Maresca, Rosalba

The study of ground motion amplification produced by surface geology is extremely interesting in the Benevento area, Southern Italy, as it is characterized by high seismic hazard. The present moderate-to-low seismicity makes the noise method appropriate to estimate the seismic site response in the area. The three components of seismic noise have been recorded in five sites in the Benevento metropolitan area characterized by different surface geology, in order to estimate the seismic site response. In evaluating site amplification effects we used the direct interpretation of amplitude spectra and standard spectral ratio techniques, evaluating sediment-to-bedrock, sediment-to-average and H/V spectral ratios. The temporal evolution of the noise spectra is analysed within one day, in order to assess the stationarity of the noise signal. The noise wavefield properties have been studied through polarization analyses in selected bands of frequency, where spectral peaks are observed to dominate, to better understand the real nature of those peaks. Results give evidence of low amplification levels, missing any correlation between spectral amplitudes and sediment thickness over the basement. We interpret this result as due to the poor impedance contrast between sediments and basement, which is characterized by low values of shear waves velocity. Moreover, sharp amplitude peaks are observed in the raw spectra of the sediment-sites, in the 2–4 Hz frequency band; a numerical simulation interprets this effect as possibly associated with a wide-scale structure, invoking the presence of a sharper impedance contrast at greater depth. At high frequencies the action of ambient noise sources, mainly active on horizontal components of motion, is retained dominant to generate the prominent peaks observed in the H/V spectral ratios; in some cases the presence of a nearsurface low-velocity layer can contribute to amplify the seismic motion generated at these frequencies.

Local microearthquakes were used to estimate site effects and source dynamic-scaling characteristics at Mt. Vesuvius, Italy. The selected data set is composed of low magnitude events (1.1 £ Md £ 3.6) recorded in 1996 and 1999 by nine digital shortperiod (1-Hz) seismic stations. Site response was evaluated by analysing data with three different approaches: 1) spectral ratios method of S-waves with respect to the average amplitude spectrum; 2) generalized inversion for site and source from the S-waves; and 3) generalized inversion from the coda waves. The results obtained with all three methods showed amplification of a factor of 1.5-2.5 in the 8-14 Hz frequency band for BKE and SGV sites and an amplification of 3 in a narrow band around 8 Hz for the CDT site. Method 2 allowed simultaneous determination of the source spectral shape for each earthquake. By assuming an w2 source model, we estimated the seismic moment Mo and corner frequency fc. The results show that most of the selected earthquakes are characterized by stress drops of 10 bars. The present results are encouraging for further investigation into the techniques for site-effect evaluation and for improving our knowledge of the scaling law of the source spectrum at Mt. Vesuvius.

Noise measurements were recorded using a dense short-period seismic array in Terzigno (Naples), a town that is located about 6 km from the Vesuvius crater. The aim of this study was to calculate a surface velocity model of the area under investigation through the application of the Spatial Autocorrelation (SPAC) method, with the hypotheses that ambient noise is stationary both in time and space, and that it is composed of surface dispersive waves. The correct knowledge of the surface structure is an important goal in site-effects studies. Correlation coefficients were calculated as functions of the azimuth on noise recorded at pairs of equally spaced stations in the frequency range of 1-8 Hz. Then, the spatial average correlation coefficients were compared to estimates over long-term recordings. The results appear to validate the hypothesis that ambient noise can be considered as a stochastic process. The correlation-frequency curves have been fitted to Bessel functions, from which the Rayleigh wave dispersion curve has been calculated. A velocity model has been derived from the dispersion curve using both trial and error and a standard inversion procedure. The results are consistent with those obtained from array measurements in the area in other studies (Scarpa et al., 2003).

Ambient vibrations were recorded by two dense short-period seismic arrays located on Mount Vesuvius. The aim of this study is to derive the surface velocity structure through the application of array techniques and to interpret it in terms of resonance effects. Mount Vesuvius, which is located in proximity to the Apennines earthquake source-zone, is one of the most dangerous volcanoes of the world. We compared different array techniques to derive the dispersive properties of the surface waves composing the noise wave field. The frequency-wavenumber (f-k) spectral method applied to the data recorded by array A furnished the Rayleigh waves and Love waves dispersive functions and proved the time-space stationarity of the noise wave field. The existence of a stochastic wave field justifies the use of the spatial autocorrelation (SPAC) method and the time stationarity autocorrelation (TSAC) method, which appear to be most effective at the lowest frequencies. The TSAC method, which time averages, is confirmed as the most user-friendly, because it does not require circular geometries. The knowledge of the surface structure is an important goal in site-effects studies and is used to determine the frequency response of the near-surface geology. Velocity models have been derived from the dispersion curves, using both trial and standard inversion procedure. The results are compared with those obtained from array measurements in adjacent areas (Scarpa et al., 2003), revealing similar mechanical properties in the cover deposits and heterogeneities down to 40 m of depth. The shear-wave velocity profiles derived at the array-sites are used to model the 1D transfer functions for vertically incident shear waves. The fundamental resonance frequencies agree with the peak frequencies observed in the microtremor horizontal-to-vertical spectral ratios. The frequency band of amplification is also compatible with the results obtained from local earthquakes at sites located at the same elevation on Mount Vesuvius (Galluzzo et al., 2009).

The Colfiorito basin is located at the boundary between Umbria and Marche districts in Central Italy. The area was struck by several strong earthquakes in the past, and during the September 1997 to April 1998 seismic sequence, the macroseismic intensity was as large as IX (MCS scale) in the region. Ambient noise measurements are carried out over fifty points in the basin; H/V spectral ratios as well as spectral ratios using a reference rock site are calculated. Polarization analyses are also performed in narrow frequency bands where spectral amplitude peaks are observed to dominate, to discriminate preferential directions of propagation in the noise wave field which could be related to source or path effects, other than site effects. In the spectral ratios, the frequencies of the most pronounced peaks generally agree with the resonance frequencies computed for the measurement sites on the basis of 1-D velocity models, at least where no strong lateral variations of the sediment thickness are present.

Abstract In this study, we describe two experiments of seismic noise measurements carried out in Naples, Italy. The site allowed measurements to be obtained both at the surface and in a tunnel that is 120-m-deep. The main goal was to compare the seismic response evaluated at the surface to the in-tunnel response, through spectral, polarization, and resonance directivity analyses. In the 1 to 20 Hz frequency band, the noise level was up to 15 dB higher at the surface than in the tunnel. The polarization properties and horizontal-to-vertical spectral ratios appear not to be influenced by the tunnel geometry or by the topography. Some preferential alignments were observed in the polarization azimuths computed at the surface, which are likely to be due to local sources, rather than morphological features. The absence of directivity effects and the low noise levels in the tunnel make this site suitable for installing seismic stations. We also studied how the subsoil structure affects the seismic motion at the surface. The dispersive properties of the Rayleigh waves were investigated using the spatial autocorrelation method. A joint inversion of the dispersion data and the horizontal-to-vertical spectral ratios provided the subsurface Vs profile. The derived model has a low velocity contrast at depth, such as to generate moderate and broad H/V spectral ratio peak amplitude. The normalized spectral ratio appears more appropriate to identify the soil-resonance frequencies.

The most recent observations of well correlated seismic phases in the high frequency coda of local earthquakes recorded throughout the world are reported. In particular the main results, obtained on two active volcanoes, Teide and Deception, using small array are described. The ZLC (Zero Lag Cross-correlation) method and polarization analysis have been applied to the data in order to distinguish the main phases in the recorded seismograms and their azimuths and apparent velocities. The results obtained at the Teide volcano demonstrate that the uncorrelated part of the seismograms may be produced by multiple scattering from randomly distributed heterogeneity, while the well correlated part, showing SH type polarization or the possible presence of Rayleigh surface waves, may be generated by single scattering by strong scatterers. At the Deception Volcano strong scattering, strongly focused in a precise direction, is deduced from the data. In that case, all the coda radiation is composed of surface waves.

The article deals with the results of extensive surveys conducted in the town of Avellino, in southern Italy, with the aim of studying the site seismic response. Avellino is a town located in the Apennine Chain in Irpinia, which is a region characterized by a high seismic hazard. Several strong earthquakes hit the town in the past, the last of which occurred on 23 November 1980 (Mw = 6.8). In the last decades since that event, background low magnitude seismicity persists, including few moderate earthquakes. In order to analyse the site seismic response, our data set was made up by borehole and downhole records, ambient noise measurements both in free-field and inside buildings, and macroseismic data related to the 1980 Irpinia-Basilicata earthquake. With the intention of ascertaining the occurrence of resonance effects influencing the distribution of the earthquake damage, we performed a correlated analysis of all the data acquired. HVSR free-field peak frequencies, joined with the data obtained from previous surveys [22], agree with the computed 1D resonance frequencies and are in the range between 1.6 and 13.0 Hz. The resonance frequencies for fifteen typical buildings in Avellino, which were partly estimated from ambient noise measurements and partly by applying the Italian regulation code, are in the range between 1.2 and 4.6 Hz, so showing that soil-structure resonance effects can be generated in a wide area of the town. Finally, we drew up a detailed damage map, related to the 1980 earthquake, which affected Avellino seriously. From our research two aspects come to light. The first regards the fact that soil-structure resonance effects can be generated in the town and were possibly associated to some buildings damaged by the 1980 earthquake. The second concerns the circumstance that the amplitudes of HVSR peaks correlate well with the rock/soil velocity contrast at depth, but do not show a relationship with the earthquake damage pattern. The results of this study will be useful in view of putting into the field suitable risk mitigation countermeasures.

In the text

The Colfiorito Basin is a small intramountain depression in the southern section of the Northern Apennine chain that is filled with Quaternary alluvial deposits. The presence of soft alluvial deposits has significantly influenced the level of local damage that was caused by two major earthquakes (ML 5.6 and 5.8) belonging to the swarm that started in September 1997. To verify the effects of the basin structure on the predominant frequency of seismic motion, ambient noise measurements were carried out in the Colfiorito Basin during two experiments in May and July of 2002. The horizontal-to-vertical spectral ratios (HVSRs) were calculated for data collected at four profiles in the basin. Array techniques were applied to determine the wave types that composed the noise, to estimate their apparent velocity and azimuth of propagation, and to calculate a velocity-dispersion curve from which a velocity-depth structure was derived. The data analysis shows a high amplification in the HVSR at low frequency. This feature is common to most of the sites, including the reference site, and it is interpreted as being due to weather disturbances. The peak frequencies of the spectral ratio calculated at the sites located in the center of the basin coincide with the theoretically estimated resonance frequencies. The arrayaveraged HVSR calculated for the array located in the middle of the plain has a pronounced peak at 0.9 Hz. This corresponds to the peak of the amplification function calculated on the basis of the velocity model deduced from the dispersion analysis. The HVSR method is instead unsuitable for the prediction of the resonance frequencies of sediments in the sites where strong lateral variations of basement topography are present. We measured apparent velocities in the range of 0.3–0.8 km/sec by applying f-k methods to array recordings. These values are compatible with the predominance of surface waves in the noise, as also confirmed by polarization analysis. Both Rayleigh and Love waves are present in the background seismic noise. The results obtained by applying the spatial autocorrelation method to the vertical component of the ground motion recorded at a 240-m-wide circular array deployed in the middle of the basin revealed the presence of Rayleigh waves, and f-k methods combined with polarization techniques revealed the presence of polarized Love waves. The wave-field analysis indicates two main propagation directions: the first is around N100 E in the frequency band of 1.0–2.0 Hz; this radiation can be interpreted as being generated at the east-southeast step borders of the basin. The second main direction is around N300 E in the frequency band of 2.0–3.0 Hz; its source may be a 180-m-deep depression located at the southwest corner of the basin.