Ground motion simulations for Dunedin and Mosgiel, Otago, New Zealand Thesis
- Author
- Kowal, A.F.
- Year
- 2022
- Publisher / Organisation
- University of Otago
- Pages
- 194
- Summary
- This research addresses the seismic hazards of the city of Dunedin and neighbouring town of Mosgiel, both located in the east of New Zealand’s Otago Province. Large (Mw ≥ 7) scenario earthquakes are developed for the local Akatore Fault (within 15 km of Dunedin and Mosgiel) and Hyde Fault (within 50 km) to provide ground-motion simulations for these two urban areas using two-dimensional basin and site response modelling. The simulations utilise numerical methods and models that explicitly incorporate the physics of the earthquake source and propagation of seismic waves in the areas of interest. The simulations use the Graves–Pitarka method, and the software platform used is the Southern California Earthquake Center Broadband Simulation Platform. A significant part of the research is dedicated to modelling site effects. Based on one-dimensional wave propagation theory, the large impedance contrast resulting from shallow soft soils overlying harder rocks may substantially amplify ground motions at high frequencies. Site response analyses are performed using the nonlinear finite element software OpenSees. The dynamic response characteristics of soft layers are simulated using a pressure-independent multi-yield plasticity model. The input ground motions are single components developed through the deconvolution of ground-motion simulations. They are modelled below the rock–soil interface and then convolved through nonlinear wave propagation site response analysis within the soil column. Two-dimensional ground-motion simulations are performed for two profile lines: the St Clair–St Kilda Beach line (the St Beach line), which runs east–west across South Dunedin, and the Taieri Basin line, which runs northwest–southeast through Mosgiel. The area surrounding the St Beach line has been the focus of recent field investigations into site response. The Taieri Basin line crosses the deepest fault-controlled sedimentary basin in East Otago. The simulations produce peak ground accelerations of up to 0.8 g for Akatore Fault earthquakes and 0.3 g for Hyde Fault earthquakes at sites positioned along the two profiles. These accelerations are well in excess of any earthquake shaking experienced in the 180-year history of Dunedin and well above the current New Zealand Loadings Standard values. Confidence in the simulation results is gained by undertaking historical and empirical ground-motion model (GMM)-based validation exercises. These comprise (1) simulations of the only instrumentally recorded earthquake of significance in the region (the Mw 4.7 2015 Lees Valley earthquake) compared with the recorded motions from that event and (2) simulations compared with empirical GMM-based ground-motion predictions for Akatore and Hyde fault earthquakes. Reasonable compatibility is demonstrated between the simulations and validation criteria for both (1) and (2), providing some confidence in the overall simulation methods developed in this thesis.