Near-source Strong Ground Motions Observed in the 22 February 2011 Christchurch Earthquake

Abstract

SUMMARYThis 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.

Changes in hot spring temperature and hydrogeology of the Alpine Fault hanging wall, New Zealand, induced by distal South Island earthquakes

“…However, the close proximity of the Mw6.3 earthquake to the city of Christchurch, the shallow source and exceptionally strong vertical ground motions resulted in high levels of damage and human casualties (e.g. Bradley & Cubrinovski 2011;Fry et al 2010;Orense et al 2011). In stark contrast, the Mw7.8 Dusky Sound earthquake of 2009 occurred in a sparsely populated region in the remote area of Fiordland, radiated a similar amount of seismic energy (Me7.3) in a predominantly offshore (south-westward) direction and produced relatively little damage and no casualties (Reyners 2009).…”

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“…This is based on the assumption that the effect of the vertical component of the ground motion, induced predominantly by vertically propagating compressional waves, is less significant, due to its smaller magnitude and higher frequency content compared to the horizontal ground motion component (Yang and Yan, 2009). However, since 1990s, strong vertical ground motions have been repeatedly observed, leading to significant damage of engineering structures in the form of vertical compression (Papazoglou and Elnashai, 1996;Yang and Sato, 2000;Bradley, 2011;Lee et al, 2013;Riches, 2015). In particular in the recent 2011 Christchurch earthquake, strong vertical ground motions were widely observed (Lee et al, 2013), which induced significant damage on crib retaining walls due to the contact loss of the crib units in the vertical direction (Riches, 2015).…”

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