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▲ ▲ Figure 2. Observed polarity for the records in the CBD in terms of peak ground acceleration, velocity, and displacement. The contoursof PGA on the map were computed by interpolation using all records in Christchurch.amplification at periods exceeding 2 sec is attributed to the factthat once liquefaction has occurred, the overlying soil “crust”oscillates with very low frequencies, causing the bulges observedin the acceleration spectra for periods of about 3 sec (see Youdand Carter 2005 for similar observations from the then-availableliquefaction-affected acceleration spectra). In addition to structuraldamage due to high spectral accelerations, important soilrelatedfailures have directly affected houses and bridges.THE POLARITY OF THE RECORDED MOTIONSThe two orthogonal components of a record are usually alignedwith the north-south and east-west directions (Figure 1) or,ideally, if the faults were known, with fault-parallel and faultnormaldirections. Mathematically there is at least one specificangle at which a certain ground motion parameter such asPGA, PGV, or PGD reaches a maximum, indicating the governingdirection for that ground motion parameter and revealinga certain polarity of the recorded motion. Polarity plots canbe useful in determining the dominant shaking direction of anearthquake and in unveiling any directivity effects (Shabestariand Yamazaki 2003).A first index of intensity is the value of peak ground acceleration(PGA), the spatial distribution of which is depicted onthe map of Figure 2. Additionally, for the records from the fourCBD stations (CCCC, REHS, CBGS, and CHHC) the maximumpeak values of ground acceleration, velocity, and displacementare calculated trigonometrically, by varying the angle by1° between 0° and 180°, resulting in asymmetric plots of positiveand negative maxima (in absolute terms). The graphs consistentlyexhibit distinct polarity in a direction that practicallycoincides with that of the fault line. Knowing the polarity ofshaking may offer information on the rupture mechanism andinsight into the dominant damage observed in the CBD area.TYPICAL SOIL PROFILE, LIQUEFACTION,ANALYSISThe Christchurch urban area, extending from Riccarton inthe west to Bexley in the east and reaching Heathcote Valleyand the Port Hills in the south, is located on the CanterburyPlains. Its dominant geomorphic feature is the river floodplains.In particular, the Avon (primarily) and Heathcote (secondarily)rivers, originating from various springs in westernChristchurch, form endless meanders through the city and theeastern suburbs as they head to the estuary near the sea.As depicted in Figure 3A, the subsoil in the CBD systematicallyconsists of profiles with random variations in layeringin the upper 15–25 m (Cubrinovski et al. 2010; Toshinawa etal. 1997). The volcanic bedrock is located at an approximate884 Seismological Research Letters Volume 82, Number 6 November/December 2011