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Comparison of Liquefaction FeaturesObserved during the 2010 and 2011 CanterburyEarthquakesR. P. Orense, T. Kiyota, S. Yamada, M. Cubrinovski, Y. Hosono, M. Okamura, and S. YasudaR. P. Orense, 1 T. Kiyota, 2 S. Yamada, 3 M. Cubrinovski, 4 Y. Hosono, 5M. Okamura, 6 and S. Yasuda 7INTRODUCTION1. Department of Civil and Environmental Engineering, University ofAuckland, New Zealand2. Institute of Industrial Science, University of Tokyo, Japan3. Department of Civil Engineering, University of Tokyo, Japan4. Department of Civil and Natural Resources Engineering, Universityof Canterbury, New Zealand5. Department of Architecture and Civil Engineering, ToyohashiUniversity of Technology, Japan6. Department of Civil and Environmental Engineering, EhimeUniversity, Japan7. Department of Civil and Environmental Engineering, Tokyo DenkiUniversity, Tokyo, JapanOn 4 September 2010, a magnitude M = 7.1 earthquake struckthe Canterbury region on the South Island of New Zealand.The epicenter of the earthquake was located near Darfield,about 40 km west of the central business district (CBD) of thecity of Christchurch and at a depth of about 10 km. Extensivedamage was inflicted on lifelines and residential houses dueto widespread liquefaction and lateral spreading in areas closeto major streams, rivers, and wetlands throughout the city ofChristchurch and the town of Kaiapoi. In the months followingthe Darfield M 7.1 earthquake, numerous aftershocks werefelt across the city.Almost six months after the Darfield mainshock, on 22February 2011, the Canterbury region was hit by a magnitudeM = 6.3 earthquake. The epicenter was located near Lyttelton,only 6 km to the southeast of the Christchurch CBD and at adepth of 5 km. In spite of its smaller magnitude, this earthquakeresulted in more damage to pipeline networks, transport facilities,residential houses/properties, and multistory buildings inthe CBD than the September 2010 event, mainly because ofthe short distance to the city and the shallower depth.Although there were no casualties after the 2010 Darfieldearthquake, which is sort of a miracle considering the magnitudeof the earthquake, the 2011 Christchurch earthquakeresulted in a significant number of casualties due to the collapseof multistory buildings and unreinforced masonry structuresin the Christchurch city center. As of 1 June 2011, 181 casualtieswere reported (New Zealand Police; http://www.police.govt.nz/list-deceased).While it is extremely regrettable that the 2011Christchurch earthquake resulted in significant casualties,engineers and seismologists now have a hard-to-find opportunityto learn the response of ground and structures to twolarge-scale earthquakes that occurred less than six monthsapart. From a geotechnical engineering point of view, it is interestingto look at the widespread liquefaction in natural sediments,re-liquefaction of ground occurring over a short periodof time, and further damage to earth structures that had beendamaged as a result of the first earthquake.Following the two earthquake events, detailed geotechnicalinvestigations were conducted by the authors as part of theJapanese Geotechnical Society (JGS) earthquake reconnaissanceteams. The reconnaissance was a collaboration betweenthe society’s New Zealand-based members and researchers dispatchedfrom Japan for this purpose. The first visit was made12–15 September 2010, while the second one was 27 February–3March 2011. This paper attempts to present a comparison ofthe two events based on the observations made by the authorsfollowing these reconnaissance trips, with emphasis on the geotechnicalimplications of liquefaction-observed damage in theaffected areas.It is worth mentioning that a series of aftershocks, the largestof which were M 5.6 and M 6.3, rattled the city on 13 June2011. These aftershocks again caused extensive liquefactionin many parts of Christchurch. As we write this paper, reconnaissancework is underway to shed more light on the damagecaused by re-liquefaction.GEOLOGIC SETTINGThe Canterbury Plains, about 180 km long and of varyingwidth, are New Zealand’s largest areas of flat land. They havebeen formed by the overlapping fans of glacier-fed rivers issuingfrom the Southern Alps, the mountain range of the SouthIsland. The plains are often described as fertile, but the soils arevariable. Most are derived from the greywacke of the mountainsor from loess (fine sediment blown from riverbeds). Indoi: 10.1785/gssrl.82.6.905Seismological Research Letters Volume 82, Number 6 November/December 2011 905