Browsing by Author "Ning, Jieyuan"

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    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.
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    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.
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    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.
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    Lg attenuation in northeast China using NECESSArray data
    (Oxford University Press on behalf of The Royal Astronomical Society, 2015) Ranasinghe, Nishath R.; Gallegos, Andrea C.; Trujillo, Andrea R.; Blanchette, Alexander R.; Sandvol, Eric A.; Ni, James; Hearn, Thomas M.; Tang, Youcai; Grand, Stephen P.; Niu, Fenglin; Chen, Yongshun J.; Ning, Jieyuan; Kawakatsu, Hitoshi; Tanaka, Satoru; Obayashi, Masayuki
    The 127 station NorthEast China Extended SeiSmic Array (NECESSArray) provides large quantities of high quality seismic data in northeast China that allow us to resolve lateral variations of Lg Q or crustal attenuation at 1 Hz (Qo) to 2.0° or greater. Using the reverse two-station/event method with 11 642 Lg path-amplitudes from 78 crustal earthquakes, we obtain a 2-D tomographic image of Lg Qo with values ranging from ∼50 to 1400. A high degree of detail in the lateral variation of Lg attenuation is revealed in our tomographic image. High Qo regions are found in the Great Xing'an, Lesser Xing'an and Songen-Zhangguangcai Ranges. Low Qo regions are observed in the Songliao, Sanjiang and Erlian Basins. The lowest Qo is found near the Wudalianchi volcanic field and other Quaternary volcanic fields, the southern Songliao Basin, the western edge of the Erlian Basin and the Sanjiang Basin. Low Qo values are measured for paths that cross sedimentary basins with thick, unconsolidated sediments. Most of the high Lg attenuation in the Songliao Basin correlates reasonably well with low crustal Rayleigh wave phase velocity anomalies. The highest attenuating regions also correlate well with regions of Holocene volcanism..
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    Upper mantle tomography in the northwestern Pacific region using triplicated P waves
    (Wiley, 2014) Takeuchi, Nozomu; Kawakatsu, Hitoshi; Tanaka, Satoru; Obayashi, Masayuki; Chen, Y. John; Ning, Jieyuan; Grand, Steve P.; Niu, Fenglin; Ni, James; Iritani, Rryohei; Idehara, Koki; Tonegawa, Takashi
    We conducted delay time tomography of the upper mantle beneath the northwestern Pacific using P data from NorthEast China Extended SeiSmic Array, F-net, and nearby available stations. To improve resolution and accuracy in the vicinity of mantle discontinuities, we extracted traveltimes of both initial and secondary phases observed at triplication distances by using a waveform fitting technique. Compared with the model obtained by using only the initial phase, the resolution just above the 410 km discontinuity is especially improved, and low-velocity anomalies beneath the Changbai Volcano are clearly observed down to the 410 km discontinuity. Compared with previous models, low-velocity anomalies atop the 410 are more pronounced. The results of this study together with the previous receiver function analysis provide further support that we have hot material beneath the Changbai Volcano