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Ambient Noise Measurements following the2011 Christchurch Earthquake: Relationshipswith Previous Microzonation Studies,Liquefaction, and NonlinearityMarco MucciarelliMarco MucciarelliBasilicata UniversityINTRODUCTIONFollowing the Christchurch 2011 earthquake, the BasilicataUniversity (Potenza, Italy) organized a field trip to NewZealand mainly to examine structural engineering issues butalso to investigate the similarity between this event and theL’Aquila 2009 quake that struck central Italy. In both casesan event with magnitude slightly above 6 occurred on a blindfault underlying an area inhabited by a population of the orderof hundreds of thousands, killing a few hundred people andseverely damaging the city center, and in both cases a site amplificationstudy was available before the event. At the same timethere were striking differences between the two earthquakesin maximum recorded acceleration, the nonlinear behavior ofsoils, and the occurrence of liquefaction.It was also an opportunity to look at some issues related tothe use of microtremor measurements, in particular:1. to verify if the soil frequencies estimated more than 15years ago by Toshinawa et al. (1997) are a persisting featureor if there were changes following the strong motionsin 2010 and 2011;2. to verify the usefulness of the soil vulnerability index proposedby Nakamura (1996) as a proxy of liquefaction susceptibility;and3. to compare the strong-motion recordings with elasticlimit soil behavior derived from ambient noise, looking forhints of hardening nonlinearity as proposed by Bonilla etal. (2005) and similarity with the observations in L’Aquila(Puglia et al. 2011).PREVIOUS STUDIES AND DATA COLLECTIONIn 1994 the Arthurs Pass Earthquake (M l = 6.6) occurredabout 100 km northwest of Christchurch. The macroseismicintensity was estimated for the city together with local siteamplifications inferred from seismic recordings and microtremors(Toshinawa et al. 1997). The authors found a satisfactorycorrelation among the results of the different techniquesand prepared a microzonation map.As for horizontal-to-vertical spectral ratio (HVSR) analysisof microtremors, Toshinawa et al. (1997) collected three setsof 40-sec-long samples at each site on a 1 by 1 km grid. Theyfound that the H/V spectral ratio of microtremors was wellcorrelated to the ground motion characteristics during earthquakesrecorded at a seismic array deployed within the city andalso correlated with the local geology. The outcropping lithologyof the Christchurch area (Brown and Weeber 1992) iscomposed of:1. Volcanic rock, in the southern part of the city.2. Holocene marine dunes, in the vicinity of the coast.3. Alluvial sand and silt deposits from the estuarine area tothe center of the city, where swamps and lagoons weredrained to reclaim land.4. Alluvial gravel area, in the westernmost part of the city.5. Transition area, with alternating deposits, located betweenthe estuarine and gravel areas.The soil fundamental frequency had higher values in the westerngravel area, starting from 5 Hz and decreasing down to 1Hz proceeding eastward in the transition area and in the sandand silt alluvium beneath the city center. The volcanic rock atthe south returned a flat response and no clear peak was identifiedin the dune area near the ocean coastline.During our field trip, we devoted three days to microtremormeasurements. It was possible to perform 43 measurements, aslisted in Table 1. We also collected 12 recordings as close aspossible to accelerometric stations that recorded the February2011 event, while the others were taken in the most damagedareas but with an effort to obtain good spatial coverage (Figure1). The data were sampled at 128 Hz using a digital threecomponenttromometer (Micromed Tromino) for an acquisitionlength of at least 12 minutes. The data were then filtered,tapered, transformed in frequency domain, smoothed with atriangular filter (n = 5), and finally averaged between horizontalcomponents and among 20-sec subsets using the geometricmean. Almost all the recordings returned HVSR peaks thatpassed the ensemble of tests proposed by the SESAME project(Chatelain et al. 2008).doi: 10.1785/gssrl.82.6.919Seismological Research Letters Volume 82, Number 6 November/December 2011 919