Site Amplification, Polarity and Topographic Effects in the Port Hills During the 2010 - 2011 Canterbury Earthquake Sequence, New Zealand

Abstract:

Significant building damage and permanent ground displacement occurred in the southern Port Hills suburbs of Christchurch during the Canterbury earthquake sequence. Damage patterns indicate that local amplification of ground motions likely contributed to the most severe effects. The Canterbury earthquake sequence provides an internationally significant case study to understand the influence of topography and local stratigraphy on ground-motion amplification in hillside areas.
We present site-response analyses based on GeoNet national strong motion stations, as well as four small-scale temporary seismic arrays installed in the Port Hills following the 2011 February Christchurch earthquake. These stations are spread across narrow north-south trending volcanic spurs with a variety of topographic shapes. The spurs are also overlain, in part, by thick (up to 10m) loess deposits. Site amplification, polarization and dependence on source back-azimuth are assessed using H/V and site-to-reference spectral ratio methods applied to aftershocks of the Canterbury earthquake sequence. Results are also compared with detailed 2D modelling at key locations.
Consistent amplification peaks at hill-top locations at 1 – 3 Hz appear to be related to slope shape, with the estimated wavelength of amplification comparable to the ridge width at a given location. The strongest amplification at these frequencies generally occurs on top of narrow, steep-sided ridges. Our results imply that relatively low-velocity material comprising the eastern Port Hills has enhanced amplification in this area. At higher frequencies > 3 Hz, significant amplification is associated with local material contrasts or sharp local convex breaks in slope.
Our results show that both local topography and material contrasts strongly influence ground motion in the Port Hills. These observations have implications for slope stability studies and engineering design in hillside areas, given that significant amplification can occur over a broad frequency range at sites generally classed as rock according to the New Zealand design standards.