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Ground motion recording station with geo-meanpeak horizontal ground accelerations from 4 Sept.2010 event (left) and 22 Feb. 2011 event (right).Christchurch CathedralFigure 2 Section LineChristchurch CBDFinite fault model for 22 Feb2011 M w = 6.2 event. Referto Beavan et al. (this issue)and Bradley and Cubrinovski(this issue) for more detail.▲▲Figure 1. Christchurch CBD relative to subsurface fault rupture of 22 February 2011 event.single-story timber-framed houses with a smaller number oftwo-story houses) spread across a large area with many parks,natural reserves, and recreation grounds. The CBD is the areaencompassed by the four avenues, Rolleston to the west, Bealeyto the north, Fitzgerald to the east, and Moorhouse to thesouth. The CBD is more densely developed, including multistorybuildings in its central area, a relatively large number ofhistoric masonry buildings dating from the late 19th and early20th century, residential buildings (typically two- to five-storystructures located north of Kilmore Street), and some industrialbuildings in the south and southeastern parts of the CBD.In total, about 3,000 buildings of various heights, constructionage, and structural systems were within the CBD boundariesbefore the 2010–2011 earthquake series. Latest estimatesindicate that about 1,000 of these buildings will have to bedemolished because of excessive earthquake damage. Figure 1outlines the boundaries of the CBD and the approximate locationof the causative fault of the 22 February 2011 earthquake.LOCAL GEOLOGYThe city of Christchurch is located on Holocene deposits ofthe Canterbury Plains, except for its southern edge, which islocated on the slopes of the Port Hills of Banks Peninsula,the eroded remnant of the extinct Lyttelton Volcano, composedof weathered volcanic rock (basalt) and thick depositsof Pleistocene loess. The river floodplain, Pacific coastline,and the Port Hills are the dominant geomorphic featuresof the Christchurch urban area. The Canterbury Plains arecomplex fans deposited by eastward-flowing rivers from theSouthern Alps, a NS-trending mountain range, into PegasusBay on the Pacific coast. The fan surfaces cover an area 50 kmwide × 160 km long. At Christchurch, surface postglacialsediments have a thickness between 15 and 40 m and overlie300–400-m-thick interlayered formations of gravels and fineto very fine grained sediments, representing deposition duringepisodic glacial and interglacial periods, and together comprisea series of ground water aquifers (Brown and Weeber 1992).As shown in Figure 2, the surface sediments are made up offluvial gravels, sands, and silts (Springston Formation, with amaximum thickness of 20 m to the west of Christchurch) orestuarine, lagoon, beach, dune, and coastal swamp deposits ofsand, silt, clay, and peat (Christchurch Formation, with a maximumthickness of 40 m on the coast at New Brighton, east ofthe CBD). The shallow soil deposits (i.e., depths of up to 15–20m) vary significantly within short distances, both horizontallyand vertically.Brown and Weeber (1992) describe the original site conditionsand development of Christchurch as follows:Originally the site of Christchurch was mainly swamplying behind beach dune sand; estuaries and lagoons,and gravel, sand and silt of river channel and flooddeposits of the coastal Waimakariri River flood plain.The Waimakariri River regularly flooded Christchurchprior to stopbank construction and river realignment.Since European settlement in the 1850s, extensivedrainage and infilling of swamps has been undertaken.894 Seismological Research Letters Volume 82, Number 6 November/December 2011