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4 Sep Mw 7.118 Oct Mw 5.013 Jun Mw 5.613 Jun Mw 6.022 Feb Mw 6.2▲ ▲ Figure 5. Vertical accelerograms from station PRPC for differentevents sorted from minimum PGA in the top panel tomaximum PGA in the bottom panel. Gray region marks twice theminimum acceleration centered about the baseline.Miyagi Nairiku earthquake of 2008 (Aoi et al., 2008; Yamadaet al., 2009). Similar asymmetric recordings from the Mw 6.9Iwate-Miyagi Nairiku earthquake of 2008 have been attributedto a “trampoline” effect (Aoi et al. 2008; Yamada et al. 2009).Aoi et al. (2008) attribute the asymmetry to the decouplingof near-surface materials during high-amplitude downwardacceleration. This occurs when the tensile forces that arise onan interface or within a granular material from downgoing particleoscillation as waves pass are larger than its tensile strength.The result is an approximate free-fall of the material. In thismodel, the high upward accelerations are caused by the compressionalresponse of the granular media to the stress of theupgoing particle oscillation. Yamada et al. (2009) suggest thelarge positive accelerations are further enhanced by “slapdown”as free-falling upper soil layers impact/interact with deeper layersthat are returning upward during the following earthquakewave cycles. Recently conducted finite element numerical modelingsupports the hypothesis that the asymmetry arises due tothe difference in response of near-surface layers to compressionand tension (Tobita et al. 2010). Figure 5 shows a series of verticalcomponent recordings from station PRPC in Christchurch.The recorded vertical acceleration from the February eventat this station was most similar to asymmetric vertical accelerationrecorded in the Iwate-Miyagi event. PRPC appears tohave experienced vertical acceleration asymmetry in numerousevents over a range of peak accelerations. Recordings from the13 June 2011 Mw 6.0 and Mw 5.6 events both exhibit slightasymmetry with PGAs as low as ~0.7 g. However, records fromthe 4 September Mw 7.1 earthquake do not appear asymmetric.Ground motions recorded at PRPC in this event reach 0.32 g.This suggests that a threshold for the non-linear effect thatcauses the asymmetry is between about 0.3 and 0.7 g at this site.In pure granular material, Tobita et al. (2010) show thatthe depth of initiation of the non-linear response is proportionalto the experienced ground acceleration. The frequencyof the response is also likely to be at least partially controlledby the depth of origin of the strong asymmetry. We will focuson station PRPC for a description of the time and frequencyof asymmetry. To quantify trace asymmetry, we normalize thedifference between the maximum (a max ) and minimum (a min )accelerations for time-windowed data (i.e., (a max – abs(a min ))/(max(abs(a max ,a min )))). Onset of dominant asymmetry occursproportionately earlier in the time series for very near-sourcestations and proportionately later for more distant stations. Atstation PRPC, onset of dominant asymmetry occurs at about3 s into the waveform (Figure 6) whereas at station HVSC,within a couple of kilometers of the rupture, onset is within thefirst second. We analyze the asymmetry of different frequenciesby applying the time-windowing to bandpass filtered data. Wedefine frequency bands in the data between 0.2–1 Hz, 1–2 Hz,2–5 Hz, 5–10 Hz, and 10–30 Hz. Asymmetry at stationPRPC is dominated by the 2–5 Hz band (Figure 6). However,we use a window half-width of 0.25 s for all considered bands.We recognize that relatively more wave cycles will contribute tothe time-window for the higher frequency bands than for thelower frequency ones. However, the asymmetry in the 2–5 Hzband is bracketed by the more symmetric 1–2 Hz and 5–10 Hzbands. This supports our suggestion that the 2–5 Hz band ofthe signal dominates the asymmetry.CONCLUSIONSThe Mw 6.2 Christchurch earthquake generated a wealth ofnear-field strong-motion data. Analysis of these data suggests850 Seismological Research Letters Volume 82, Number 6 November/December 2011