Here - Stuff

▲▲Figure 8. Comparison between normalized S-transform and HVSR at CBGS (see text for details).ometer is located inside the building, directly on the concretefoundation slab visible outside the door and under the bench. Inaddition to cracks and liquefaction, a settlement of the stationoccurred with a few centimeters displacement. This place is theideal candidate for strong nonlinear effects, which are indeedvisible in Figure 8. Figure 8 includes information derived fromaccelerometric and noise recordings, showing the accelerometricrecording on the top left, the S-transform on the bottom left,and the HVSR on the bottom right panel; most of the energyof the largest horizontal component of motion is at frequencieslower than the fundamental one determined by HVSR. Itis also worth noting that between 15 to 20 s, the time-domaintrace and the S-transform show high-frequency accelerationevidence of hardening nonlinearity of the kind first describedby Bonilla et al. (2005), due to hysteretic dilatant behavior ofnon-cohesive, partially saturated soils (for more details on accelerometricrecordings and liquefaction studies, see Bradley andCubrinovski (2011, page 853 of this issue) and Smyrou et al.(2011, page 882 of this issue).CONCLUSIONSDuring a quick field survey in Christchurch after the February2011 earthquake it was possible to collect microtremor measurementsclose to accelerometric stations that recorded theevent in the most damaged areas and with overall good spatialcoverage. This allowed us to compare current measurementswith the microzonation map produced by Toshinawa et al.(1997), and in particular with their map of HVSR frequenciesand peak values. There is a general agreement in the frequencymap, with frequency decreasing from the western part of thecity moving toward the estuarine area. The main differencewith the previous study is that we were able to identify frequenciesbelow 1 Hz that returned the maximum HVSR amplitudein the ocean coastline area but were also visible in other partsof the city. Another difference is that the HVSR amplitudemap always provided higher values in this study with respect toToshinawa et al. (1997).Christchurch was affected by severe and widespread soilliquefaction. The fact that the soil frequency remained stablewith respect to measurements performed before the ongoingearthquake sequence prompted us to check the reliability ofthe soil vulnerability index K g introduced by Nakamura (1996)for estimating earthquake damage on the ground surface andpreviously used to map the soil susceptibility to liquefactionbut never tested after an earthquake. No clear correlation hasbeen found in this study between K g and the occurrence of liquefaction.Possible explanations for this are (1) the NakamuraSeismological Research Letters Volume 82, Number 6 November/December 2011 925