Sandra M. Richwalski

Analysis and Modeling of hvsr in the Presence of a Velocity Inversion: The Case of Venosa, Italy

The aim of this work is to check the stability of the horizontal-to-vertical spectral ratios (HVSRs) calculated at the Venosa station site (Italy). This site lies over a layer of anthropogenic fill (4 m thick), a rigid layer of conglomerates (15 m thick), and a thick layer of clays (about 300 m thick) above the seismic bedrock. The velocity inversion, which takes place at the conglomerates-clays interface, is of main importance for the amplification behavior of this site. We have analyzed nearly 2 years of data, composed of 244 triggered noise records and 44 earthquakes. The results obtained by the two data sets show different site-response characteristics. In particular, the earthquake HVSR is not deamplified in the frequency range 1-8 Hz like the triggered noise HVSR. To find out the origin of this difference, we modeled both the triggered noise and the earthquakes, taking advantage of an improved ver- sion of the Thompson-Haskell propagation matrix method. The differences between triggered-noise- and earthquake-amplification functions might be explained by the difference in composition and propagation of the seismic wave fields. Moreover, we show that the nonlinear behavior of the anthropogenic fill might explain the presence of the misfit of the resonance frequency attributed to this layer between triggered noise and earthquakes.

The Importance of Converted Waves in Comparing H/V and RSM Site Response Estimates

The application of the horizontal-to-vertical (H/V) spectral ratio technique to earthquake recordings can lead to significant differences in the estimate of the site response compared to that produced by the reference-site method (RSM). In particular, whereas the estimates of the resonance frequency from the two methods are fairly consistent, the levels of amplification are not. Using numerical modeling based on an improved Thompson–Haskell propagator matrix method, we were able to isolate the contribution of pure and converted waves to the site response. We show that the conversion of body waves at the sediment–bedrock interface leads to differences in the site response estimates obtained by the H/V method and the RSM. Such differences are consistent with observations in the field. In particular, the lower level of amplification obtained by the H/V method at frequencies higher than the fundamental one is due to a transfer of energy onto the vertical component caused by S - to P -wave conversion. Applying the RSM technique to the vertical as well as the horizontal component of the P -wave window, we obtain stable estimates of the fundamental resonance frequency that are consistent with 1D resonance for vertically incident P and S waves, respectively. The amplification from the P -wave window of the horizontal component might be considered as a lower boundary.

Rayleigh wave dispersion curves from seismological and engineering-geotechnical methods: a comparison at the Bornheim test site (Germany)

Active and passive procedures for estimating the local seismic response from surface-wave measurements are compared for a test site in the Bornheim area (Germany), where independent geophysical and geological information is available. Recording was done using geophones, as well as seismometers, in various configurations. Five popular and standardized techniques were used for analysing the data: multichannel analysis of surface waves (MASW), the refraction microtremor technique (ReMi), the extended spatial autocorrelation technique (ESAC) and frequency–wavenumber analysis (beam-forming and maximum likelihood methods). The resulting surface wave dispersion curves are largely consistent, but differ in their respective low-frequency ranges due to the resolving capabilities of the respective acquisition geometries. Two joint inversions of dispersion and H/V curves, one for the lower frequency range (2.3–9.2 Hz) and the other for the complete range (2.3–45 Hz) of the dispersion curves resulted in fairly similar S-wave profiles, but increasing the frequency range allowed better estimates for the lower velocities at shallow depths. The results also compare well with borehole information. The site responses obtained from the two S-wave profiles are very similar, even at higher frequencies. The use of combined procedures (geotechnical-engineering and seismological) allows a high quality estimation of the S-wave velocity structure to be obtained, both at shallow and large depth. However, if a combined approach is not possible, for site response estimation at sites with sedimentary cover thicker than 30 to 50 m and where knowledge of the average S-wave velocity is more important than higher resolution estimates at shallower depths, the use of passive seismological 2D arrays is strongly recommended.

Comparison of Empirical and Numerical Site Responses at the Tito Test Site, Southern Italy

The town of Potenza (Southern Italy) is one of the test sites for preparing ground-motion scenarios within the framework of the Italian Dipartimento Protezione Civile-Instituto Nazionale di Geofisica e Vulcanologia (DPC-INGV) 2004-2006 pro- jects. An area in the neighboring village of Tito was selected to evaluate different techniques for estimating site effects involving a 40-m-deep instrumented borehole. This two-sensor vertical array records teleseismic, regional, and local seismicity. Close to the borehole, three seismological microarrays (utilizing short-period sen- sors and digitizers with a high dynamic range) were installed in May 2005 to record seismic noise. Differing acquisition geometries allowed the checking of any depen- dency in the derived dispersion curves based on the adopted analysis method (ex- tended spatial autocorrelation (ESAC) and frequency wave-number (F-K)). In gen- eral, the ESAC method appears to provide more reliable results in the low-frequency range. Furthermore, the soil-velocity profiles obtained from the microarray data were compared with the S-wave velocity profile derived from down-hole measurements. A good agreement was observed in the depth range well constrained by the data. Finally, empirical site responses were compared with those calculated numerically from the S-wave velocity profiles obtained from the microarray data. Although this comparison did not resolve a preference among the derived models, it showed the importance of downgoing waves in modifying the site response at the Tito site.

Multi-component wavefield separation applied to high-resolution surface seismic data

Abstract A detailed characterisation of the subsurface using high-resolution multi-component seismic data requires additional processing. We adapted a frequency-domain wavefield separation method and applied it in an iterative way to subsets of a nine-component CMP survey recorded in The Netherlands. The method is well suited to remove the Rayleigh-wave. Further decomposition of the data in the case of two-component wavefield separation made it possible to group P- and S-wave sections and attempt an interpretation of events based on zero-offset arrival times, computed from existing velocity–depth profiles. Three-component separation failed beyond the extraction of the Rayleigh wave because events were not sufficiently present on all receiver components to allow for a description in terms of all polarisation parameters. The absence of the Love wave due to subsurface conditions simplified the processing of the data subset, with source and receivers oriented in a crossline-horizontal direction, to a depth converted stack, which agrees with results found earlier. The detected dipping of a peat layer was later confirmed by cone penetration tests (CPT).