distance asthenospheric flow helps explain the widespread occurrence of global intraplate magmatism in regions far from deeply‐rooted mantle plumes throughout Earth history.

Stress State of the Stable Part of the Pacific Plate Predicted by a Numerical Model of Global Mantle Flow Coupled with Plate Motion

Abstract The Pacific plate, which is the largest oceanic plate on Earth, has implications for the general understanding of plate dynamics, including the origin of intraplate stress and the driving force for plate motion. However, this is currently limited by the scarcity of geophysical and geological observational data. In this study, an instantaneous global mantle flow calculation was performed to predict the intraplate stress field and stress regimes on the Pacific plate using a geodynamic model based on the density anomaly structure of the mantle converted from a seismic tomography model incorporating subducting plates. The numerical results demonstrate that the southern part of the Pacific plate is dominated by a normal faulting regime. In contrast, the northern part is dominated by a thrust faulting regime, in which the tensional stress axes in the older and stable part of the Pacific plate tend to be oblique to the direction of plate motion. This suggests that the stress state of the Pacific plate is almost neutral (i.e., neither compressional nor tensional) along the direction of plate motion. Furthermore, shallow positive buoyancy-induced asthenospheric flow is essential for reproducing the observed plate motion of the Pacific plate.

Interconnection of magma sources beneath the Katmai volcanic system inferred from seismic tomography and petrology

The Katmai volcanic cluster in Alaska hosts eight volcanoes, most of which are presently active, including Novarupta, whose eruption in 1912 is considered as the largest in the World during the 20th and 21st centuries. We present a new seismic tomographic model for the upper crust beneath the Katmai Group calculated with the use of ∼173,000 arrival times of the P and S waves from ∼11,000 local earthquakes. The resulting distribution of the Vp/Vs ratio reveals a common deep magma source for the entire system below the depth of 5 km centered in the area of Katmai Pass. The tomographic model also demonstrates interconnections of this source with most of the active volcanoes of the group. Based on this model, we propose a possible mechanism for the Novarupta eruption in 1912. The high-Vp/Vs anomaly beneath Novarupta at depths below 5 km may represent a trace of intrusion of low-density volatile-rich rhyolite magma that was made additionally buoyant by vesiculation. This hypothesis is supported by our petrological analysis showing that amphibole is a stable phase in 1912 rhyolite, which require a deeper source than previously suggested. This ascent was accompanied by draining of denser andesite and dacite magmas from under Mount Katmai, which caused the Katmai edifice to collapse. A shallow anomaly of low Vp/Vs ratio right below Novarupta may trace the conduit filled with pumice-type highly porous rocks remained after the eruption.

Deep Mantle Influence on the Cameroon Volcanic Line

AbstractThe origin of the Cameroon Volcanic Line (CVL), which is difficult to explain with traditional plate tectonics and mantle convection models because the volcanism does not display clear age progression, remains widely debated. Existing seismic tomography models show anomalously slow structure beneath the CVL, which some have interpreted to reflect upper mantle convective processes, possibly associated with edge‐driven flow related to the neighboring Congo Craton. However, mid‐ and lower mantle depths are generally not well resolved in these models, making it difficult to determine the extent of the anomalous CVL structure. Here, we present a new P‐wave velocity model for the African mantle, developed with the largest collection of travel‐time residuals recorded across the continent to date and an adaptive model parameterization. Our extensive data set and inversion method yield high resolution images of the mantle structure beneath western Africa, particularly at the critical mid‐ and lower mantle depths needed to further evaluate processes associated with the formation of the CVL. Our new model provides strong evidence for a connection between the African Large Low Velocity Province, centered in the lower mantle beneath southern Africa, and the continental portion of the CVL. We suggest that seismically slow material generated near the core‐mantle boundary beneath southern Africa moves northwestward under the Congo Craton. At the northern edge of the craton, the hot, buoyant material rises through the upper mantle, causing the CVL volcanism. Consequently, CVL magmatism can be linked to large‐scale mantle processes rooted in the deep mantle.

Constraining 3-D variations in mantle attenuation using normal modes: forward modelling and sensitivity tests

SUMMARY Robust estimates of the Earth’s temperature, composition and partial melt are difficult to obtain from seismic tomography models based only on wave velocities. Seismic attenuation is able to add crucial additional information and allows us to constrain 3-D variations in temperature as well as answer fundamental questions regarding the presence of water and partial melt within the mantle. A major problem with measuring attenuation is that scattering and focusing effects need to be included in order to distinguish between intrinsic attenuation (transformation of energy to heat) and scattering (redistribution of energy). Here, we will use whole Earth oscillations or normal modes, exploiting the fact that small scale scattering becomes less important at longer periods. In addition, focussing is implicitly included by simultaneously measuring the elastic and anelastic splitting function of a given normal mode. Normal mode data has rarely been used before to constrain 3-D variations in attenuation, hence we first need to assess the feasibility of using whole Earth oscillations to image 3-D variations in attenuation in Earth’s upper and lower mantle. Here, we use simple models in forward calculations to evaluate the sensitivity of normal modes to 3-D attenuation variations, and find observable differences in normal mode spectra when 3-D variations in mantle attenuation are included. We also test if 3-D attenuation variations can be recovered using splitting function measurements for synthetic normal mode spectra and investigate potential problems with unaccounted earthquake magnitude variations, noise and wide-band cross-coupling between modes. We find that we are able to recover input synthetic anelastic splitting functions, even when they are an order of magnitude smaller than the elastic splitting functions. Biases in the earthquake seismic moment solutions may lead to ‘ghost’ anelastic splitting functions being recovered, especially for high degree structure. However, this ‘ghost’ signal is only relevant if the size of the input anelastic splitting function is 50 per cent smaller than the scaled amplitude signal of expected upper mantle attenuation. Random noise and normal mode coupling also lead to spurious effects in the anelastic splitting functions. However, they manly affect high degree anelastic structure in a non-branch consistent manner. Thus, we are able to robustly recover low anelastic degree structure (smax ≤ 4) in all tested cases.

A naive Bayesian method to chase mantle plumes in global tomography models

SUMMARY This study provides a quantitative approach to search for mantle plumes in global seismic tomography models without any preconceived notions about the associated mantle velocity anomalies, other than the assumption that the plumes are not significantly deflected horizontally by more than 6°, anywhere in the mantle. We design identification tests with a reasonable detection threshold while keeping false alarms at a level lower than 5 per cent. This is based on a naive Bayesian clustering analysis, which is possible thanks to the varimax principal component analysis that provides components of the tomography models that are much more independent than the original number of depth slices in the models. We find that using such independent components greatly reduces detection errors compared to using an arbitrary number of depth slices due to correlations between the different slices. We detect a wide range of behaviour of the seismic velocity profiles underneath the hotspots investigated in this study. Moreover, we retrieve locations away from hotspots that have similar seismic velocity profile signatures to those underneath some hotspots. Hence, it is not possible to obtain a unique definition of seismic velocity anomalies that are associated with hotspots and thus care needs to be taken when searching for mantle plumes beneath hotspots using prior assumptions about the velocity anomalies that might be associated with them. On the other hand, we establish a probability distribution of the seismic velocity profiles that is specific to a sublist of hotspots. Overall, the mantle plume zones identified in our analysis do not appear to surround the Africa and Pacific large low shear velocity provinces (LLSVPs), but are rather within them. This rules out the idea that LLSVPs correspond to compact, dense piles with mantle plumes rising from their edges. Instead, our analysis suggests two possible options that either the LLSVPs: (1) correspond to bundles of thermochemical mantle plumes or (2) are made up of compact piles topped by a bundle of plumes.

Moho and LAB Across the Western Alps (Europe) From P and S Receiver Function Analysis

AbstractIn spite of numerous active and passive seismological investigations, the existence of continuous or interrupted continental subduction below the Western Alps is still open to debate. Many of the observations focus on the Moho or the deeper part of the mantle, while reliable information on the Lithosphere‐Asthenosphere Boundary (LAB) below the Alpine region is scarce. Exploiting the data from the dense, broadband AlpArray Seismic Network we present a set of Receiver Function (RF) measurements on the Moho and LAB of a region encompassing the Western Alps, which includes the Ivrea Geophysical Body (IGB), a fragment of mantle placed at a few kilometers depth at the collision margin between Eurasia and Adria plates. We derive seismic velocity profiles of the crust‐uppermost mantle below each station down to about 250 km, through the joint inversion of P and S RF. We constrain the lateral variations of the Moho and LAB topographies across the colliding plates, and quantify the errors related to our measurements. Our results allow us to considerably expand the published data of the Moho depth and to add a unique set of new measurements of the LAB. Our observations show that Eurasia and Adria lithospheres have a comparable thickness (on average 90–100 km), and are colliding below the IGB, and that Eurasia is not presently continuously subducting below Adria. These observations suggest that there is a gap between the superficial (continental) European lithosphere and the deep (oceanic) lithosphere, confirming the discontinuous structure imaged by some seismic tomography models.

The Crust and Upper Mantle of the East China Sea (Seismic Tomography and Gravity Models)

The Crust and Upper Mantle of the East China Sea (Seismic Tomography and Gravity Models)

Slab remnants beneath the Myanmar terrane evidencing double subduction of the Neo-Tethyan Ocean.

Closure of the Neo-Tethyan Ocean is one of the most significant tectonic events of the Cenozoic, forming the longest continental collision belt on Earth and influencing global climate and biodiversity. However, whether late Mesozoic subduction of the Neo-Tethyan Ocean occurred along one single or a double subduction system remains controversial. Here, upper mantle imaging from seismic tomography and waveform modeling in the Myanmar region reveals two prominent, parallel, slab-like structures with high seismic velocities that trend to the north-south and dip to the east. The western high-velocity zone has been observed previously and represents the modern subducting slab. The eastern zone has not been previously reported and exhibits high-velocity anomalies of 1.0 to 2.5% to a depth of ~300 km. This zone likely represents a remnant of another Neo-Tethyan oceanic slab that subducted ~40 million years ago. Double subduction of the Neo-Tethyan Ocean during the late Mesozoic to early Cenozoic requires reevaluation of previous tectonic models.

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.