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0000111122223333Depth Depth (m) (m)45Depth (m) (m)45Depth (m) (m)45Depth (m) (m)456666777788889109(e) Spot No.5(d) Spot No.4(b) Spot No.2(c) Spot No.31010100 10 20 30 400 10 20 30 400 10 20 30 400 10 20 30 40Conversion N-valueConversion N-valueConversion N-valueConversion N-valueConverted N-value Converted N-value Converted N-value Converted N-value9Bexley Bexley Dallington Dallington9▲ ▲ Figure 6. Converted SPT N-value profiles from Swedish weight sounding tests conducted by the University of Canterbury and JGSreconnaissance teams in September 2010.N = 0.002W SW + 0.067N SW , (1)where W SW (kg) is the weight less than 100 kg and N SW is thenumber of half-rotations for every meter of penetration. W SWis counted when penetration occurs with dead-load less than100 kg. Note that this equation is applicable to gravel, sand,and sandy soils.Typical results of SWS tests in Christchurch are shown inFigure 6. The locations of the test sites are indicated in Figure3. It can be seen from the strength-depth profiles that in theseareas, layers of about 5 m or thicker exist with high potential toliquefy (very loose silt/sand layer with SPT N-value < 5). Thepresence of loose sandy deposits in many areas in Christchurchhas also been confirmed through dynamic cone penetrometer(DCP) tests and spectral analysis of surface waves(SASW) tests conducted by the Geotechnical Extreme EventsReconnaissance (GEER) team (Green et al. 2011, page 927of this issue).Immediately following the two earthquakes, reconnaissancework was performed to investigate the extent and featuresof the damage. Figures 7A and 7B show the distributionof liquefaction observed in the suburbs east of the CBD followingthe September 2010 and February 2011 earthquakes,respectively. These maps, which were constructed from on-footinvestigations and drive-through surveys with the help of theUniversity of Canterbury reconnaissance team, may be incompletedue to the limited time spent by the team in the areafollowing each earthquake. As mentioned earlier, the shorterdistance to the city and the shallower depth of the February2011 earthquake resulted in more significant and more widespreaddamage in these areas than the September 2010 earthquake.The circle and square data points plotted in the figurescorrespond to the maximum distance from the Avon River atwhich lateral spreading was observed in the north and southbanks, respectively, based on ground inspection. It is worthnoting that while major liquefied sites in the September 2010earthquake were concentrated along the Avon River, liquefactionwas observed in the 2011 earthquake across a wider area,i.e., not only in the eastern suburbs but in the north and in theCBD as well.Considering the short time interval between the two largeearthquakes, the 2011 earthquake induced additional damageto many facilities that suffered liquefaction-induced damageafter the 2010 earthquake and had not been repaired. Figures8A and 8B show the condition of a river embankment adjacentto the Avon Rowing Club (east bank of Avon River) after the2010 earthquake and 2011 earthquake, respectively. The widthof crack openings on the shoulder and the settlement of thecrown became larger due to the re-liquefaction of the foundationground of the embankment. Extensive re-liquefaction wasobserved in the entire Porritt Park in 2011, where almost halfof the green grassy area was covered by sand boils, similar tothat observed after the 2010 earthquake, as shown in Figure 9.The southern portion of the Bexley suburb was formerlya swamp and formed part of the Bexley wetlands. It had beenreclaimed in the late 1990s by filling the area, and the subdivisionwas built over it. Interviews with homeowners indicatethat the area was fairly new, with some houses built as recent asfive years ago (Orense et al. 2011). The September 2010 earth-910 Seismological Research Letters Volume 82, Number 6 November/December 2011