Full Seismic Waveform Tomography of the Japan region using Adjoint Methods
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This thesis presents a preliminary full-waveform tomographic model of the Japan region based on spectral element wave propagation, adjoint techniques and seismic data from dense station networks. The shallow Earth structure of the Japan region has been the subject of con- siderable tomographic investigation. The islands of Japan exist in an area of significant plate complexity: subduction related to the Pacific and Philippine Sea plates is responsible for the majority of seismicity and volcanism in Japan, whilst smaller micro-plates contribute significant local intricacy. In response to the need to monitor and understand the motion of these plates and their associated faults, numerous seismograph networks have been established. This study utilises the 84 station F-net network of Japan, and the 55 station BATS network of Taiwan. This exceptional coverage is used to construct a 3-dimensional model of the Japan region from the full-waveform inversion of over 15,000 individual component seismograms, from 51 events that occurred between 1997 and 2012. Data is modelled using spectral-element simulations of seismic wave propagation at a regional scale over an area from 120◦ −150◦E and 20◦ −50◦N to a depth of around 500 km. The differences between observed and synthetic waveforms are quantified using instantaneous phase misfits allowing for the separation of both phase and amplitude measurements whilst exploiting the complete waveform at periods of 50 − 100 seconds. Fr ́echet kernels for these misfits are calculated via the adjoint method and subsequently used in an iterative non-linear conjugate-gradient optimisation scheme. Finally, custom smoothing algorithms are employed to remove the singularities of the Fr ́echet kernels, and artefacts introduced by the heterogeneous coverage in oceanic regions of the model. The resulting S-velocity model has limited resolving capability below a depth of 200 km, but is the most geographically extensive model of this region to date and represents a significant improvement over currently available surface-wave models. The sensitivity distribution of the data to Earth structure is investigated and the model compared to the 3-dimensional slab model, Slab1.0. The model provides a solid foundation on which to construct a future generation of tomographic models of this region. The model is eventually intended to further our understanding of both the complex regional tectonics and the finite rupture processes of large earthquakes.