New developments in ambient noise analysis to characterise the seismic response of landslide-prone slopes

Abstract

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 <i>P</i> 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

Assessment of the Seismic Site Amplification in the City of Ivanec (NW Part of Croatia) Using the Microtremor HVSR Method and Equivalent-Linear Site Response Analysis

“…Geosciences 2019, 9,312 19 of 25 response peak amplification in an area mostly dominated by coarse-grained clastic sediments (soil category B based on EC8) and which could possibly be prone to topographic site effects. Clearly, site response peak amplifications are correlate better with VS30 ( Figure 11) and input seismic ground motions (0.09 g and 0.19 g) than HVSR peak amplitudes, although similar distribution may be observed when the three maps are compared to each other [22,23]. Soft soil can play a very important role in the amplification (or de-amplification) of ground motion when propagated from bedrock to the surface (Figure 12d).…”

“…These maps clearly distinguish two major seismic microzones: the highest site response peak amplifications that occurs in the alluvial basin of Bednja River (also covering the central city area) where soft soils such as sands and gravels are dominant (designated as soil category C based on EC8 V S30 classification), and the zone exhibiting a smaller site response peak amplification in an area mostly dominated by coarse-grained clastic sediments (soil category B based on EC8) and which could possibly be prone to topographic site effects. Clearly, site response peak amplifications are correlate better with V S30 (Figure 11) and input seismic ground motions (0.09 g and 0.19 g) than HVSR peak amplitudes, although similar distribution may be observed when the three maps are compared to each other [22,23]. Soft soil can play a very important role in the amplification (or de-amplification) of ground motion when propagated from bedrock to the surface (Figure 12d).…”

“…The seismic site amplification map for the city of Ivanec was derived from individual HVSR and site response analyses using a spatial interpolation method as applied for maps of fundamental frequencies ( Figure 6), sediment thickness ( Figure 9) and V S30 (Figure 11). Since the HVSR peak amplitude is only an indication of amplification in natural state and is related to impedance contrast between sediments and bedrock, the question remains as to the qualitative validity of HVSR peak amplitudes and differences between whole spectral HVSR amplitudes and seismic site response (amplification) for the local site when subjected to earthquake influence [21][22][23][24]. For these reasons, all three maps in Figure 13, i.e., the map of HVSR peak amplitude and maps of site response peak amplification for PGA ROCK = 0.09 g (95yrp) and 0.19 g (475yrp) are shown together to better perceive differences between HVSR peak amplitude and seismic site response (amplification).…”

“…The microtremor horizontal-to-vertical spectral ratio (HVSR) methodology expressed by the natural/fundamental soil frequency and HVSR spectral peak amplitude, introduced by Nogoshi and Igarashi [11] and extensively revised by Nakamura [12], has been used in numerous studies for estimating local seismic ground response, particularly in Slovenia, Italy, and Croatia [13][14][15][16][17][18][19][20]. The estimated HVSR site frequency and peak amplitude provides only an indication of the initial soil site frequency and amplification without earthquake activity [21][22][23][24]. One-dimensional (1-D) equivalent-linear (EQL) site response analysis estimates the effects of local soil conditions on ground shaking, based on one-dimensional wave propagation theory [5].…”

Characterization of the 3‐D fracture setting of an unstable rock mass: From surface and seismic investigations to numerical modeling

“…The interpretation of these data is not straightforward as for the cited nearly 2-D case studies, in which the vibration motion at the first resonance frequency (f1) was systematically found to have the highest spectral amplitude and to be oriented perpendicularly to the main fracture set separating the unstable and stable Lévy et al, 2010;Burjánek et al, 2012;Del Gaudio et al, 2013;Bottelin et al, 2013a]. The vibration directions are here complicated by the presence of two distinct unstable blocks, which are separated by a complex 3-D fracture setting and constrained to a sloping basal discontinuity.…”