Earthquake News
Geotechnical Damage due to the 2011 Christchurch,
New Zealand
Suguru Yamada, Assistant Professor, University of Tokyo
Rolando Orense, Senior Lecturer, University of Auckland
Misko Cubrinovski, Associate Professor, University of Canterbury
INTRODUCTION
On 22 February 2011 ，a magnitude Mw 6.3 earthquake occurred with an epicenter located near Lyttelton at about 10km from Christchurch in Canterbury region on the South Island of New Zealand (Figure 1). Since this earthquake occurred in the midst of the aftershock activity which had continued since the 4 September 2010 Darfield Earthquake occurrence, it was considered to be an aftershock of the initial earthquake. Because of the short distance to the city and the shallower depth of the epicenter, this earthquake caused more significant damage to pipelines, traffic facilities, residential houses/properties and multi-story buildings in the central business district than the September 2010 Darfield Earthquake in spite of its smaller earthquake magnitude. Unfortunately, this earthquake resulted in significant number of casualties due to the collapse of multi-story buildings and unreinforced masonry structures in the city center of Christchurch. As of 4 April, 172 casualties were reported and the final death toll is expected to be 181. While it is extremely regrettable that Christchurch suffered a terrible number of victims, civil and geotechnical engineers have this hard-to-find opportunity to learn the response of real ground from two gigantic earthquakes which occurred in less than six months from each other. From geotechnical engineering point of view, it is interesting to discuss the widespread liquefaction in natural sediments, repeated liquefaction within short period and further damage to earth structures which have been damaged in the previous earthquake. Following the earthquake, an intensive geotechnical reconnaissance was conducted to capture evidence and perishable data from this event. The team included the following members: Misko Cubrinovski (University of Canterbury, NZ, Team Leader), Susumu Yasuda (Tokyo Denki University, Japan, JGS Team Leader), Rolando Orense (University of Auckland, NZ), Kohji Tokimatsu (Tokyo Institute of Technology, Japan), Ryosuke Uzuoka (Tokushima University, Japan), Takashi Kiyota (University of Tokyo, Japan), Yasuyo Hosono (Toyohashi University of Technology, Japan) and Suguru Yamada (University of Tokyo, Japan)
GEOLOGICAL AND TECTONIC SETTING
The Canterbury Plains, about 180 km long and of varying width, are New Zealand’s largest areas of flat land. They have been formed by the overlapping fans of glacier-fed rivers issuing from the Southern Alps, the mountain range of the South Island. The plains are often described as fertile, but the soils are variable. Most are derived from the greywacke of the mountains or from loess (fine sediment blown from riverbeds). In addition, clay and volcanic rock are present near Christchurch from the Port Hills slopes of Banks Peninsula. The city of Christchurch is located at the coast of the Canterbury Plains adjacent to an extinct volcanic complex forming Banks Peninsula. Most of the city was mainly swamp, behind beach dune sand, and estuaries and lagoons, which have now been drained (Brown et al., 1995). The simplified geographical and geological information are shown in Figures 1, 2 and 3.
Canterbury has abundant water, in the rivers which carry mountain rainfall to the coast, and in aquifers. Beneath the plains, layers of porous gravels are interspersed with impermeable finer sediments. Near Ashburton, bedrock is at a depth of 1,600 meters (Wilson, 2009). Unlike most urban water supplies, Christchurch’s water comes from aquifers beneath the city. The aquifers are recharged by rainfall and by river seepage. They have been tapped to irrigate farmland and for town water supplies.
Download ISSMGE Bulletin – Volume 5 Issue 2 (pp. 27-45)
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