Stefano Parolai

ASSESSMENT OF THE NATURAL FREQUENCY OF THE SEDIMENTARY COVER IN THE COLOGNE AREA (GERMANY) USING NOISE MEASUREMENTS

Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique.

Denoising of Seismograms Using the S Transform

The growing number of urban seismological experiments within the framework of seismic hazard studies has increased the necessity of effective tools for denoising seismograms. Because of the frequency dependency of seismic noise and the nonstationarity of the recorded signal, tools that can effectively take into account the frequency-time variation of the seismic recordings are more suitable for fulfilling this task. The S transform is an invertible time-frequency spectral localization technique that combines elements of wavelet transforms and short-time Fourier transforms. In this study, a customized thresholding technique is applied to the S -transform coefficients for obtaining an optimally (in the sense of the maximum increase of the signal-to-noise ratio with a minimal loss of information) denoised seismogram. Tests performed with synthetic data allow us to calibrate the optimal denoising procedure parameters and show the effectiveness of the proposed method when compared with standard filtering techniques. The application of a combination of denoising with time-frequency filtering on real data shows the potential of the method for extracting lower amplitude dispersive arrivals from seismograms.

SITE EFFECTS BY H/V RATIO: COMPARISON OF TWO DIFFERENT PROCEDURES

In this article, H/V ratio are evaluated using different spectral techiniques applied to both earthquake and microtremor data (Nakamura technique). In particular, in order to avoid numerical instability, two different numerical techiniques are taken into account: (a) a smoothing procedure applied to the spectra of the seismogram components and (b) a regularization method applied to the H/V ratio (Landweber scheme). The data set consists of more than 70 earthquake events recorded by three component sensors displaced in the town of Fabriano (Central Italy) during the Umbria-Marche sequence started on September 1997. The local magnitudes range between 2.7 and 4.4, while the epicentral distances range between nearly 30 and 60 km. The stations were set to continuous recording so that a huge amount of microtremors was stored. The results are compared in terms of predominant frequencies and amplification levels in order to point out the influence of the adopted methods. The H/V ratio provides similar results if applied to a smoothed version of both earthquake and microtremor spectra, confirming that Nakamura technique is a cheap and a fast method to collect information on the site amplification effects. Moreover, the results relevant to earthquake data seem not to depend on the method used to stabilize the H/V ratio, whereas those relevant to microtremor data does. The explanation of this fact is suggested by the behaviour of the Landweber filter showing that the predominant frequency detected by means of microtremor data lies in a high instability region of the spectra.

Application of the Generalized Inversion Technique (GIT) to a Microzonation Study: Numerical Simulations and Comparison with Different Site-Estimation Techniques

The reliability of the results obtained by applying the Generalized Inversion Technique (GIT) to site response estimates is assessed by means of numerical simulations. This study was developed in the framework of the UMBRIA-MARCHE microzonation project. The analysis with synthetic data demonstrates that the generalized inversion technique is capable of revealing the site effects. Furthermore, this approach shows that a priori information relevant to the Q quality factor is needed if a source estimate is required. We also analyze data recorded by five seismological stations installed in Fabriano (central Italy) with a minimum interstation distance of 100 m. The results computed with the generalized inversion technique are compared with those obtained by the reference site method and the receiver function technique. Whereas the first two methods provide highly consistent site-response estimates, the receiver function technique provides lower levels amplification. Furthermore, the time duration used in the analysis seems to be a critical parameter for the receiver function technique alone. Manuscript received 15 April 1999.

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.

Statistical Analysis of Noise Horizontal-to-Vertical Spectral Ratios (hvsr)

Statistical properties of the horizontal to vertical spectral ratios (hvsr) applied to noise recording are analyzed in order to define optimal strategies for numerical processing and identification of possible artifacts. To this purpose, two time series have been analyzed: one constituted by environmental seismic noise in the presence of a genuine physical signal and one relative to pure instrumental noise, both obtained with the same experimental apparatus. By means of suitable statistics, some guidelines for the hvsr analysis are provided. A statistical test proposed by Albarello (2001) for the identification of artifacts in the hvsr function has been analyzed and invalidated.

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.

Influence of Soil-Layer Properties on k Evaluation

In this article, we investigate, by means of simulations, the reliability of the attenuation parameter k when it is evaluated in the presence of site amplification effects. We consider a single soil layer overlying the bedrock, and we assume that k describes the attenuation due to propagation through the soil layer. We generate synthetic spectra for events with magnitudes from 2 to 6, taking into account different soil-layer properties. In particular, we consider 1D site transfer functions that account for fundamental resonance frequencies varying from 1 to 12 Hz and S -wave velocities ranging from 360 to 1000 m/sec. A least-squares fit to determine the slope of the high-frequency decay is performed over spectral windows having different widths. The results show that site resonance at both intermediate (4 Hz ≤ f 0 f 0 ≥ 9 Hz) frequencies leads to unreliable k estimates due to the spectral windows selected to perform the least-squares fit not being wide enough to average out local peaks resulting from site amplification effects. This is an important result because, in the analysis of weak motion data, the exploitable frequency band is usually no wider than those adopted in this work. The results relevant to the simulations that consider high fundamental resonance frequencies, close to or inside the spectral windows adopted to perform the fit, indicate that the contribution of k to the high-frequency decay can be completely masked even if an excellent fit is obtained. In these cases, the soil properties estimated from k values may be unreliable, especially when high impendance contrasts exist. For fundamental resonance frequencies well below the frequency range selected for least-squares fitting, fairly good results can be obtained.

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.

GFZ Wireless Seismic Array (GFZ-WISE), a Wireless Mesh Network of Seismic Sensors: New Perspectives for Seismic Noise Array Investigations and Site Monitoring

Over the last few years, the analysis of seismic noise recorded by two dimensional arrays has been confirmed to be capable of deriving the subsoil shear-wave velocity structure down to several hundred meters depth. In fact, using just a few minutes of seismic noise recordings and combining this with the well known horizontal-to-vertical method, it has also been shown that it is possible to investigate the average one dimensional velocity structure below an array of stations in urban areas with a sufficient resolution to depths that would be prohibitive with active source array surveys, while in addition reducing the number of boreholes required to be drilled for site-effect analysis. However, the high cost of standard seismological instrumentation limits the number of sensors generally available for two-dimensional array measurements (i.e., of the order of 10), limiting the resolution in the estimated shear-wave velocity profiles. Therefore, new themes in site-effect estimation research by two-dimensional arrays involve the development and application of low-cost instrumentation, which potentially allows the performance of dense-array measurements, and the development of dedicated signal-analysis procedures for rapid and robust estimation of shear-wave velocity profiles. In this work, we present novel low-cost wireless instrumentation for dense two-dimensional ambient seismic noise array measurements that allows the real–time analysis of the surface-wavefield and the rapid estimation of the local shear-wave velocity structure for site response studies. We first introduce the general philosophy of the new system, as well as the hardware and software that forms the novel instrument, which we have tested in laboratory and field studies.