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(1996) formulation is based on just the fundamental frequencypeak, while HVSR in Christchurch often returns two peakscorresponding to resonant strata at different depths; and (2)the K g parameter is dependent on the square of HVSR amplitude,which is quite unstable as discussed before.Finally, the same technique based on S-transform, whichafter the L’Aquila 2009 earthquake did not point out significantevidence of nonlinearity, here shows clear signs of energyat frequencies lower than the fundamental one in the elasticdomain (softening nonlinearity) in the coda of accelerogramsfrom CBGS; at the same time it is possible to recognize hintsof hardening nonlinearity due to hysteretic dilatant behaviorof soils.Future research will include a second, more detailed mappingof soil frequency using HVSR and comparison betweenelastic and nonlinear behavior at all the accelerometric stations,including the recordings of the September 2010 Darfieldearthquake and the June 2011 Christchurch earthquake.ACKNOWLEDGMENTSMany thanks are due to the staff of Canterbury University(Christchurch) who helped with logistical assistance, insightfulfield trips, and stimulating discussions, and in particular toStefano Pampanin, Misko Cubrinovski, Tobias Smith, WengKam, and Umut Akguzel. Thanks to Rocco Ditommaso forthe S-transform calculations. The paper was prepared duringa stay at GFZ–Helmholtz Zentrum, Potsdam, and benefitedfrom comments from colleagues after a seminar presentation.REFERENCESBeroya, M. A. A., A. Aydin, R. Tiglao, and M. Lasala (2009). Use ofmicrotremor in liquefaction hazard mapping. Engineering Geology107, 140–153.Bonilla, L. F., R. J. Archuleta, and D. Lavallée (2005). Histeretic anddilatant behavior of cohesionless soils and their effects on nonlinearsite response: Field data observation and modeling. Bulletin of theSeismological Society of America 95, 2,373– 2,395.Bradley, B. A., and M. Cubrinovski (2011). Near-source strong groundmotions observed in the 22 February 2011 Christchurch earthquake.Seismological Research Letters 82, 853–865.Brown, L. J., and J. H. Weeber (1992). Geology of the Christchurch UrbanArea. Institute of Geological and Nuclear Sciences, Map 1, 1 sheet+ 104 pp. Lower Hutt, New Zealand: GNS Science.Chatelain, J.-L., B. Guillier, F. Cara, A.-M. Duval, K. Atakan, and theSESAME Working Group (2008). Evaluation of the influenceof experimental conditions on H/V results from ambient noiserecordings. Bulletin of Earthquake Engineering 6 (1), 33–74.Cubrinovski, M., and M. Taylor (2011). Liquefaction Map V.1.0 22Feb. 2001 earthquake, http://db.nzsee.org.nz:8080/en/web/chch_2011/geotechnical/-/blogs/liquefaction-map-drive-throughreconnaissance.Last accessed 19 September 2011.Di Giacomo, D., M. R. Gallipoli, M. Mucciarelli, S. Parolai, and S. M.Richwalski (2005). Analysis and modeling of HVSR in the presenceof a velocity inversion: The case of Venosa, Italy. Bulletin of theSeismological Society of America 95, 2,364–2,372.Nakamura, Y. (1996). Real-time information systems for hazard mitigation.In Proceedings of the 10th World Conference in EarthquakeEngineering, paper # 2134.Parolai, S., and S. M. Richwalski (2004). The importance of convertedwaves in comparing H/V and RSM site response estimates. Bulletinof the Seismological Society of America 94 (1), 304–313.Puglia, R., R. Ditommaso, F. Pacor, M. Mucciarelli, L. Luzi, and M.Bianca (2011). Frequency variation in site response as observedfrom strong motion data of the L’Aquila, 2009 seismic sequence.Bulletin of Earthquake Engineering 9 (3), 869–892; doi:10.1007/s10518-011-9266-2.Smyrou, E., P. Tasiopoulou, İ. E. Bal, and G. Gazetas (2011). Groundmotions versus geotechnical and structural damage in the February2011 Christchurch earthquake. Seismological Research Letters 82,882–892.Stockwell, R. G., L. Mansinha, and R. P. Lowe (1996). Localization ofthe complex spectrum: The S transform. IEEE Transactions onSignal Processing 44, 998–1,001.Toshinawa, T., J. J. Taber, and J. B. Berrill (1997). Distribution ofground-motion intensity inferred from questionnaire survey, earthquakerecordings, and microtremor measurements—A case studyin Christchurch, New Zealand, during the 1994 Arthurs Passearthquake, Bulletin of the Seismological Society of America 87 (2),356–369.Department of Structural Engineering, GeotechnicalEngineering, Engineering GeologyBasilicata UniversityViale dell’Ateneo Lucano, 1085100 Potenza Italymarco.mucciarelli@unibas.it926 Seismological Research Letters Volume 82, Number 6 November/December 2011