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▲ ▲ Figure 2. Representative subsurface cross-section of Christchurch CBD along Hereford Street (modified from I. McCahon, personalcommunication, 19 July 2011).Brown and Weeber also state that surface deposits are activelyaccumulating and that the present-day river channel depositsare excluded from the above-mentioned Christchurch andSpringston formations.Canterbury has an abundant water supply through riversand streams and rich aquifers. The dominant features ofpresent-day Christchurch are the Avon and Heathcote rivers,which originate from springs in western Christchurch, meanderthrough the city, and feed the estuary at the southeast endof the city. As illustrated in Figure 3, the Avon River meandersthrough the CBD while the Heathcote River flows south of theCBD. The groundwater table is deepest at the west end of thecity (i.e., about 5 m depth), becoming progressively shallowereastward (i.e., within 1.0–1.5 m of the ground surface for mostof the city east of the CBD), and approaching the ground surfacenear the coastline. The water table is generally within 1.5to 2.0 m of the ground surface within the CBD.GROUND MOTION CHARACTERISTICSThe 4 September 2010 M w = 7.1 Darfield earthquake was causedby a complex rupture of several fault segments, the largest andnearest to Christchurch being on the Greendale fault about20 km west of the CBD. A maximum horizontal peak groundacceleration (PGA) of 0.24 g was recorded in the CBD, and thePGA decreased generally with distance downstream along theAvon River. The M w = 6.2, 22 February 2011 Christchurchearthquake was less than 10 km from the CBD along the southeasternperimeter of the city in the Port Hills (Figure 1). Theclose proximity of this event caused higher-intensity shaking inthe CBD as compared to the Darfield earthquake. Several ofthe recordings exhibited forward-directivity significant velocitypulses. In the CBD, horizontal PGAs of between 0.37 gand 0.52 g were recorded. There are four strong motion stationslocated within or very close to the CBD (Figure 1). The recordedPGAs at these four stations are summarized in Table 1 for thefive earthquakes producing highest accelerations (Bradley andCubrinovski 2011, page 853 this issue).For the shallow part of a deposit, the variation in therecorded PGA values corresponds closely with variations in thecyclic stress ratio (CSR) for each of these events. Magnitudescaling factors can then be applied to adjust each calculated CSRvalue to an equivalent value for an M w = 7.5 event (CSR M7.5 ) assummarized in Table 1 for the geometric mean horizontal valuesof the PGA (Bradley and Cubrinovski 2011, page 853 ofthis issue). The data show that in addition to the high PGAs duringthe 22 February 2011 earthquake (PGA = 0.37–0.52 g), theCBD buildings were subjected to significant PGAs in the rangeof 0.16–0.27 g in four additional events. The highest adjustedCSR 7.5 values of 0.14–0.20 were obtained for the M w = 6.2, 22February 2011 earthquake, which were about 1.6 times the correspondingCSR-values from the M w = 7.1, 4 September 2010Darfield earthquake. At many sites in Christchurch liquefactionre-occurred during these earthquakes, which in conjunctionwith the numerous smaller aftershock records providesinvaluable data both in terms of thresholds for liquefactiontriggering and CSR levels responsible for producing damagingliquefaction. In the CBD itself, only isolated areas liquefied duringmultiple events. Instead, widespread liquefaction occurredonly in the CBD during the 22 February 2011 earthquake.LIQUEFACTION IN THE CBDImmediately after the 22 February 2011 earthquake (i.e., from23 February to 1 March) an extensive drive-through reconnaissancewas conducted to map liquefaction and to document theSeismological Research Letters Volume 82, Number 6 November/December 2011 895