Geotechnical reconnaissance of the 2010 Darfield (Canterbury) earthquake

Abstract

On 4 September 2010, a magnitude Mw 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicentre of the earthquake was located in the Darfield area about 40 km west of the city of Christchurch. Extensive damage was inflicted to lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers and wetlands throughout Christchurch and Kaiapoi. Unreinforced masonry buildings also suffered extensive damage throughout the region. Despite the severe damage to infrastructure and residential houses, fortunately, no deaths occurred and only two injuries were reported in this earthquake. From an engineering viewpoint, one may argue that the most significant aspects of the 2010 Darfield Earthquake were geotechnical in nature, with liquefaction and lateral spreading being the principal culprits for the inflicted damage.
 Following the earthquake, an intensive geotechnical reconnaissance was conducted to capture evidence and perishable data from this event. The surveys were performed on foot, by car and from a helicopter over a period of six days. A broad-brush field reconnaissance was conducted in the first two days, followed by pin-point investigations at specific locations including detailed site inspections and field testing using: Dynamic Cone Penetration Test (DCPT), Swedish Weight Sounding (SWS), and Spectral Analysis of Surface Waves (SASW).
 This paper summarizes the observations and preliminary findings from this early reconnaissance work.

Paleoliquefaction in Christchurch, New Zealand

“…The youthful, unconsolidated nature of the fine sands to silt underlying eastern Christchurch combined with high water tables (1-2 m depth) and localized artesian water pressures pose a long-recognized high liquefaction hazard (Elder et al, 1991). This was confirmed during the Canterbury earthquake sequence (Cubrinovski and Green, 2010). Liquefaction may also have been exacerbated in parts of the eastern suburbs by leakage of underlying artesian aquifers through breached aquitards .…”

“…This indicates that moderate-tolarge (M 5-7.9) events can trigger lique faction at distances greater than that predicted from empirical global compilations of M w versus "distance to most distal liquefaction feature" (Ambraseys, 1988;Galli, 2000;Cubrinovski and Green, 2010), particularly in highly susceptible sediments or in areas prone to seismic amplification. Therefore, it is possible that the 1869 earthquake triggered minor liquefaction at an approximate epicentral distance of 6 km in highly susceptible sediments.…”

“…Liquefaction during the Canterbury earthquake sequence was reported at distances of ~8 km from the epicenter following the 19 October 2010 M w 5.0 aftershock (Cubrinovski and Green, 2010) and ~14 km from the epicenter of the 16 April 2011 M w 5.0 aftershock . This indicates that moderate-tolarge (M 5-7.9) events can trigger lique faction at distances greater than that predicted from empirical global compilations of M w versus "distance to most distal liquefaction feature" (Ambraseys, 1988;Galli, 2000;Cubrinovski and Green, 2010), particularly in highly susceptible sediments or in areas prone to seismic amplification.…”

“…The most severe liquefaction-induced damage was reported following the September 2010 M w 7.1 Darfield main shock, and the February 2011 M w 6.2 (i.e., 2011 Christchurch earthquake), June 2011 M w 6.0, and December 2011 M w 5.9 aftershock earthquakes ( Fig. 1; Cubrinovski and Green, 2010;Cubrinovski et al, 2011;Quigley et al, 2013). As a result of extensive land and infrastructure damage, more than 6000 residential properties in eastern Christchurch were purchased by the central government (http:// cera .govt .nz /residential -red -zone).…”

“…Lique fied sediment may be ejected to the ground surface via feeder dikes, that commonly utilize fractures in the sedimentary cover overlying the liquefied stratum. Surface ejecta commonly manifests as sand blows, blistering of the surface by near-surface sediment injection, and vertical (subsidence) or lateral (lateral spreading) ground deformation (Seed and Idriss, 1982;Sims and Garvin, 1995;Tuttle and Barstow, 1996;Obermeier, 1996;Galli, 2000;Idriss and Boulanger, 2008;Cubrinovski and Green, 2010;Tuttle and Hartleb , 2012;Quigley et al, 2013). Surface liquefaction features may be rapidly (i.e., within hours to months) reworked into forms that are difficult to distinguish from eolian, fluvial, or estuarine deposits (Sims and Garvin, 1995;Reid et al, 2012;Quigley et al, 2013), complicating the geologic identification of prehistoric features.…”