Seismic Wave Modelling and Basin-Edge Effects in the Lower Hutt Valley, New Zealand

The study of seismic wave modelling and basin-edge effects is crucial for understanding earthquake dynamics, particularly in sedimentary basins. This section introduces the fundamental concepts and hypotheses guiding the research. The primary aim is to explore how geological and seismic conditions influence the amplification of seismic waves at basin edges. The research focuses on the Lower Hutt Valley, a region characterized by complex geological formations. The introduction sets the stage for a detailed examination of the mechanisms behind seismic wave propagation and the resultant effects on ground motion. It emphasizes the importance of understanding these phenomena for effective earthquake preparedness and risk mitigation. The findings are expected to contribute significantly to the field of civil engineering, particularly in designing structures that can withstand seismic events. As stated in the thesis, 'The aim of this thesis is to determine the geological and seismic conditions that control the occurrence and nature of basin-edge effects.' This highlights the research's relevance to both academic inquiry and practical applications in engineering.

A comprehensive review of existing literature on seismic response in sediment-filled basins is presented. This section categorizes previous studies into various themes, including multi-dimensional basin resonance and numerical modelling techniques. The literature indicates that basin-edge effects can significantly alter seismic wave behavior, leading to localized amplification. The review identifies gaps in current research, particularly regarding the specific conditions that lead to different types of edge effects. It discusses the significance of numerical studies on hypothetical basins and the analytical solutions available for SH waves at corners and wedges. The findings underscore the necessity for further investigation into the unique characteristics of the Lower Hutt Valley. The literature review serves as a foundation for the subsequent sections, providing context and justification for the research conducted in this thesis. Notably, it emphasizes that 'occurrences of localized amplification at the edge of the layer are categorized into three different classes based on their mechanisms of development.' This classification is vital for understanding the implications of seismic activity in urban planning and infrastructure development.

The methodology section outlines the computational methods employed in the research, focusing on finite element analysis and numerical techniques for investigating seismic wave propagation. The use of the Archimedes software for two-dimensional analysis is highlighted, along with the procedures for mesh generation and material damping. The section details the testing procedures and the analytical estimates of expected responses. It emphasizes the importance of rigorous testing to validate the models used in the study. The methodology is designed to ensure that the results are reliable and applicable to real-world scenarios. The findings from the simulations are crucial for understanding the seismic response of the Lower Hutt Valley. As noted, 'A high degree of similarity is found' between the modelling results and actual recorded ground motions, reinforcing the validity of the approach taken. This section is essential for establishing the credibility of the research and its potential applications in civil engineering.

The results section presents the findings from the finite element modelling of seismic waves at the edge of the Lower Hutt Valley. It discusses the observed basin-edge effects and their implications for ground motion during seismic events. The analysis reveals that Love waves generated at the edges are a primary cause of spatially varying ground motions. The discussion includes a comparison of modelling results with actual weak ground motions recorded in the valley, demonstrating a significant correlation. This section also addresses the implications of the findings for predicting seismic responses during strong ground shaking. The results contribute to a deeper understanding of how geological features influence seismic activity, which is critical for urban planning and disaster preparedness. The thesis concludes that 'the use of the two-dimensional elastic SH-wave analysis for predicting seismic response during strong ground shaking' is a valuable tool for engineers and planners. This highlights the practical applications of the research in enhancing community resilience to earthquakes.