Ambient Noise Measurements following the 2011 Christchurch Earthquake: Relationships with Previous Microzonation Studies, Liquefaction, and Nonlinearity

Abstract

Following the Christchurch 2011 earthquake, the Basilicata University (Potenza, Italy) organized a field trip to New Zealand mainly to examine structural engineering issues but also to investigate the similarity between this event and the L'Aquila 2009 quake that struck central Italy. In both cases an event with magnitude slightly above 6 occurred on a blind fault underlying an area inhabited by a population of the order of hundreds of thousands, killing a few hundred people and severely damaging the city center, and in both cases a site amplification study was available before the event. At the same time there were striking differences between the two earthquakes in maximum recorded acceleration, the nonlinear behavior of soils, and the occurrence of liquefaction. It was also an opportunity to look at some issues related to the use of microtremor measurements, in particular: 1. to verify if the soil frequencies estimated more than 15 years ago by Toshinawa et al. (1997) are a persisting feature or if there were changes following the strong motions in 2010 and 2011; 2. to verify the usefulness of the soil vulnerability index proposed by Nakamura (1996) as a proxy of liquefaction susceptibility; and 3. to compare the strong-motion recordings with elastic limit soil behavior derived from ambient noise, looking for hints of hardening nonlinearity as proposed by Bonilla et al. (2005) and similarity with the observations in L'Aquila (Puglia et al. 2011). In 1994 the Arthurs Pass Earthquake ( Ml = 6.6) occurred about 100 km northwest of Christchurch. The macroseismic intensity was estimated for the city together with local site amplifications inferred from seismic recordings and microtremors (Toshinawa et al. 1997). The authors found a satisfactory correlation among the results of the different techniques and prepared a microzonation map. As for horizontal-to-vertical spectral ratio (HVSR) analysis of microtremors, Toshinawa …