Abstract
This manuscript provides a critical examination of the ground motions recorded in the near-source region resulting from the 22 February 2011 Christchurch earthquake. Particular attention is given to reconciling the observed spatial distribution of ground motions in terms of physical phenomena related to source, path and site effects. The large number of near-source observed strong ground motions show clear evidence of: forward-directivity, basin generated surface waves, liquefaction and other significant nonlinear site response. The pseudo-acceleration response spectra (SA) amplitudes and significant duration of strong motions agree well with empirical prediction models, except at long vibration periods where the influence of basin-generated surface waves and nonlinear site response are significant and not adequately accounted for in empirical SA models. Pseudo-acceleration response spectra are also compared with those observed in the 4 September 2010 Darfield earthquake and routine design response spectra used in order to emphasise the amplitude of ground shaking and elucidate the importance of local geotechnical characteristics on surface ground motions. The characteristics of the observed vertical component accelerations are shown to be strongly dependent on source-to-site distance and are comparable with those from the 4 September 2010 Darfield earthquake, implying the large amplitudes observed are simply a result of many observations at close distances rather than a peculiar source effect.
Highlights
On 22 February 2011 at 12:51pm local time, a moment magnitude Mw6.3 earthquake occurred beneath the city of Christchurch, New Zealand, causing an unparalleled level of damage in the country’s history, and the largest number of causalities since the 1931 Hawkes Bay (Napier) earthquake
The 22 February 2011 Mw6.3 Christchurch earthquake imposed severe ground motion intensities, which were in excess of the current seismic design spectra and those experienced in the 4 September 2010 Darfield earthquake, over the majority of the Christchurch region
It was seen that forward directivity due to the rupture propagation was evident at Pages Road (PRPC), such effects were not predominant over the region due to the inferred misalignment between the rupture font and slip vector
Summary
This manuscript provides a critical examination of the ground motions recorded in the near-source region resulting from the 22 February 2011 Christchurch earthquake. The large number of near-source observed strong ground motions show clear evidence of: forward-directivity, basin generated surface waves, liquefaction and other significant nonlinear site response. The pseudo-acceleration response spectra (SA) amplitudes and significant duration of strong motions agree well with empirical prediction models, except at long vibration periods where the influence of basin-generated surface waves and nonlinear site response are significant and not adequately accounted for in empirical SA models. The characteristics of the observed vertical component accelerations are shown to be strongly dependent on source-to-site distance and are comparable with those from the 4 September 2010 Darfield earthquake, implying the large amplitudes observed are a result of many observations at close distances rather than a peculiar source effect
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