Marco Mucciarelli

RELIABILITY AND APPLICABILITY OF NAKAMURA'S TECHNIQUE USING MICROTREMORS: AN EXPERIMENTAL APPROACH

Nakamuras technique has been widely discussed from the theoretical point of view. Very little information is available as far as the experimental, on-the-field standpoint is concerned. A series of experiments under controlled conditions were set up to assess the reliability and applicability range of this microtremor technique. Since this approach to microzonation is particularly appealing in urban environments, great care was devoted to evaluate the influence of factors such as anthropic noise, concrete or asphalt coverings and soil-structure interaction. Environmental factors (wind) and the influence of sensors were also considered. The results were compared with the seismic response from four earthquakes. The set of experiments shows that under proper measurement conditions Nakamuras technique provides stable and reliable results, but great care has to be devoted to avoid factors that may significantly and adversely affect the results.

The Stability of the Horizontal-to-Vertical Spectral Ratio of Triggered Noise and Earthquake Recordings

The aim of this work is to analyze the stability of the amplification function obtained by the horizontal-to-vertical spectral ratio (HVSR) for a sedimentary site with a simple geomorphological situation. We have estimated the stability of the HVSR analyzing two years of data, composed of 674 triggered noise records (man-made seismic waves strong enough to reach the threshold set for earthquake detection) and 132 earthquakes (local, regional, and teleseismic events). The resonance peaks obtained with the two different data sets converge on an average both in frequency and amplitude. We examined and rejected the possible presence of periodicity of the fundamental frequency in the time sequences and its relevant amplification obtained by the HVSR of both triggered noises and earthquakes. Then, we performed a correlation analysis between these sequences and other parameters. In particular they have been correlated with signal amplitude, rainfall, and magnitude (for earthquakes only). A weak, negative correlation has been estimated between the rainfall and fundamental frequency and between the fundamental peak9s amplification and magnitude. Finally, we validated the HVSR transfer function with a 1D model using a V s profile obtained with the noise analysis of surface wave technique (Louie, 2001).

Comparison of site classification from VS30, VS10, and HVSR in Italy

The aim of this work is to verify if (1) the average of shear-wave velocity from the surface to 30 m depth (VS30) is a good proxy of site amplification in a country with complex geology like Italy, (2) the grouping of VS30 in different soil classes and relevant spectra in the Italian seismic codes is adequate, and (3) shallow shear-wave profiles (VS10) could be a more economical tool for site classification. To answer the first two questions, we examined 40 estimates of VS30 derived from microzonation projects we performed in Italy (27 obtained with downhole measurements, plus 13 ve- locity profiles obtained with surface techniques). In all the sites, we installed seismic instrumentation to record earthquakes and to estimate site response using horizontal to vertical spectral ratios (HVSR). The comparison between HVSR and VS30 showed in about one-third of the sites that VS30 is not a good proxy of observed amplification effects if the site does not have a monotonically increasing velocity profile. The reason VS30 does not provide satisfactory estimates in Italy is linked to peculiar geological settings that are widespread in the country. We then compared the observed ampli- fications from earthquake HVSR data with the ones provided by the Italian seismic code, noting a substantial underestimation by the code, a somewhat unsettling situa- tion because HVSR is usually considered to be a lower bound for amplification es- timated with other techniques. Finally, we studied 45 VS downhole profiles to 30 m depth performed also at sites where earthquake recordings are not available. On this data set, we noticed that VS10 could predict site classification with the same performances of VS30. We consider al- ternative soil classification schemes that include soil frequency besides the velocity profile. In this two-parameter approach, VS10 could be substituted for VS30.

Deformation pattern in the central Mediterranean and behavior of the African/Adriatic promontory

Abstract The Adriatic platform has been described in the literature both as an Africa promontory, moving in close connection with the main continent and as an independent microplate. This evident incongruity is mainly due to the wide spectrum of kinematic hypotheses proposed on the basis of paleomagnetic data and to the fact that the Adriatic-Africa transition is not marked by any clear decoupling fracture or by any interruption of lithological facies along the marginal belts (Apennines-Maghrebides and Dinarides-Hellenides). In this work it is argued that the counterclockwise rotation of the Adriatic plate, driven by Africa pushing beneath the Calabrian Arc and southern Tyrrhenian, may coherently account for all major Plio-Quatemary events in the central Mediterranean.

Analysis of RC Building Dynamic Response and Soil-Building Resonance Based on Data Recorded during a Damaging Earthquake (Molise, Italy, 2002)

During the 2002 seismic sequence in Molise (Italy), the town of Bonefro suffered moderate damage ( I MCS = VII) except for two reinforced concrete (RC) buildings. These buildings are located on soft sediments, close to each other and very similar in design and construction. The main difference is the height: the most damaged one (European Macroseismic Scale damage 4) has four stories, whereas the less damaged (EMS damage 2) has three stories. The M 5.4 shock on 31 October damaged both of them. The second shock on 1 November ( M 5.3) increased the damage on the four-story building substantially, just while a 5-min. seismic recording was taken. We analyzed the recorded data by four different techniques: short-time fourier transform (STFT), wavelet transform (WT), horizontal-to vertical spectral ratio (HVSR), and horizontal-to-vertical moving window ratio (HVMWR). All the results agree upon the estimate of the main building frequency before the second shock and upon the shift of frequency due to damage. All the fundamental frequencies (pre-, during, and postdamage) are in the range 2.5-1.25 Hz. The fundamental frequency of the less damaged building was estimated at about 4 Hz. To test if the soil-building resonance effect could have increased the damage, we also evaluated the soil fundamental frequency by three different techniques: noise HVSR, strong motion HVSR of seven aftershocks, and 1D modeling based on a velocity profile derived from noise analysis of surface waves (NASW) measurements. The results are again in good agreement, showing that resonance frequencies of the soil and of the more damaged building are very close.

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.

Estimates of Site Seismicity Rates Using Ill-defined Macroseismic Data

A new approach to the problem of site seismic hazard analysis is proposed, based on intensity data affected by uncertainties. This approach takes into account the ordinal and discrete character of intensities, trying to avoid misleading results due to the assumption that intensity can be treated as a real number (continuous distribution estimators, attenuation relationships, etc.). The proposed formulation is based on the use of a distribution function describing, for each earthquake, the probability that site seismic effects can be described by each possible intensity value. In order to obtain site hazard estimates where local data are lacking, the dependence of this distribution function with the distance from the macroseismic epicenter and with epicentral intensity is examined. A methodology has been developed for the purpose of combining such probabilities and estimating site seismicity rates which takes into account the effect of uncertainties involved in this kind of analysis. An application of this approach is described and discussed.

An attenuation study using earthquakes from the 1997 Umbria-Marche sequence

We studied spatial and temporal characteristics of seismic attenuation inCentral Italy using S- and coda- waves recorded by the MarchesanSeismograph Network from earthquakes located in the epicentral area ofthe 1997 Umbria-Marche sequence. The amplitude decay of the S waveswith distance was defined calculating empirical attenuation functions at 15frequencies between 1 and 25 Hz. We analyzed separately foreshocks andaftershocks and we found the same attenuation functions, suggesting thatthe possible temporal variations could be confined in a small area. Thefrequency dependence of QS was approximated by the equation QS=18 · f2.0between 1 and 10 Hz. At higher frequencies (10–25 Hz), the frequencydependence of Qs weakens, having an average value of QS=990. We also estimated Q from coda waves (QC) using the single-scattering models of Aki andChouet (1975) and Sato (1977). We found that QC=77 · f0.6, (between 2 and 20Hz) at the western side of the mountain chain, using either foreshocks oraftershocks. This relation is consistent with previous estimates of QCreported for the Central Apennines. For a volume sampling the Colfioritobasin, the Apennines and the Marche region we found that QC=55 · f0.8,indicating highattenuation below the mountain belt. To detect small temporal changes ofQ, we calculated spectral ratios of 5 temporal doublets located in theepicentral area and recorded at the closest station. We found temporalchanges of Q that vary from 27% to 56%, depending on the locationof the doublets. This variability suggests that the temporal change ofattenuation may depend on the spatial variation of Q and perhaps on thespatial distribution of tectonic stress in the epicentral area.

Interferometric Analysis of Strong Ground Motion for Structural Health Monitoring: The Example of the L’Aquila, Italy, Seismic Sequence of 2009

Abstract Structural health monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures. The usage of recording systems exploiting wireless communication technology is particularly suitable for SHM, especially for rapid response following earthquakes. In this study, both of these issues are combined, and we report on the application of seismic interferometry to SHM using a dataset of seven earthquakes collected using a novel wireless system of accelerometers during the L’Aquila, Italy, seismic sequence in 2009. We show that interferometric analysis allows the estimation of the shear-wave velocity of seismic phases propagating throughout a structure, and, most important for SHM purposes, allows the monitoring of the velocity variations during the aftershock sequence. Moreover, innovatively we apply the S transform to the building response functions retrieved by interferometry to estimate the fundamental resonance frequency and the quality factor Q .

Selection of Natural and Synthetic Accelerograms for Seismic Vulnerability Studies on Reinforced Concrete Frames

The choice of seismic input utilized in the evaluation of structural response is determined by the scope of the analysis, namely, the design of new buildings or damage scenarios on existing buildings. Further, a decision has to be made regarding the intensity measure better able to represent the damage potential of the earthquake. This work reports the results of a large set of nonlinear dynamic analyses on structural types representing reinforced concrete buildings widely present in the Italian and European built environments. Seismic input comes from both natural recordings and synthetic data. The maximum interstory drift has been selected as the response parameter better able to represent the structural and nonstructural damage level. The use of a computer code able to generate spectrum-compatible accelerograms showed that synthetic data provide output that is closer to a natural recording when the accelerograms are not forced to converge to a code response spectrum. The Housner intensity proved to be ...