Sturgeon, William Howard; (2022) Using surface wave amplitude measurements to constrain elastic and anelastic Earth structure. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

When an earthquake ruptures, seismic waves propagate through the Earth. We can utilise information from seismic waves to image Earth structure, a process called seismic tomography. Traditional methods have focused on imaging Earth’s elastic structure, but a knowledge of the Earth’s anelastic structure allows for a more detailed mapping of variations in temperature, composition and potentially the presence of water. Anelasticity can be determined through the study of seismic attenuation, the energy loss experienced by seismic waves as they propagate, through grain-boundary friction. However, mapping anelasticity typically involves the analysis of seismic wave amplitudes, which are much more challenging to observe and model than other observables typically used in seismology. Here, we build a global 3-D tomography model of seismic attenuation of the Earth’s mantle, using innovative and novel data sets and modelling techniques, with the aim of further increasing the resolution of Earth structure and hopefully image new features. We utilise a huge global dataset of fundamental mode surface wave amplitude anomaly measurements (over 6,500,000 measurements). The data are modelled using a normal-mode formalism and then inverted using a Monte Carlo inversion method. We invert for shear-wave velocity structure in the crust of the western USA using data from the USArray, which are converted into local surface wave amplification measurements in the period range T∼ 38-114 s. It is the first time that surface wave amplification measurements are used alone to constrain crustal structure. We find that there are many similarities to other seismic studies of the region, but also some enhanced features. Similarities include a high-velocity anomaly beneath the Columbia Basin and Colorado Plateau, and a low-velocity anomaly beneath the Basin and Range province. On the other hand, unlike previous studies, the High-Lava Plains in south-eastern Oregon are highlighted as a low-velocity anomaly. We then use the global amplitude dataset to build frequency-dependent 2-D attenuation maps. We build maps at 18 wave periods from 38-275 s, providing sensitivity throughout the upper mantle. Similarly to previous global attenuation models, we observe low attenuation beneath the continents, particularly the stable continental cratons, and high attenuation beneath the oceans, particularly in the Pacific. Extensive synthetic testing allowed the model to be parameterised up to a maximum spherical harmonic degree of 20, which is higher than the most recent models that used a maximum degree of 16. As a result, we can image for example smaller cratons, including the Kalahari and Congo cratons in Africa and the North China craton. Finally, a 3-D depth-dependent global attenuation tomography model is built using the new 2-D attenuation maps along with a Monte Carlo inversion algorithm. It is the first time that a non-linear inversion algorithm is used in global attenuation studies, enabling uncertainty quantification. The resultant model, QsSF21, observe many similar features to previous attenuation models. Including a strong correlation of low attenuation beneath surface tectonics at ∼100 km depth and a broad high attenuation feature in the Pacific ocean, thought to be due to several thermal plumes. Interestingly, in the top ∼150 km, high attenuation is found beneath mid-ocean ridges, but this changes to low attenuation at greater depth, promting the need for further investigation.

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