Site amplification, polarity and topographic effects in the Port Hills during the Canterbury earthquake sequence
Authors: A E Kaiser, C Holden, C I Massey, GNS Sciences
Paper number: 3784 (EQC 12/626)
Abstract
Significant permanent ground displacement and severe building damage occurred in the southern Port Hills suburbs of Christchurch during the Canterbury earthquake sequence, most notably during the Mw 6.2 February and Mw 6.0 June 2011 earthquakes. High levels of building damage and/or slope cracking occurred on hill tops in the epicentral region, indicating 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 data from four small-scale temporary seismic arrays installed in the Port Hills following the 2011 February Christchurch earthquake. These arrays span narrow north-south trending volcanic spurs with a variety of topographic shapes. The spurs are also overlain, in part, by thick (up to 10m) soil (loess) deposits. Ground motion amplification and polarization is assessed at each seismometer location during aftershocks of the Canterbury earthquake sequence.
Our analyses of small to moderate magnitude earthquakes, show that amplification of ground motion in the 1–3 Hz frequency range appears to be related to slope shape, in particular the ridge width. The strongest amplification at these frequencies tends to occur on top of narrow, steep-sided ridges (e.g., Redcliffs). We speculate that such effects may have been important during the larger events of the Canterbury sequence.
Ground-motion amplification and polarization at frequencies > 3 Hz varies strongly over small distances (tens to hundreds of metres). In this frequency range, amplification is inferred to arise from local material properties (i.e., soil or weakened rock) and/or slope morphology (e.g., sharp convex breaks in slope).
At hillside locations, peak ground acceleration values (PGA) differed between neighbouring seismometers in the same array by up to 2.5 times (for horizontal motion) and up to 3 times (for vertical motion). Our results indicate that significant variation in horizontal and vertical PGA can occur across small distances at sites classed as rock, but the differences can vary significantly depending on the particular earthquake recorded.
Our results show that both local topography and local soil/rock properties 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 (NZS1170). Furthermore, much population and infrastructure in New Zealand is sited on hillside locations in areas with high seismic hazard (e.g., Wellington city).
Technical Abstract
Significant permanent ground displacement and severe building damage occurred in the southern Port Hills suburbs of Christchurch during the Canterbury earthquake sequence, most notably during the Mw 6.2 February and Mw 6.0 June 2011 earthquakes. High levels of building damage and/or slope cracking occurred on hill tops in the epicentral region, indicating 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 data from four small-scale temporary seismic arrays installed in the Port Hills following the 2011 February Christchurch earthquake. These arrays span generally 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 and polarization is assessed using H/V and site-to-reference spectral ratio methods applied to aftershocks of the Canterbury earthquake sequence. Earthquakes included in the analysis ranged in magnitude from M1.5–M5.2, hence results are based on weak-motion records
Our analyses of weak-motion scenarios, show that amplification peaks at 1–3 Hz appear to be related to slope shape. In particular, the estimated wavelength associated with this amplification is comparable to the ridge width at a given location. The strongest amplification at these frequencies generally occurs on top of narrow, steep-sided ridges (e.g., Redcliffs). We speculate that such effects may have been important during the larger events of the Canterbury sequence, during which recorded near-field ground motions were particularly rich in these frequencies.
Amplification and polarization at frequencies > 3 Hz show significant complexity over small distances (tens to hundreds of metres). In this frequency range, amplification is inferred to arise from local material impedance contrasts, and/or slope morphology, e.g., sharp convex breaks in slope.
At hillside locations, mean PGA ratios between individual array stations and a chosen reference station ranged up to a factor of 2.5 times and 3 times for horizontal and vertical motions respectively. However large inter-event variation was generally observed. Our results indicate that significant variation in horizontal and vertical PGA can occur across small distances at sites classed as rock and also from event to event. It is important to note that PGA amplification ratios derived from weak motions in this study may not be valid at stronger levels of shaking.
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 (NZS1170). Furthermore, much population and infrastructure in New Zealand is sited on hillside locations in areas with high seismic hazard (e.g., Wellington city).
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