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▲▲Figure 5. Site N20 showing HVSR as a single, low frequency peak and site N25 with a similar peak plus another one at higher frequency.ment with those determined 17 years earlier provides an opportunityto test the K g coefficient on soil that has not changed itscharacteristics as assessed with microtremor measurements.The map of this coefficient is provided in Figure 6. Thedotted area in the figure indicates the extent of the most severeliquefaction episodes as reported by Cubrinovski and Taylor(2011). The lack of correlation is evident in the most affectedarea, where both high and low values of K g are present, andthe same happens in those zones with no evidence of liquefaction.More detailed analyses are needed, however, before rulingout the K g parameter, since the original formulation is basedon the idea that just one peak is visible in the HVSR curve,corresponding to the fundamental frequency. As stated above,our measurement sometimes returned two peaks correspondingto two resonant strata at different depths, so it would beimportant to have more data about the depth of the layer whereliquefaction occurred.Our third activity was about the possible difference insoil behavior between the elastic, weak-motion domain andthe strong-motion signals. In this paper I report preliminaryresults, since a comprehensive study on all the recording stationsis still underway.To check for variation of soil fundamental frequency thatcould be taken as a possible indication of nonlinear behavior,our research group has recently developed a technique that wasfirst applied to the recordings of the L’Aquila, 2009 earthquake(Puglia et al. 2011). The basic idea is the comparison betweenthe soil fundamental frequency estimated using the HVSRtechnique and the time-frequency behavior during strongmotion estimated using the S-transform technique (Stockwellet al.1996). This transform is particularly useful for analyzing asystem that changes its dynamic characteristics over time sinceit provides information about the local spectrum of a genericsignal overcoming the limitations derived from the assumptionsof the stationarity of a signal (as is the case for the shorttimeFourier transform).Using the same frequency scale it is possible to comparethe S-transform with the HVSR to verify whether the fundamentalfrequency obtained from the noise recording remainsconstant during the S-waves phase, until the coda-waves andto the end of the signal. The S-transform is normalized to themaximum of each time step. The normalized S-transformallows us to better identify the site fundamental frequency,while the standard S-transform is more useful for identifyingthe instant-by-instant frequency content of the signal, itschange over time, and the related energy.The example given here is for station CBGS at ChristchurchBotanical Gardens (Figure 7). The dashed red line highlights thecracks in the ground and the sand left by liquefaction, still clearlyvisible two months after the February 2011 quake. The acceler-Seismological Research Letters Volume 82, Number 6 November/December 2011 923