Detection of directivity in seismic site response from microtremor spectral analysis

Abstract

Abstract.Recent observations have shown that slope response to seismic shaking can be characterised by directional variations of a factor of 2-3 or larger, with maxima oriented along local topography features (e.g. maximum slope direction). This phenomenon appears influenced by slope material properties and has occasionally been detected on landslide-prone slopes, where a down-slope directed amplification could enhance susceptibility to seismically-induced landsliding. The exact conditions for the occurrence of directional amplification remain still unclear and the implementation of investigation techniques capable to reveal the presence of such phenomena is desirable. To this purpose we tested the applicability of a method commonly used to evaluate site resonance properties (Horizontal to Vertical Noise Ratio -HVNR or Nakamura's method) as reconnaissance technique for the identification of site response directivity. Measurements of the azimuthal variation of H/V spectral ratios (i.e. between horizontal and vertical component) of ambient microtremors were conducted in a landslide-prone study area of central Italy where a local accelerometric network had previously provided evidence of directivity phenomena on some slopes. The test results were compared with average H/V spectral ratios obtained for low-to-moderate earthquakes recorded by the accelerometric stations. In general, noise and seismic recordings provided different amplitudes of spectral ratios at similar frequencies, likely because of differences in signal and instrument characteristics. Nevertheless, both kinds of recordings showed that at sites affected by site response directivity major H/V peaks have orientations consistent (within 20 • -30 • ) with the direction of maximum shaking energy. Therefore, HVNR appears to be Correspondence to: V. Del Gaudio (delga@geo.uniba.it) a promising technique for identifying seismic response directivity. Furthermore, in a comparative test conducted on a slope mantled in part by a deep-seated landslide we detected spectral peaks with orientations close to the maximum slope direction, whereas no evidence of directivity was found outside the slide boundaries. This indicates the influence of the landslide body on seismic response directivity.

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

“…These new SSR results appear consistent with those obtained from the first ambient noise recording campaign carried out in July 2007 using a Tromino tomograph prototype (Del Gaudio et al, 2008). The azimuthal variation of HVNR values in Fig.…”

“…The presence of H /V peaks at site resonance frequencies is explained by assuming that horizontal and vertical components of noise wave field have a comparable amplitude at the substratum (within an approximation factor of 2) and that only horizontal components are significantly amplified by the effect of shallow layers. Some preliminary tests on landslide areas showed that an analysis of azimuthal variation of H /V spectral ratios can reveal the occurrence and orientation of directional resonance possibly related to sliding directions (Del Gaudio et al, 2008).…”

“…Del Gaudio et al (2008) considered the distribution of HVNR values as function of azimuth and frequency and proposed an approach based on the detection of multiple major peaks having coherent orientation (within 30 • ) and satisfying the following significance criteria:…”

Supervised Method of Landslide Inventory Using Panchromatic SPOT5 Images and Application to the Earthquake-Triggered Landslides of Pisco (Peru, 2007, Mw8.0)

“…The source magnitude controls the area affected by landslides [1] relating the ground motion decay with distance to the epicenter or to the fault plane (e.g., [3]). However this generic law is affected by different factors: (1) a significant over-abundance of landslides in the hanging walls of ruptured thrust faults relative to the footwalls rate are reported (e.g., [4,5]), showing the effect of the dynamics of the fault rupture on the landslide distribution; (2) the greater erosion rate due to precipitations observed after the earthquake [8,10] shows that earthquakes are found to interact non-linearly with precipitations; (3) the site effect, or amplification of the seismic waves due to shear-wave velocity contrast between the landslide material and the stable material [11,12] or topographical particularities like ridges [9]. This topographic control on co-seismic landslides has also been pointed out through different parameters: convexity [7], slopes, proximity from ridges and crests [6,9].…”

Seismic site response of unstable steep slope using noise measurements: the case study of Xemxija Bay area, Malta

“…Recordings of ambient noise and the use of the HVSR technique have recently had widespread use in studying landslides (e.g. Del Gaudio et al, 2008;Burjánek et al, 2010;Del Gaudio and Wasowski 2011;Burjánek et al, 2012).…”