R-3 Repository :: Browsing by Author "Niu, Fenglin"

Browsing by Author "Niu, Fenglin"

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3D Shear Velocity Structure of the Caribbean—Northwestern South America Subduction Zone From Ambient Noise and Ballistic Rayleigh Wave Tomography
(Wiley, 2024) Miao, Wenpei; Cornthwaite, John; Levander, Alan; Niu, Fenglin; Schmitz, Michael; Li, Guoliang; Dionicio, Viviana; Prieto, German
The Caribbean-South America subduction zone is a flat subduction zone, with Laramide-style thick-skinned uplifts occurring in the Merida Andes, Sierra de Perija Range, and Santa Marta Massif. Geodetic measurements and historical seismicity show this region is storing strain energy and is capable of a mega-thrust earthquake (M ≥ 8.0). Previous seismic investigations of the lithosphere and upper mantle in this area are either very large scale, very local, or only peripheral to this area; therefore, details of the Caribbean plate subduction geometry beneath the Maracaibo block remain unclear. In this study, we used a new data set acquired by the Caribbean-Merida Andes seismic experiment (CARMA), which comprised 65 temporary broadband stations and 44 permanent stations from the Colombian and Venezuelan national seismic networks. We jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8–30 s band and ballistic Rayleigh wave phase velocities in 30–80 s band to construct a 3-D S-wave velocity model in the area between 75°–65°W and 5°–12°N. The 3-D model reveals a general increase in crust thickness from the trench to the southeast. An anomalous area is the Lake Maracaibo, which is underlaid by the thinnest crystalline crust in the region. This observation may indicate that the Maracaibo block is experiencing a contortion deformation within the crust. We also identified a high velocity anomaly above the subducting Caribbean slab, likely representing a detached piece of eclogitized Caribbean large igneous province from the base of the Maracaibo block. Additionally, our Vs model clearly indicates a slab tear within the subducted Caribbean slab, approximately beneath the Oca-Ancon Fault.
A Displaced Lower Mantle Source of the Hainan Plume in South China Revealed by Receiver Function Imaging of the CEArray
(Wiley, 2024) Zhang, Yan; Niu, Fenglin; Ning, Jieyuan; Yu, Chunquan
We analyzed 49,592 teleseismic receiver functions (RFs) recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand the origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast-marching method and a high-resolution 3-D velocity model (FWEA18) derived from full waveform inversion in computing P-to-S conversion times to better image the 410- and 660-km discontinuities. Our results indicate that the common-conversion-point stacking of RFs using 3-D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410-km with a few small uplifted patches, and showed that the 660-km beneath the western Yangtze Craton is depressed by 10–25 km, which is likely caused by the stagnant Paleo-Pacific slab. The 660-km beneath the southern Cathaysia Block has a 5–15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ∼150 km and is located beneath the central Pearl River Mount Basin near Hong Kong. We speculate that the topographic low occurs within the Hainan plume with a temperature excess of ∼300–400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410-km. The southwest channel is likely the source that feeds the Hainan volcanoes.
A New Approach to the High-Resolution Linear Radon Transform based on Compressive Sensing Theory: Application on Teleseismic Wavefields
(2013-04-19) Aharchaou, Mehdi; Levander, Alan R.; Niu, Fenglin; Baraniuk, Richard G.
The development of new tools for high-resolution seismic imaging has been for many years one of the key challenges faced by earthquake and exploration seismologists. In order to make data amenable to imaging analysis, pre-processing steps are of great importance. This thesis proposes a new method for pre-processing teleseismic data based on the linear radon transform implemented according to compressive sensing theory – a novel theory about acquiring and recovering the sparsest signals (with minimum significant coefficients) in the most efficient way possible with the help of incoherent measurements. The LRT works by mapping data into a sparsity-promoting domain (called the radon domain) where the desired signals can be easily isolated, classiﬁed, ﬁltered and enhanced; and where noise can be attenuated or completely removed. The performance of the LRT is enhanced in terms of both high-resolution and computational cost by formulating the problem as an inverse problem in the frequency domain. This work shows that, unlike the common wisdom, irregularity in spatial sampling of teleseismic wavefields can be beneficial because it provides the incoherency needed to solve the compressive sensing problem and therefore recover the sparsest solutions in the radon domain. The inverse problem formulation yields the added advantage of automatic spatial interpolation and phase isolation after data reconstruction, and enables to regularize the problem by imposing sparsity constraint (instead of smoothness, which is the constraint usually adopted). We discuss and investigate the resolving power and applicability of convex and non-convex types of regularizers inspired from compressive sensing theory, and we establish a lower bound on the number of measurements needed to resolve certain time dips related to signals of interest within the data. We finish by applying the method to synthetic and recorded datasets and show how we do signal extraction, noise removal and spatial interpolation on teleseismic wavefields.
A novel and efficient engine for P-/S-wave-mode vector decomposition for vertical transverse isotropic elastic reverse time migration
(Society of Exploration Geophysicists, 2022) Zhang, Lele; Liu, Lu; Niu, Fenglin; Zuo, Jiahui; Shuai, Da; Jia, Wanli; Zhao, Yang
Wave-mode decomposition plays a very important role in elastic reverse time migration (ERTM). Improved imaging quality can be achieved due to reduced wave-mode crosstalk artifacts. The current state-of-the-art methods for anisotropic wavefield separation are based on either splitting model strategy, low-rank approximation, or lower-upper (LU) factorization. Most of these involve expensive matrix computation and Fourier transforms with strong model assumptions. Based on the anisotropic-Helmholtz (ani-Helmholtz) decomposition operator and decoupled formulations, we develop a novel and efficient P-/S-wave-mode vector decomposition method in vertical transverse isotropic (VTI) media with application in ERTM. We first review the basics of classical Helmholtz decomposition and isotropic decoupled formulations. In addition, the ani-Helmholtz decomposition operator is built from the P- and S-wave polarizations of the Christoffel equation in VTI media. We then derive novel decoupled formulations of anisotropic P-/S-waves based on the obtained ani-Helmholtz operator. Moreover, we use the first-order Taylor expansion to approximate the normalization term from the derived decoupled formulations and obtain an efficient ani-Helmholtz decomposition approach, which produces vector P- and S-wavefields with correct units, phases, and amplitudes. Compared with the previous studies, our approach mitigates model assumptions, avoids intricate calculations, such as LU factorization or low-rank approximation, and only needs three fast Fourier transforms at each time step. In addition, the graphic processing unit technique is used to dramatically accelerate various functions of ERTM, such as wavefields extrapolation, decomposition, and imaging. Three synthetic examples demonstrate the effectiveness and feasibility of our proposed approach.
Adjoint traveltime tomography unravels a scenario of horizontal mantle flow beneath the North China craton
(Springer Nature, 2021) Dong, Xingpeng; Yang, Dinghui; Niu, Fenglin; Liu, Shaolin; Tong, Ping
The North China craton (NCC) was dominated by tectonic extension from late Cretaceous to Cenozoic, yet seismic studies on the relationship between crust extension and lithospheric mantle deformation are scarce. Here we present a three dimensional radially anisotropic model of NCC derived from adjoint traveltime tomography to address this issue. We find a prominent low S-wave velocity anomaly at lithospheric mantle depths beneath the Taihang Mountains, which extends eastward with a gradually decreasing amplitude. The horizontally elongated low-velocity anomaly is also featured by a distinctive positive radial anisotropy (VSH > VSV). Combining geodetic and other seismic measurements, we speculate the presence of a horizontal mantle flow beneath central and eastern NCC, which led to the extension of the overlying crust. We suggest that the rollback of Western Pacific slab likely played a pivotal role in generating the horizontal mantle flow at lithospheric depth beneath the central and eastern NCC.
An explicit time evolution method for acoustic wave propagation
(Society of Exploration Geophysicists, 2014) Liu, Huafeng; Dai, Nanxun; Niu, Fenglin; Wu, Wei
Cost-effective waveform modeling is the key to practical reverse time migration (RTM) and full-waveform inversion (FWI) implementations. We evaluated an explicit time evolution (ETE) method to efficiently simulate wave propagation in acoustic media with high temporal accuracy. We started from the constant-density acoustic wave equation and obtained an analytical time-marching scheme in the wave number domain. We then formulated an ETE scheme in the time-space domain by introducing a cosine function approximation. Although the ETE operator appears to be similar to the second-order temporal finite-difference (FD) operator, the exact nature of the ETE formula ensures high accuracy in time. We further introduced a set of optimum stencils and coefficients by minimizing evolution errors in a least-squares sense. Our numerical tests indicated that ETE can achieve similar waveform accuracy as FD with four times larger time steps. Meanwhile, the compact ETE operator keeps the computation efficient. The efficiency and capability to handle complex velocity field make ETE an attractive engine in acoustic RTM and FWI.
Constraining crustal velocity and anisotropic structures of basins and margins with surface wave and receiver function data
(2022-04-22) Miao, Wenpei; Niu, Fenglin
My four projects focus different regions in the world, including the passive margin of the Gulf of Mexico, the northwestern South American-Caribbean subduction zone, the southeastern Tibetan region, and the Tanlu Fault zone. Using the surface wave data and receiver function data, I construct detailed shear velocity models and anisotropic structures of those regions. Moreover, my results help improve the understandings of regional tectonic evolutions and can also be used for future seismic hazard predictions. My first project focuses on the passive margin of Gulf of Mexico. We have developed an S-wave model of the south-central US focusing on the Gulf Coast sedimentary and its crust to understand continental rifting and regional tectonics. The model was derived by a joint inversion of Rayleigh wave phase velocities, Z/H ratios and P-coda data. Our constructed model shows very strong spatial correlation between surface tectonic units and velocity structure. Crustal thinning towards the coast is more obvious in the SE direction, which confirms the general view that continental rifting initiated in a NW-SE direction, and later shifted to the NNE-SSW direction as seafloor spreading started. We also observe a high velocity anomaly in the lower crust and the uppermost mantle beneath the southeast Texas coast that is coincident with the Houston magnetic anomaly, which provides evidence for mafic igneous intrusive rocks and suggests a magmatically active rift margin. My second project targets the northwestern South American-Caribbean subduction zone. The Caribbean plate subducts beneath northwestern South America at a shallow angle due to a large igneous province that added up to 12 km of buoyant crust. In this project, we jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8-30s band and ballistic Rayleigh wave phase velocities in 30-80s band to construct a 3D S-wave velocity model in the area from 75o-65o west and 5o-12o north. Our results show the overriding Maracaibo block is contorted by the subducted Caribbean plate and the South-American plate. The lowermost Maracaibo mantle lithosphere was displaced and accumulated on the subducted Caribbean slab surface during the flat-slab subduction, ceasing volcanisms. Our Vs model also provides clear evidence of a slab tear with the subducted Caribbean slab approximately below the Oca-Ancon fault. My third project focuses the southeastern margin of the Tibetan plateau, which formed during the convergence between the Eurasia plate and the India plate. Due to the complex tectonic evolution, the SE Tibetan region is one of the most seismic-active regions all over the world. Over the past two decades, two devastating earthquakes (the 2008 Mw 7.9 Wenchuan earthquake and the 2013 Mw 6.6 Lushan earthquake) occurred in the Longmenshan fault zone and caused huge loss of life and property. In depth knowledge of sedimentary structure within a basin is critical for understanding the tectonic evolution, estimating petroleum resources, and predicting strong ground motions that caused by earthquakes. In this project, we apply a recently developed method that uses the frequency-dependent apparent P-wave particle motion to investigate the basins’ sedimentary structure in the SE Tibetan region. We measured the particle motions of teleseismic P waves recorded by a total of 653 stations and found the apparent P-wave splitting (APS) times pattern in the SE Tibetan region is different from previous studies in the Songliao and Bohai bay basins and it might due to different tectonic evolution histories and sediment structures. We used a grid-search strategy to invert the sediment thickness (Z0) and surface S-wave velocity (β0). The estimated sediments thickness is consistent with surface geological settings, such as the Sichuan, Xichang, Chuxiong and Simao basins. Comparing to the ambient noise studies in the same region, our results show a deeper sediment thickness for basins like Sichuan, Xichang, Chuxiong and Simao basins. A more accurate sediment thickness model could greatly improve the predictions of ground motion and seismic hazards. My fourth project seeks to constrain the crustal and upper mantle seismic anisotropy using P-to-S converted waves at the Moho (Pms) and core-mantle boundary (SKS) recorded by broadband arrays deployed across the Weifang segment of the Tanlu fault zone. The Tanlu fault zone is the most prominent fault system in east China, an area with a large population and economy. One of the most devastating earthquakes in recent history of China, the M8+ Tancheng earthquake occurred in the central segment of this fault. We gathered Pms geographically and measured crustal seismic anisotropy using a joint analysis of radial and transverse receiver functions. The measured crustal anisotropy inside the fault zone shows a fast direction of ~NNE, parallel to the fault orientation. Right east to the fault zone, the fast axis rotates by almost 90 degree to ESE. Meanwhile, SKS splitting data showed a consistent ESE fast direction, parallel to the absolute plate motion direction, suggesting that it is likely caused by asthenospheric flow and the frozen flow fabric of the contemporary mantle lithosphere. The crustal anisotropy within the fault zone could be caused by aligned micro cracks and foliated minerals due to long-lasting shear motion within the fault zone.
Constraints on rigid zones and other distinct layers at the top of the outer core using CMB underside reflected PKKP waves
(Springer, 2012) Niu, Fenglin; Kelly, Cindi; Huang, Jianping
Crustal Anisotropy Beneath the Trans-North China Orogen and its Adjacent Areas From Receiver Functions
(Frontiers Media S.A., 2021) Xu, Xiaoming; Ding, Zhifeng; Li, Li; Niu, Fenglin
As an important segment of the North China Craton, the Trans-North China Orogen (TNCO) has experienced strong tectonic deformation and magmatic activities since the Cenozoic and is characterized by significant seismicity. To understand the mechanism of the crustal deformation and seismic hazards, we determined the crustal thickness (H), Vp/Vs ratio (κ) and crustal anisotropy (the fast polarization direction φ and splitting time τ) beneath the TNCO and its adjacent areas by analyzing receiver function data recorded by a dense seismic array. The (H, κ) and (φ, τ) at a total of 309 stations were measured, respectively. The Moho depth varies from ∼30 km beneath the western margin of the Bohai bay basin to the maximum value of ∼48 km beneath the northern Lüliang Mountain, which shows the positive and negative correlations with the elevation and the Bouguer anomaly. The average φ is roughly parallel to the strikes of the faults, grabens and Mountains in this study area, whereas a rotating distribution is shown around the Datong-Hannuoba volcanic regions. Based on the φ measured from the Moho Ps and SKS/SKKS phases, we propose that the crustal deformation and seismic hazards beneath the TNCO could be due to the counterclockwise rotation of the Ordos block driven by the far-field effects of the India-Eurasian collision.
Crustal structure and deformation beneath the NE margin of the Tibetan plateau constrained by teleseismic receiver function data
(Oxford University Press, 2016) Wang, Qiong; Niu, Fenglin; Gao, Yuan; Chen, Yuntai
We analysed a large amount of teleseismic receiver function data recorded by 172 broadband stations in the NE Tibetan plateau and its surrounding areas to investigate the crustal velocity and anisotropy structure beneath the margin. We first applied the modified H–κ stacking technique to measure the crustal thickness and average Vp/Vs ratio, and then employed a joint inversion scheme to measure azimuthal anisotropy of the crust beneath each station. The observed crustal thickness and Vp/Vs ratio exhibit large variations across the study area, varying from 32 to 75.6 km and from 1.601 to 1.864, respectively. We also found significant azimuthal anisotropy within the crust beneath 12 stations, with a splitting time between 0.36 and 1.06s. The fast polarization directions align well with surface structures, and follow the directions of the maximum horizontal tensile stress. The low Vp/Vs ratio and the strong azimuthal anisotropy observed beneath the margin suggest that whole crustal shortening might be the dominant mechanism for producing the thick crust in NE Tibet. We compared the measured seismic anisotropy with those measured from XKS (SKS, PKS and SKKS), and found that crustal anisotropy appears to play an important role in explaining the amount of XKS splitting times. More importantly, the Moho Ps and the XKS share similar fast polarization directions, suggesting a vertically coherent deformation within the lithosphere beneath the margin.
Data driven and machine learning based methods for Coulomb failure stress calculation, velocity model evaluation and receiver function selection
(2022-04-22) Xiong, Neng; Niu, Fenglin
In recent years, machine learning has become a very popular field of study in geophysics and seismology. As a data intensive field, seismology provides a great opportunity for the development of data driven method. Machine learning, with its great ability to find and extract hidden information from data, gives a new perspective to solve existing problems in seismology in a statistical way. Utilizing the great power of data science, my projects create data driven and machine learning based methods to tackle existing problems in Coulomb stress calculation, velocity model evaluation, and receiver function selection, in the purpose of providing more precise, robust, and efficient solutions. My first project aims at providing a more accurate estimation of Coulomb failure stress changes (ΔCFS) in regions with poor understanding of the structure and seismic station coverage. Applied to the 2015 Mw 7.8 Gorkha earthquake, my designed method uses ΔCFS calculated from aftershocks, with known moment tensor solutions, as true value, to constrain the predicted value of maximum Coulomb failure stress changes occurred with the optimum focal mechanisms and provide a better ΔCFS estimation for areas with no receiver fault information. Based on the calculation result, this study confirm that a detailed rupture model, that fits the realistic geometry of the fault, is crucial to ΔCFS calculation of thrust fault earthquakes. My result also indicates a significant stress accumulation in the unruptured area south and west to the Gorkha earthquake. Next, my thesis shifts the focus to traditional machine learning method, using unsupervised clustering analysis to tackle the problem of velocity model evaluation. Comparing to the forward 3-D waveform simulation, which is often used for velocity model validation, my second project provides a much more efficient solution using K-means clustering and applies to two velocity models in Southern California (CVM-H15.1 and CVM-S4.26). This is done by first calculating synthetic surface wave velocity dispersion curves input velocity models, and then clustering the synthetic and observed velocity dispersion curves independently into certain number of groups. The velocity model is rated by estimating the similarity between spatial patterns obtained from the synthetic and observed dispersion data. Comparing to the forward 3-D waveform simulation, this new evaluation scheme is extremely efficient and no longer limited by the source-receiver configuration. My studies also suggests that the CVM-S4.26 fits the observed dispersion maps better than the CVM-H15.1 in term of the clustering pattern. Last, my thesis targets on solving the problem of receiver function (RF) selection. Traditional human handpicked receiver function lacks established picking criteria and requires significant amount of time. My third project incorporates the feature engineering process to extract meaningful features from the RF data sets, in the purpose of understanding what are the characteristics that separate the good and bad RFs. Build on the selected features, I apply the fuzzy C-means clustering method to automatically picking the RFs and achieves a high F1 score of ~90% for the testing data sets. My study identifies 4 features with great separation between good and bad RFs, which can be used as the guide metrics for RF selection. The clustering based classifier could further eliminate the need for human picking, saving significant amount of time for large seismic studies.
Detection of the low velocity layer atop the 410-km discontinuity beneath Northeast China with Slowness based CCP stacking
(2016-04-15) Pang, Xiaojiao; Niu, Fenglin
The structure and tectonics of Northeast China are dominantly affected by the interactions of Pacific plate and Eurasia plate. The mantle transition zone (MTZ) structures are also influenced due to this interaction. The importance of the MTZ in advancing understanding of mantle dynamics is highlighted by the Transition Zone Water Filter (TZWF) model, which predicts the existence of a partial-melt layer atop ‘410’, hereafter referred to as 410-LVL (Low Velocity Layer). In recent years, efforts to investigate the 410-LVL are accelerating. Regardless of the veracity of the TZWF model, it is clear that a more complex role for the MTZ in thermochemical mantle convection is emerging and that higher resolution seismic mapping of lateral variations in mantle layering will provide valuable constraints on the thermal and chemical processes active in the MTZ. The presence of a global layer of partial melt above the ‘410’ discontinuity would modify material circulation in the Earth mantle and may help to reconcile geophysical and geochemical observations. We collect the data in Northeast China mainly from NECESSArray and CEArray. By first screening the raw data and then generating the receiver functions from the selected data, we are able to use the common conversion point (CCP) method to do the stacking of the receiver functions. After the stacking, the common features will emerge. However, we are not clear whether some of the features are real structures or processing artifacts. We introduce the slowness based CCP stacking method to further confirm the existence of 410-LVL. With additional support from the statistical analysis of the results, we are able to finalize the detections of the 410-LVL. The LVL is not related to a particular type of geodynamical environment atop the ‘410’: it is found globally with various geological settings. The NE China area is dominantly affected by the subduction of Pacific plate under Eurasia plate. The stagnant slab has the ability to add a large amount of subducted fluid into the mantle. Slab dehydration creates the 410-LVL, along with the potential for subsequent triggering of wet mantle upwelling.
Dynamics and Evolution of Solar Eruptive Prominences
(2014-04-24) Zhu, Chunming; Alexander, David; Toffoletto, Frank R.; Niu, Fenglin
Solar eruptive phenomena, including eruptive solar prominences/filaments, solar flares and Coronal Mass Ejections (CMEs), have severe impact on the Earth’s space environment and human activities: so-called Space Weather. The dynamics and evolution of the prominences/filaments are important for our understanding of the initiation processes that drive CMEs and lead to drastic energy release in the solar flares. This thesis focuses on recent progresses on the destabilization and subsequent eruption of the prominences/filaments, via three primary case studies that elucidate the most important activities occurring in the eruptive prominence: eruption of a bifurcated solar filament, interchange reconnection facilitating a filament eruption, and the interaction of two distinct filaments with subsequent production of solar flares. In Chapter 2, we study a partial eruption of a bifurcated filament which exhibited clear and strong kinking motion of the filament axis (∼ 120◦ rotation). Seven mass transfer events are identified and are thought to also transfer magnetic flux from the lower to upper branch, leading to the generation of ideal instabilities, that subsequently triggered the eruption of the upper branch. In Chapter 3, we present evidence of interchange reconnection driven by the interaction of an erupting filament with a nearby coronal hole that leads to the eruption of this filament. Kinking motions in this filament serves to bring the magnetic field of its eastern leg in close contact with the unipolar magnetic field of the coronal hole where it drives the reconnection that governs the subsequent evolution of the fila- ment and coronal hole boundary. The observed EUV brightenings and bi-directional flows in the contact layer formed by this interaction, along with the occurrence of type III radio bursts that are strongly related to escaping electrons along open fields, provide corroborative evidence for the occurrence of reconnection at this location. A consequence of this interaction was the development of a complex CME, that displayed both open and closed features: we believe this is the first time such a CME configuration has been observed directly in association with a filament eruption. In Chapter 4, an interaction between two filaments, rarely reported before, is identified and studied. This complex interaction is responsible for the production of a hard x-ray coronal source as part of a C3.0 class solar flare. The observed hard x-ray coronal source occurring between the two filaments, driven by a convergence of the filaments, and a newly formed hot plasma layer, indicate that magnetic reconnection occurred between the magnetic fields associated with both filaments. The eruption of the filaments later led to the onset of a much larger solar flare, class M2.9, as expected from the standard flare model. It is interesting to note that both loop shrinkage and supra-arcade downflows (SADs) are present during this M2.9 flare.
Effects of shallow density structure on the inversion for crustal shear wave speeds in surface wave tomography
(2016-12-01) Xing, Guangchi; Niu, Fenglin
Surface wave tomography routinely uses empirically scaled density model in the inversion of dispersion curves for shear wave speeds of the crust and uppermost mantle. An improperly selected empirical scaling relationship between density and shear wave speed can lead to unrealistic density models beneath certain tectonic formations such as sedimentary basins. Taking the Sichuan basin east to the Tibetan plateau as an example, we investigate the differences between density profiles calculated from four scaling methods and their effects on Rayleigh wave phase velocities. Analytical equations for 1-D layered models and adjoint tomography for 3-D models are used to examine the trade-off between density and S-wave velocity structures at different depth ranges. We demonstrate that shallow density structure can significantly influence phase velocities at short periods, and thereby affect the shear wave speed inversion from phase velocity data. In particular, a deviation of 25 per cent in the initial density model can introduce an error up to 5 per cent in the inverted shear velocity at middle and lower crustal depths. Therefore one must pay enough attention in choosing a proper velocity-density scaling relationship in constructing initial density model in Rayleigh wave inversion for crustal shear velocity structure.
Estimating sedimentary and crustal structure using wavefield continuation: theory, techniques and applications
(Oxford University Press, 2014) Tao, Kai; Liu, Tianze; Ning, Jieyuan; Niu, Fenglin
Receiver function techniques are widely used in imaging crustal and mantle structure beneath a seismic station. The weak P-to-S conversions at deep seismic structures are usually masked by strong shallow reverberations when unconsolidated sediments are present below the station, making it nearly impossible to utilize receiver function techniques. We develop a method to estimate sediment and crustal structures beneath a seismic station based on wavefield downward continuation and decomposition method. The method parametrizes velocity structure beneath the station with a stack of constant velocity layers overlying a homogeneous half-space, and approximates the teleseismic P wave and its coda by the structural response to an incoming plane P wave. Our method is based on the principle that the upgoing S wavefield is absent in the half-space, and searches for the optimum velocity and thickness of the layers that give the minimum S-wave energy flux from the half-space to the layers. An iterative grid-search algorithm from the top to the bottom layers is employed to implement the search. In this study, we only use models comprising either only one crust layer or two layers (sediment+crust) with a half-space mantle, although models with more layers are also implementable. The method is especially useful in resolving seismic structure beneath a station sitting on unconsolidated sediments. It not only can be used to determine the sediment thickness and velocity structure, but also provides an effective way to generate subsurface receiver functions, which are formed by deconvolving the upgoing P wavefield from the upgoing S waves at the top of hardrock crust, and thus are free from shallow reverberations. The technique is applied to various synthetic data generated with different types of velocity model and noise levels, and appears to have good capability in recovering the input models. We further applied this method to teleseismic data recorded at a station inside the Songliao Basin in northeast China. The estimated sediment thickness and velocity agrees well with the results of previous activesource studies. The subsurface receiver functions also show a superior power in exposing the Moho Ps conversions, resulting in a well-defined peak in the H-? domain, which are absent in the regular receiver function data.
Fluid Controls on the Heterogeneous Seismic Characteristics of the Cascadia Margin
(Wiley, 2018) Delph, Jonathan R.; Levander, Alan; Niu, Fenglin
The dehydration of oceanic slabs during subduction is mainly thermally controlled and is often expressed as intermediate‐depth seismicity. In warm subduction zones, shallow dehydration can also lead to the buildup of pore‐fluid pressure near the plate interface, resulting in nonvolcanic tremor. Along the Cascadia margin, tremor density and intermediate‐depth seismicity correlate but vary significantly from south to north despite little variation in the thermal structure of the Juan de Fuca Plate. Along the northern and southern Cascadia margin, intermediate‐depth seismicity likely corresponds to increased fluid flux, while increased tremor density may result from fluid infiltration into thick underthrust metasediments characterized by very slow shear wave velocities (<3.2 km/s). In central Cascadia, low intermediate‐depth seismicity and tremor density may indicate a lower fluid flux, and shear wave velocities indicate that the Siletzia terrane extends to the plate interface. These results indicate that the presence of thick underthrust sediments is associated with increased tremor occurrence.
Full-waveform inversion of triplicated data using a normalized-correlation-coefficient-based misfit function
(Oxford University Press, 2017) Tao, Kai; Grand, Stephen P.; Niu, Fenglin
In seismic full-waveform inversion (FWI), the choice of misfit function determines what information in data is used and ultimately affects the resolution of the inverted images of the Earth's structure. Misfit functions based on traveltime have been successfully applied in global and regional tomographic studies. However, wave propagation through the upper mantle results in multiple phases arriving at a given receiver in a narrow time interval resulting in complicated waveforms that evolve with distance. To extract waveform information as well as traveltime, we use a misfit function based on the normalized correlation coefficient (CC). This misfit function is able to capture the waveform complexities in both phase and relative amplitude within the measurement window. It is also insensitive to absolute amplitude differences between modeled and recorded data, which avoids problems due to uncertainties in source magnitude, radiation pattern, receiver site effects or even miscalibrated instruments. These features make the misfit function based on normalized CC a good candidate to achieve high-resolution images of complex geological structures when interfering phases coexist in the measurement window, such as triplication waveforms. From synthetic tests, we show the advantages of this misfit function over the cross-correlation traveltime misfit function. Preliminary inversion of data from an earthquake in Northeast China images a sharper and stronger amplitude slab stagnant in the middle of the transition zone than FWI of cross-correlation traveltime.
Hydraulic Injection‐Induced Velocity Changes Revealed by Surface Wave Coda and Polarization Data at a Shale Play Site in Southwest China
(Wiley, 2020) Zhang, Yan; Niu, Fenglin; Tao, Kai; Ning, Jieyuan; Chen, Haichao; Tang, Youcai
We investigated temporal variations of seismic wave velocity associated with hydraulic fracturing using Green's functions computed from ambient noise data. In October and November of 2014, we set up a broadband array at a shale play site inside the Sichuan basin where a pilot horizontal drilling and hydraulic injections were conducted. We first computed cross‐correlation functions using continuous data recorded by 21 three‐component broadband sensors deployed around the treatment well. We then employed a running window correlation‐based coda wave interferometry technique to measure apparent velocity changes from the daily Green's functions of all the station pairs in the frequency range of 1 to 3 Hz. We found significant velocity changes right after the hydraulic fracturing, which exhibited a clear direction‐dependent pattern. S wave velocity along raypaths parallel to the well trajectory showed a clear increase while those perpendicular exhibited a small decrease. The anisotropic changes in seismic velocity observed here were also confirmed from surface wave horizontal particle motion data. By comparing our observations with normal stress changes calculated with a half‐space elastic model, we speculate that stress changes induced by the hydraulic fracturing were likely to be responsible for the observed anisotropic changes in seismic velocity. Our results suggest that time‐lapse seismic imaging with ambient noise data provides a promising probe for monitoring geomechanical changes related to exploitation of unconventional oil and gas resources.
Imaging the Pacific Plate and transition zone beneath eastern Asia with receiver functions
(2012) Wang, Xinling; Niu, Fenglin
In this thesis, I applied receiver function techniques to the data recorded by national and regional broadband seismic networks of the China Earthquake Administration to image the mantle transition-zone structure beneath eastern Asia. I analyzed a total of 37427 receiver-function data. Significant topography on both the 410-km and 660-km discontinuities was clearly imaged in the 3D volume of CCP stacked images that cover an area of 102.5°-122.5°E and 22.0°-42.°N. 3D crustal and mantle velocity models were used in computing the Ps time move-outs to better image the absolute depths of the two discontinuities. The 660-km discontinuity is depressed up to 25 km along the east coast of China, indicating the presence of the subducted Pacific slab in the region. A double 660-km was observed beneath the Yellow Sea. The 410-km is depressed by as much as 15 km beneath the Quaternary Datong volcano located at the northeastern edge of the Ordos plateau in north China.
Improving upper-mantle receiver function imaging with slowness weighted stacking and fast-marching based 3D Pds traveltime
(2018-04-19) Guan, Zhe; Niu, Fenglin
Common-conversion-point (CCP) stacking of receiver functions is a widely used technique to image velocity discontinuities in the mantle. The CCP imaging technique assumes that receiver functions are composed solely of P to S conversions at velocity boundaries, whose depths can be mapped out through their arrival times. The multiple reflections at shallow boundaries with large velocity contrasts, such as the base of unconsolidated sediments and the Moho, can lead to artificial structures in the CCP images. We develop a refined CCP stacking method that uses relative slowness as a weighting factor to suppress the multiples (slowness weighted CCP stacking; SWCCP). We conduct extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images. We apply this technique to receiver function data of NECESSArray in China and the transportable array in western US, and find most of the events in the depth range of 200-400 km shown in the regular CCP images are eliminated. The SWCCP images, on the other hand, reveal a clear negative event under some parts of the two arrays, indicating the presence of low velocity layer above the 410-km discontinuity, which was reported by previous studies. In CCP stacking of receiver functions, most of the current studies computed 3D relative Pds traveltime corrections by integrating traveltime anomalies along 1D ray paths. This ray-tracing approach is generally time consuming and less accurate when prominent velocity anomalies exist and effects of the 3D ray paths become significant. In this study we introduce a new scheme that utilizes the fast-marching method eikonal solver to improve both the efficiency and accuracy of 3D Pds traveltime computation. We first employ a 1D ray tracing method and the iasp91 model to calibrate the accuracy of the new scheme and optimize the parameters of the numerical solver. We then apply the new scheme to compute a massive number of Pds traveltimes using two 3D synthetic models, one with a high-velocity slab and another one with a low-velocity plume, and compare these 3D traveltimes with those computed with the ray-tracing approach. We found 3% ray paths in the slab model and 12% ray paths in the plume model show a 3D traveltime difference of more than 0.5s. We apply the proposed scheme to the TA receiver functions that sample the transition zone structure beneath the Yellowstone hotspot and find that CCP stack using 3D Pds traveltimes computed by eikonal solver has better focused P660s than that by the ray-tracing method. Finally we illustrate that the computational times can be reduced by 1 to 2 orders of magnitude with the new scheme to compute the 3D Pds traveltimes of 20,000-200,000 receiver functions.

Browsing by Author "Niu, Fenglin"

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