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▲ ▲ Figure 2. A) shows three-component accelerogram from station PRPC (see Figure 1 for location) for the 22 February event, scaledto units g. B) shows their corresponding spectra.the proximity of the February event to Christchurch and theeffects of strong source directivity. However, not all featurescan be explained by source effects alone. The dense near-sourcedata from the 22 February earthquake have provided us with avaluable opportunity to study the response of the shallow subsurfaceto extreme ground motions in very fine detail.The way ground responds to an earthquake is a result ofthe earthquake rupture process, the path that the waves takebetween the source and the surface, and the response of theshallow materials below the ground. We know that the topfew meters of the ground in Christchurch played an importantrole in the shaking. This role is evident in low-frequencysignals resulting from liquefaction. Many of the poorly consolidated,low shear-wave velocity soils liquefied at shallowdepths with less than 0.1 g peak horizontal accelerations andexperienced deep liquefaction at around 0.2 to 0.3 g accelerations.The influence of the shallow subsurface is also exhibitedby the existence of energetic high-frequency signals resultingfrom the interaction of the waves with both the water table andunconsolidated soils prior to liquefaction. A marked featureof the strong-motion seismograms recorded at several nearsourcesites in Christchurch during the earthquake sequence isthe much higher frequency content of the vertical componentcompared to the corresponding horizontal recordings (Figure2). We believe that this phenomenon is due to the presence ofa shallow water table dramatically attenuating the propagationof high-frequency shear waves. A rigorous numerical demonstrationof such water effect would require the calculation ofseismic wave propagation in layered media in which one of thelayers is a porous, elastic solid containing an incompressible,inviscous fluid (Biot 1956a, Biot 1956b). However, for thiswork, we simulate such an effect in an indirect way by model-Seismological Research Letters Volume 82, Number 6 November/December 2011 847