Two-dimensional site effects in Wellington and the Hutt Valley - similarities to Kobe

Abstract

Central Wellington and Lower Hutt City are both situated on soft alluvial basins bounded by stiff bedrock.  The basins tend to trap seismic energy, creating an amplified and longer duration response.  The potentially active Wellington Fault, which forms the western edge of the Hutt Valley and Wellington Harbour, represents a major seismic hazard to the area with a characteristic earthquake of magnitude 7.1 – 7.8 expected within the next few hundred years. 

Here we use a 2-D elastic finite-element method (FEM) to simulate the propagation of seismic SH waves through typical geological cross-sections of the Lower Hutt Valley and Te Aro Basin in Wellington. 

The results allow us to estimate the intensity and spatial distribution of ground motions during a major rupture of the Wellington Fault.  Strong low-frequency shaking, characterised by surface wave propagation and 2-D modes of resonance, is expected over the entire width of both the Lower Hutt Valley and the Te Aro Basin.  Amplification is both frequency- and positon- dependent, with peak values of Fourier spectral ratio (FSR) reaching a value of 12 in Lower Hutt, and 19 in Wellington.  Peak ground accelerations (PGA) vary between 0.8-1.3g in Lower Hutt, and 0.7-1.2g in Wellington.  Vertical resonance of shallow soft layers near the sides may yield FSR up to 14, and PGA up to 0.7g.  The aptly named basin-edge effect, speculated to be the cause of a belt of severe shaking during the 1995 Kobe earthquake, is observed in Lower Hutt simulations.  It produces strong pulses of energy on the seismogram trace as vertically incident body waves construct with horizontally propagating edge waves.  A maximum acceleration of 1.4g is predicted along a strip of land 70-200m away–from and parallel-to the Wellington Fault. Planners and engineers need to be aware of the risk of extra-strong shaking associated with particular resonant frequencies and locations within Wellington and the Lower Hutt Valley.