Seismic full-waveform inversion of the crust-mantle structure beneath China and adjacent regions

Ma, Jincheng (2023): Seismic full-waveform inversion of the crust-mantle structure beneath China and adjacent regions. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

Since the late 1970s, seismic tomography has been emerging as the pre-eminent tool for imaging the Earth’s interior from the meter to the global scale. Significant recent advances in seismic data acquisition, high-performance computing, and modern numerical methods have drastically progressed tomographic methods. Today it is technically feasible to accurately simulate seismic wave propagation through realistically heterogeneous Earth models across a range of scales. When seismic waves propagate inside the Earth and encounter structural heterogeneities with a certain scale, wave propagation speed changes, reflection, and scattering phenomena occur, and interconversions between compressional and shear waves happen. The combined effect of multiple heterogeneities produces a highly complicated wavefield recorded in the form of three-component (vertical, radial, transverse) seismograms. The ultimate objective of seismic imaging is to utilize the full information from waveforms recorded at seismic stations distributed around the globe in a broad frequency range to characterize detailed tomographic images of Earth’s interior by fitting synthetic seismograms to recorded seismograms. The full-waveform inversion technique based on adjoint and spectral-element methods can be employed to maximumly exploit the information contained in these seismic wavefield complexities to determine the fine-scale structural heterogeneities from which they originated across various orders of magnitude in frequency and wavelength. Over the past two decades, the three-dimensional crust and mantle structure beneath the broad Asian region has attracted much attention in seismic studies due to its complicated and unique geological setting involving active lithospheric deformation, intracontinental rifting, intraplate seismotectonics, volcanism and magmatism, continent-continent collision, oceanic plate deep subduction, and mantle dynamics. To enhance our understanding of the subsurface behavior of cold subducting slabs and hot mantle flows and their dynamic relation to the tectonic evolution of the overriding plates as mentioned above, we construct the first-generation full-waveform tomographic model SinoScope 1.0 for the crust-mantle structure beneath China and adjacent regions. The three-component seismograms from 410 earthquakes recorded at 2,427 seismographic stations are employed in iterative gradient-based inversions for three successively broadened period bands of 70 - 120 s, 50 - 120 s, and 30 - 120 s. Synthetic seismograms were computed using GPU-accelerated spectral-element simulations of seismic wave propagation in 3-D anelastic models, and Fréchet derivatives were calculated based on an adjoint-state method facilitated by a checkpointing algorithm. The inversion involved 352 iterations, which required 18,600 wavefield simulations. The simulations were performed on the Xeon Platinum ‘SuperMUC-NG’ at the Leibniz Supercomputing Center and the Xeon E5 ‘Piz Daint’ at the Swiss National Supercomputing Center with a total cost of ~8 million CPU hours. SinoScope 1.0 is described in terms of isotropic P-wave velocity (Vp), horizontally and vertically polarized S-wave velocities (Vsh and Vsv), and mass density (ρ) variations, which are independently constrained with the same data set coupled with a stochastic L-BFGS quasi-Newton optimization scheme. It systematically reduced differences between observed and synthetic full-length seismograms. We performed a detailed resolution analysis by repairing input random parametric perturbations, indicating that resolution lengths can approach the half-propagated wavelength within the well-covered areas. SinoScope 1.0 exhibits strong lateral heterogeneities in the crust and uppermost mantle, and features correlate well with geological observations, such as sedimentary basins, Holocene volcanoes, Tibetan Plateau, Philippine Sea Plate, orogenic belts, and subduction zones. Estimating lithospheric thickness from seismic velocity reductions at depth reveals significant variations underneath the different tectonic units:~50 km in Northeast and North China, ~70 km in South China, ~90 km in the South China Sea, Philippine Sea Plate, and Caroline Plate, and 150-250 km in the Indian Plate. The thickest (200-250 km) cratonic roots are present beneath the Sichuan, Ordos, and Tarim basins. The large-scale lithospheric deformation is imaged as low-velocity zones from the Himalayan orogen to the Baikal rift system in central Asia. A thin horizontal layer of ~100-200 km depth extent below the lithosphere points toward the existence of the asthenosphere beneath East and Southeast Asia, with heterogeneous anisotropy indicative of channel flows. This provides independent, high-resolution evidence for the low-viscosity asthenosphere that partially decouples plates from mantle flow beneath and allows plate tectonics to work above. Beneath the Indo-Australian Plate, we observe distinct low-velocity anomalies from a depth of ~200 km to the bottom of the mantle transition zone (MTZ), continuously extending northward below western China from the lower MTZ down to the top of the lower mantle. Furthermore, we observe an enhanced image of well-known slabs along strongly curved subduction zones, including Kurile, Japan, Izu-Bonin, Mariana, Ryukyu, Philippines, Indonesia, and Burma. Broad high-velocity bodies persist from the lower MTZ to 1000 km depth or deeper beneath the north of the Indo-Australian Plate. They might be pieces of the ancient Tethyan slab sinking down to the lower mantle before the Indo-Australian Plate and Eurasian Plate collision. The deep geodynamic processes controlling the large-scale tectonic activity of the broad Asian region are very complicated and not yet well understood, which is a source of much debate. In this thesis, the main focus is on deciphering the three-dimensional seismic structure and dynamics of the lithosphere and mantle beneath China and adjacent regions with the help of the high-resolution full-waveform tomographic model. More importantly, in the subsequent works of geodynamic inversion, it provides the improved seismological basis for a sequential reconstruction of the late Mesozoic and Cenozoic plate motion history of the broad Asian region and the present-day mantle heterogeneity state estimates that, in turn, allow one to track material back in time from any given sampling location through retrodicting past mantle states.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Keywords: seismology, geodynamics, full-waveform inversion, lithosphere, mantle, anisotropy
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 22. Februar 2023
• 1. Bericht­erstatter:in: Bunge, Hans-Peter