Calibration of Wellington 3D ground shaking model

Abstract

This thesis calibrates the Hutt Valley section of the previously developed Wellington 3D ground shaking model, using weak motion records from seven earthquakes recorded during the Lower Hutt deployment of seismometers. The Hutt Valley section is approximately one-third of the whole 3D model, but exhibits the highest shaking for a presumed Wellington Fault earthquake. We use 24 weak motion sites in Lower Hutt. The sites sample the full range of soil types and depths in the region, from bedrock to thick soft sediments.

In this research, the focal mechanisms of the 15 events recorded by the four portable deployments are solved by the combined amplitude ratio and first motion method, using all the available data from New Zealand Statement Network (NZSN), SNZO, and the portable deployment.

A new method named the 1D + 3D hybrid modelling technique was developed to simulate the ground motion in Hutt Valley to compare with the recorded ground shaking data from the Lower Hutt portable deployment. The method combines 1D modelling between the source and the bottom of the valley with 3D modelling from the valley bottom to the surface.

The discrete wavenumber (DWN) method and general reflection and transmission coefficient matrices are used with a 1D velocity model to calculate the stress and velocity wavefield at the bottom of the Hutt Valley sediments. A double couple, point source model with a modulated ramp time function is used as the earthquake focus in the 1D modelling. The finite difference (FD) scheme is then used with the 2D Hutt Valley shaking hazard model to calculate the velocity wavefield at the free surface of the Hutt area, using the time domain stress and velocity wavefield at the bottom of Hutt Valley sediments as input. Stress and velocity synthetics are determined at each 40 m of the grid. To compare with the observed seismogram data in the Lower Hutt deployment, the points within the model corresponding to the recording sites were selected and the velocity time series for those sites were calculated.