Ground motion simulations for the Dunedin - Mosgiel area

Abstract

The research addresses Dunedin-Mosgiel seismic hazard. We use scenario earthquakes from local active faults and 2D basin modelling to develop ground motion simulations for the two urban areas. The simulations utilise numerical methods and models that explicitly incorporate the physics of the earthquake source and the propagation of seismic waves in the areas of interest .The simulation method used is that of Graves and Pitarka, and the software platform used is the SCEC BBP. A major part of the research is dedicated to modelling site effects. Based on 1D wave propagation theory the large impedance contrast resulting from shallow soft soils overlying the harder rocks may cause strong amplification of ground motions at high frequencies. Site response analyses are performed with the nonlinear finite element software OpenSees. The dynamic response characteristics of soft layers are simulated by a pressure dependent multi yield plasticity model, developed specifically for modelling the behaviour of cohesionless soils. The input ground motions are single components developed by deconvolution of ground motion simulations, so they are modelled below the rock-soil interface, and then convolved via nonlinear wave propagation site response analysis within the soil column. Two profile lines are selected for 2D-based ground motion simulations, which represents a progression from the 1D analyses: The two lines are, namely: the Kettle Park line, which runs east-west across south Dunedin; and the Taieri basin line, which runs northwest-southeast through Mosgiel. The area of Kettle Park has been the focus of previous QuakeCoRE-funded field investigations of site response. The Taieri basin line crosses the deepest fault-controlled sedimentary basin in the greater Dunedin area.