3-dimensional modelling of the effects of near-surface ground structure in central Christchurch, New Zealand, on the passage of seismic waves

Abstract

To understand how near-surface ground structure affects patterns of 3-dimensional seismic wave amplification in simple sedimentary situations, wavefield modelling has been performed for the top 20 m of soft soil, gravel and sand beneath a 1 km x 1 km area of the central business district of Christchurch. Detailed information from over 100 well logs is available about soil types and layer thicknesses in this area, including results from penetration tests. Weakly consolidated material is restricted to the top 20m, with the 3 layers present there each varying in thickness by at least 10 m, though in general the rates of spatial variation in thickness are subdued. A model of the distribution of elastic properties in these layers has been developed from published values based on empirical relationships between penetration measurements and elastic moduli, and to demonstrate the sensitivity of the wavefield modelling to the distribution of S wave velocities, which vary from 100 m/s to 350 m/s, a second model of the distribution of elastic properties has been constructed by increasing the S wave velocities by 50 m/s.

In this first such study, the 3-dimensional wavefield modelling has been performed in the frequency domain, using the recently-developed Riccati equation approach. This calculates solutions for a set of incident waves from which solutions for other incident disturbances can be obtained by weighted superposition of these fundamental solutions.