Browsing by Author "Miao, Wenpei"

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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.
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    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.
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    Sedimentary and crustal structure of the US Gulf Coast revealed by Rayleigh wave and teleseismic P coda data with implications for continent rifting
    (Elsevier, 2022) Miao, Wenpei; Niu, Fenglin; Li, Guoliang; Levander, Alan
    We have developed an S-wave model of the south-central US focusing on the Gulf Coast sedimentary basin 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. The surface- and body-wave measurements were made, respectively, from ambient noise and teleseismic earthquakes recorded by 215 USArray stations in a rectangular area of 100°–87° west and 28°–37° north. We employed a cross-convolution function and H-κ analysis to better constrain sedimentary and Moho structure. We find that the southern edge of the Ouachita fold-and-thrust belt (OFTB) appears as a boundary in measured phase velocities, Z/H ratios, sediment basement depths, Moho depths, average crustal Vs and Vp/Vs ratios. The model shows southeastward thickening of the sedimentary basin, accompanied by thinning of the crystalline crust. The Moho gradient suggests that early rifting between North America and the Yucatan block commenced in a SE direction and involved most of the Pangea crust south of the OFTB boundary (i.e., Gondwana crust). We believe that a high velocity feature in the lowermost crust and upper mantle parallel to the southeast Texas coast was emplaced as a mafic body and is the source of the Houston magnetic anomaly. The seismic structures of the crust and uppermost mantle observed beneath the Mississippi Embayment and the Mississippi Valley Graben are consistent with plume induced Cretaceous uplift of the Mississippi Embayment as North America passed over the Bermuda hotspot.
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    Sharp Changes of Crustal Seismic Anisotropy Across the Central Tanlu Fault Zone in East China
    (Wiley, 2023) Miao, Wenpei; Niu, Fenglin; Chen, Haichao
    Both seismic and geodetic data suggested that the ∼120-km long Weifang segment of the Tanlu fault zone, a large-scale active strike-slip system at east China, is a seismic gap with no obvious along-strike shear motion at surface. Measuring crustal deformation around the segment is crucial to constrain stress/strain buildup and potential seismic risk at the fault. We measured 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 across the Weifang fault segment. 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° to ESE. The crustal anisotropy within the fault zone could be caused by aligned microcracks and foliated minerals due to long-lasting shear motion inside the fault zone.