Geotechnical aspects of the 2010 Darfield and 2011 Christchurch earthquakes, New Zealand, and geotechnical damage to structures and lifelines

The Darfield earthquake (M_w 7.1; M_L 7.2) occurred along a previously unknown strike-slip fault in Canterbury Province, New Zealand on 4 September 2010. The second (Christchurch) earthquake (M_w = 6.2) on 22 February 2011, caused by a thrust fault, affected the same region. Both earthquakes caused extensive liquefaction, associated ground deformation and severe damage to residential houses and utilities while instability occurred on the Port Hills. In this study, observations by the authors at different parts of the earthquake-affected region after both earthquakes are described, analyzed and discussed. The grain-size distribution of the liquefied soils shows a good agreement with empirical bounds of liquefiable soils. In addition, comparisons of the observations with the empirical relations linking the earthquake magnitude and liquefaction limit distance, show good agreement. The factors of safety against liquefaction are very low when computed by three different techniques using the data from available geotechnical boreholes. Good correlations are obtained between the displacements due to lateral spreading and those estimated from the aerial photogrammetry method, and between the observations with empirical magnitude and epicentral limit distance relations for coherent and disrupted slopes. Estimations using an empirical relation proposed by the authors are compared with lateral spreading and it is concluded that the thickness of liquefiable layers ranges between 4 and 8 m. As the houses and suburban buildings were light, the effect of liquefaction and lateral spreading caused severe damage but not collapse. However, heavy concrete structures settled and tilted and empty buried tanks floated. It is of note that these two earthquakes and major aftershocks greater than magnitude 5 showed that the same areas could liquefy multiple times if the conditions for liquefaction are satisfied.