The Mw= 6.3 earthquake of February 22 was the strongest seismic event in a series of damaging aftershocks in and around Christchurch after the Darfield earthquake on 4 September 2010. The source of the Darfield earthquake was in a sparsely populated area and thus it caused no loss of life. Serious damage was mainly due to extensive liquefaction. By contrast, the Christchurch earthquake was generated on a fault in close proximity to the city, resulting in a death toll of 181 people. The Canterbury Plains are covered with river gravels that hide any evidence of past fault activity in this region. The newly revealed Greendale fault was therefore completely unknown. Only a portion of it was revealed on the ground surface during the Darfield earthquake. The second fault (the one that ruptured in February 2011) appears to be a continuation of the first, although no fault structure directly connecting the faults has been recognized. There is a debate among seismologists at this point whether this is a different fault from Greendale one or not (NHRP 2011a; NHRP 2011b; Geonet 2011). Due to its magnitude, shallow depth and close proximity to the city, the February earthquake proved particularly destructive for the central business district (CBD) of Christchurch, where buildings suffered extensive damage. Thanks to a dense network of strong ground motion stations, a large number of records have been obtained, which provide valuable information on the event and offer the possibility of relating the extent of damage to actual measurements of ground shaking. Apart from the southern part of the city on the hills and the Lyttelton port area, Christchurch is built on deep estuarine soil, which has been shaped in the last thousands of years by the ever-changing riverbed. Fine sands—the dominant soil type—and the high ground water level contributed to widespread liquefaction in one or both earthquake events. Often accompanied by lateral spreading, liquefaction amplified the level of damage, resulting in the failure of structures in the CBD and surrounding areas, as will be explained below.

The older buildings in the city center, many of which are made of unreinforced masonry with timber floors, were mostly built in the late 19th and early 20th century, following English architectural style and construction practice and with no consideration of the high seismicity of the region. However, some of these buildings had been retrofitted in recent years. In contrast, many of the modern buildings in the CBD were designed in accordance with recent seismic codes, although their foundation systems were not always suitable for the adverse effects stemming from liquefaction. Thus, despite the fact that liquefied layers beneath the CBD restricted somewhat the amplitude of already significantly high accelerations, the increased velocities and displacements due to soil softening magnified the demands on long-period structures. Both structural and geotechnical aspects are investigated here in an effort to broadly explain and quantify the observed damage.