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(A)(B)Spectral Acc, Sa (g)North/West10 010 -110 -2Z 1.0=1000mChristchurchMesozoicbasement rockFault ParallelFault NormalHoriz gmTertiary VolcanicRockHypocenterZ 1.0=300mStation:CHHCR rup=3.8 km10 -2 10 -1 10 0 10 1Period, T (s)▲ ▲ Figure 8. A) Schematic illustration of waveguide effectsoccurring in the sedimentary basin underlying Christchurch (notto scale); and B) influence of basin depth on pseudo-spectralacceleration ordinates predicted empirically compared withthat observed at Christchurch Hospital (CHHC). The predictionshown is for the horizontal geometric mean and dashed linesrepresent the 16th and 84th percentiles.horizontal pseudo-response spectra at Christchurch hospital(CHHC), located at a source-to-site distance of R rup = 3.8 kmon the hanging wall. Also shown in Figure 8B is the predictedmedian, 16th, and 84th percentile response spectra for the siteusing the Bradley (2010) empirical model for two differentvalues of a proxy for basin depth. The Bradley (2010) modelis based on the Chiou and Youngs (2008) model with NewZealand–specific modifications. Basin effects are accountedfor in the model through the use of the parameter Z 1.0 , whichrepresents the depth to sediments with shear wave velocity,V s = 1.0 km/s. For site class D conditions (a nominal 30-maverage shear wave velocity of V s,30 = 250 m/s) the default valueof Z 1.0 is on the order of 300 m. Figure 8B illustrates that spectralamplitudes at CHHC for periods greater than 0.3 secondsare underpredicted using this default Z 1.0 value. The fact thatthe thickness of gravels in the Christchurch basin is known tobe greater than 500 m implies that Z 1.0 would be significantlygreater than 500 m. Figure 8B also illustrates the predictedspectral amplitudes, using a value of Z 1.0 = 1,000 m, where itcan be seen that the empirical prediction of long-period spectralamplitudes is significantly increased compared with thoseusing Z 1.0 = 300 m, in line with the observed amplitudes.The increase in amplitude of horizontal ground motionat long periods illustrated at Christchurch hospital (CHHC)was also observed at numerous other locations in the region.Significant amplitude Rayleigh surface waves are also clearlyevident in the vertical component of ground motion observedat larger source-to-site distances where body wave amplitudesare smaller (e.g., stations SMTC and CACS in Figure 6).Near-source Forward DirectivityIn the near-source region ground motions may exhibit forwarddirectivity effects due to the rupture front and direction of slipbeing aligned with the direction toward the site of interest.While the finite fault model of Beavan et al. (2011, page 789of this issue; see also Figure 2) does not provide information onthe temporal evolution of rupture, based on the central locationof the inferred hypocenter, the direction of slip is not wellaligned with an elliptically inferred rupture front. As a result, itis expected that rupture directivity effects will only be importantover a small surface area, relative to other possible rupturescenarios (Aagaard et al. 2004).Figure 9A illustrates the velocity time history at PagesRoad (PRPC), where forward directivity effects can be seen inthe fault-normal component. Figure 9B illustrates the velocitytime history at Christchurch hospital (CHHC) where avelocity pulse in the fault normal component is not clearly evident,and the large velocity amplitudes are the result of surfacewaves as previously noted. Figure 9C illustrates the observedand predicted pseudo-acceleration response spectra at CHHCwith and without the consideration of directivity effects. Thedirectivity effect was estimated empirically using the model ofShahi and Baker (2011). It can be seen that the predicted effectof forward directivity is relatively small (compared to the basindepth effect in Figure 8B) because of the small propagation distancefrom the hypocenter along the fault plane toward the site(which gives a low probability of observing a velocity pulse inthe model of Shahi and Baker 2011).The effects of near-source directivity fling step were notexamined here, both because on the near-source soil sites staticdeformations may be the result of ground failure, and alsobecause, as previously noted, the standard record processingadopted removes such long-period signals.Vertical Ground MotionFigure 6 illustrates that significant vertical ground motion amplitudeswere recorded in the near-source region in Christchurch,with peak vertical accelerations exceeding 0.6 g at sevenstrong motion stations and, in particular, values of 2.21 g and1.88 g observed at Heathcote Valley (HVSC) and Pages Road(PRPC), respectively. The vertical acceleration time histories atthese two sites also exhibit the so-called trampoline effect (Aoiet al. 2008; Yamada et al. 2009) caused by separation of surficialsoil layers in tension, limiting peak negative vertical accelerationsto approximately –1 g. Such large vertical accelerations canbe understood physically, first due to the relatively steep dip ofthe fault plane (δ = 69°), which results in a large component offault slip oriented in the vertical direction. Furthermore, at soilsites in sedimentary basins in particular, large vertical accelerationsat near-source locations can result from the conversion ofinclined SV waves to P waves at the sedimentary basin interface,Seismological Research Letters Volume 82, Number 6 November/December 2011 859