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▲ ▲ Figure 2. Horizontal peak ground accelerations recorded at strong motion sites in Christchurch during the Darfield and Christchurchearthquakes, and the locations of bridges highlighted in this paper (Google Inc. 2011).central business district (CBD), through the CBD, and to theeastern edge of the city where it enters the Avon-Heathcoteestuary. East of the CBD the Avon River widens as it nears theestuary. The Heathcote is a smaller river and runs from west toeast in the southern part of the city before entering the estuary.GROUND MOTION CHARACTERISTICSOn 4 September 2010, the M w 7.1 Darfield earthquake struck40 km west of the Christchurch CBD at a focal depth of 11 km(Gledhill et al. 2011). The highest recorded ground motionswere near the epicenter, having a maximum horizontal PGA of0.76 g (geometric mean of the horizontal components, appliesto all horizontal PGAs stated herein) and a maximum verticalPGA of 1.26 g. These large vertical accelerations are typicalof the near-source strong motion recordings for this event. Amaximum horizontal PGA of 0.25 g and maximum verticalPGA of 0.22 g were recorded in the Christchurch CBD, andthe PGA generally decreased with distance downstream alongthe Avon River. The largest vertical PGA in the central andeastern areas of Christchurch was 0.32 g at Pages Road pumpingstation.The M w 6.2 2011 Christchurch earthquake was centeredless than 10 km from the Christchurch CBD along the southeasternperimeter of the city in the Port Hills (Figure 1). Theclose proximity and shallow depth of this event caused higherintensity shaking in Christchurch as compared to the Darfieldearthquake. In the city, ground motions were characterized bylarge vertical accelerations resulting from the close proximity tothe fault plane, steeply dipping oblique thrust faulting mecha-Seismological Research Letters Volume 82, Number 6 November/December 2011 951