We report on new developments in the application of ambient noise analysis applied to investigate the dynamic response of landslide-prone slopes to seismic shaking, with special attention to the directional resonance phenomena recognised in previous studies. These phenomena can be relevant for seismic slope susceptibility, especially when maximum resonance orientation is close to potential sliding directions. Therefore, the implementation of an effective technique for site response directivity detection is of general interest. In this regard methods based on the calculation of horizontal-to-vertical noise spectral ratio (HVNR) are promising. The applicability of such methods is investigated in the area of Caramanico Terme (central Italy), where ongoing accelerometer monitoring of slopes with different characteristics offers the possibility of validation of HVNR analysis. The noise measurements, carried out in different times to test the result repeatability, revealed that sites affected by response directivity persistently show major peaks with a common orientation, consistent with the resonance direction inferred from accelerometer data. In some cases such a directivity turned out parallel to maximum slope direction, but this cannot be considered a systematic feature of slope dynamic response. At sites where directivity is absent, the HVNR peaks do not generally show a preferential orientation, with rare exceptions that could be linked to the presence of temporarily active sources of polarised noise. The observed variations of spectral ratio amplitude can be related to temporal changes in site conditions (e.g. groundwater level/soil water content variations affecting P wave velocity and Poisson's ratio of surficial layer), which can hinder the recognition of main resonance frequencies. Therefore, we recommend conducting simultaneous measurements at nearby sites within the same study area and repeating measurements at different times in order to distinguish significant systematic polarisation caused by site-specific response directivity from polarisation controlled by properties of noise sources. Furthermore, an analysis of persistence in noise recordings of signals with systematic directivity showed that only a portion of recordings contains wave trains having a clear polarisation representative of site directional resonance. Thus a careful selection of signals for HVNR analysis is needed for a correct characterisation of site directional properties.
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