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(A)(B)▲▲Figure 8. A) Representative CPT sounding for the north bank of the Kaiapoi River; B) liquefaction evaluation of the site for both theDarfield and Christchurch earthquakes.(A)(B)▲ ▲ Figure 9. A) Representative CPT sounding for the south bank of the Kaiapoi River; B) liquefaction evaluation of the site for both theDarfield and Christchurch earthquakes.the levees resulted from lateral spreading, more so than deepseated slumping and settlement/bearing capacity failures. Ofthese three failure modes, lateral spreading can occur even if arelatively thin layer liquefies, while deep seated slumping andsettlement/bearing capacity failures require a thicker layer toliquefy. This is likely the reason for the disparity between the r 2values for the T vs. DI plots for the Darfield and Christchurchevents. In the case of the Darfield earthquake, the levees weresubjected to relatively intense shaking and the thickness of theliquefied layer was large. However, because lateral spreadingcan occur on even a thin liquefied layer, the r 2 value for the Tvs. DI plot was very low (i.e., r 2 = 0.147). In contrast, the leveeswere subjected to less shaking during the Christchurch earthquakeand the liquefied layers were relatively thin where liquefactionoccurred. However, even these relatively thin liquefiedlayers were thick enough for lateral spreading to occur, whichresulted in damage to the levees and a relatively high value of r 2for the T vs. DI plot (i.e., r 2 = 0.625). The implication of this isthat liquefaction severity indices that account for both the factorof safety against liquefaction and thickness of the liquefiedlayer, such as the liquefaction potential index (LPI) (Iwasakiet al. 1982), may not be appropriate for evaluating the risk ofdamage from liquefaction where lateral spreading is the primaryfailure mode.SUMMARY AND CONCLUSIONSThe seismic stability of the levees in the Christchurch, NewZealand, area is critically important to the flood protectionfor the region. Overall, the levee system performed well duringboth the M w 7.1 Darfield and M w 6.2 Christchurch earthquakes.However, portions of the levees along the easternSeismological Research Letters Volume 82, Number 6 November/December 2011 947