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0.8PRPC Z0.40Normalized (Max PGA + Min PGA)-0.4-0.80.80.40 1 2 3 4 5 6 7 8 9 100.2 - 1 Hz1 - 2 Hz2 - 5 Hz5 - 10 Hz10 - 30 Hz0-0.4-0.80 1 2 3 4 5 6 7 8 9 10time (s)▲▲Figure 6. A) Normalized sum of maximum and minimum PGA for windows of the vertical recording at PRPC. Windows have a half-widthof 0.25 seconds and are sampled every 0.1 s. Bottom) Data windowed as above after band-pass filtering. The band between 2–-5 Hz (darkblue) is dominantly positively asymmetric between about 2.5 and 7.5 seconds (shown as shaded region).that non-linear effects in the near-surface layers such as thoseproposed by Aoi et al. (2008) and Yamada et al. (2009) are presentin many of the urban recordings. These are manifest in asymmetryin the vertical recordings. We find asymmetric behaviorin records from smaller events as well, and bracket initiation ofvertical component acceleration asymmetry at between 0.3 and0.7 g. Furthermore, a multiple-filter approach to analyzing thetime signals recorded at one of the eastern city stations suggeststhat the dominant bandwidth carrying the asymmetric accelerationsis between 2 and 5 Hz. Further analysis of these datain conjunction with detailed site response analyses and numericalmodeling has the potential to greatly increase our understandingof this phenomenon. We have noted a difference inthe frequency content of the recorded vertical and horizontalaccelerations from the earthquake. Based on numerical modeling,we propose that the lack of high-frequency energy on thehorizontal components can largely be attributed to shear waveattenuation in the shallow, water-saturated sediments. Futurework is aimed at fully implementing Biot’s theory into themodeling, explicitly describing the complete fluid interactionof the water-saturated soils in Christchurch.Many of the strong-motion recordings have vertical componentsthat are rich in high-frequency energy and horizontalcomponents that are dominated by low-frequency waves. Wemodel attenuation with a discrete wavenumber method as aproxy for this effect. The characteristic difference in energycontent appears to be the result of attenuation of high-frequencyshear waves in the shallow subsurface.REFERENCESAki, K., and K. L. Larner (1970). Surface motion of a layered mediumhaving an irregular interface due to incident plane SH waves.Journal of Geophysical Research 75, 933–954.Aki, K., and P. G. Richards (1980). Quantitative Seismology. SanFrancisco: W. H. Freeman and Company, 556 pp.Aoi, S., T. Kunugi, and H. Fujiwara (2008). Trampoline effect in extremeground motion. Science 322 (5902), 727–730, doi:10.1126/science.1163113.Seismological Research Letters Volume 82, Number 6 November/December 2011 851