Thesis Defense: Wenpei Miao, Ph.D. Candidate
Department: Earth, Environmental and Planetary Sciences
Defense Date: April 14, 2022
Time: 1:00 p.m.
Location: KWGL 123
Constraining crustal velocity and anisotropic structures of basins and margins with surface wave and receiver function data
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 Coase 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 microcracks and foliated minerals due to long-lasting shear motion within the fault zone.