Gabriele Ferretti

An Improved Method for the Recognition of Seismic Families: Application to the Garfagnana–Lunigiana Area, Italy

With the aim to find a more objective way to detect seismic families, we applied a series of successive steps to constrain the results of a waveform similarity analysis. The evaluation of similarity was carried out on the waveforms recorded in the period 1999–2003 by the stations operating in the Garfagnana area, located in northern Tuscany (Italy). The algorithm is based on the cross-correlation technique applied in a process that overcomes the limit of one order of magnitude between events to be compared through a bridging technique. In practice, if two couples of events (A, B) and (B, C), each exceeding the correlation threshold, share a common quake (B), then all three events are attributed to the same family even if the match between A and C is below a value chosen as a reference for similarity. To avoid any subjective choice of threshold for cross-correlation values, the results from the computation algorithm are submitted to a routine that gives increasing reliability to them if they are confirmed by the three components of the seismogram and if the number of families detected by each station is confirmed by more recordings. This latter constraint is made possible by the geometry of the recording network, with interdistances between stations of the order of 40–50 km. The process finally leads to the recognition of 27 families detected and confirmed by, on average, 3 stations that represent 40% of the recording capabilities. Since the performances of the recording network have been very odd in the past, especially in the early years of operation, the reliability of the detection is much higher, as in most cases the stations that detected the families were the only ones to be effectively recording. The methodology proved to be more efficient than other methods applied in the past; moreover, the results could be probably improved even more if, instead of doing a one-run process, it would be borne as a trial-and-error approach.

Assessing the Effectiveness of Soil Parameters for Ground Response Characterization and Soil Classification

The average shear wave velocity over the top 30 m of a soil profile (VS,30) represents an usual parameter for soil classification in a modern building code for seismic design. In this work the ground response of about 100 soil profiles in Tuscany and Molise (Italy) is studied through 1-D numerical simulations in order to evaluate the reliability of European and Italian soil classifications based on the VS,30 criterion. The amplification factor, Fa , defined here as the ratio of the pseudo-velocity response spectrum intensity (Housner 1952) at the surface, S Is , to the pseudo-velocity response spectrum intensity at the rock outcrop, S Ir , is related to some soil parameters, such as VS,30, the fundamental frequency of vibration of the soil column, F0, and seismic impedance contrast, Iw . Analyzing the standard deviation of the residual obtained from regression analyses of Fa versus VS,30, F0, and Iw shows that F0 is the most helpful parameter for the prediction of Fa . Hence F0 appears to be more appropriate than VS,30 and Iw for the characterization of the seismic response of a site and, therefore, should not be disregarded in building code soil classifications.

Analysis of Site Amplification Phenomena: An Application in Ripabottoni for the 2002 Molise, Italy, Earthquake

The geophysical working group of the University of Genoa conducted a field experiment to analyze site amplification effects in Ripabottoni, a village in the Molise region of Italy. We used both noise and earthquake recordings, combined with detailed geologic and geotechnical surveys, to define site amplification phenomena. The site effects determination was obtained using the Nakamura technique and the H/V spectral analysis applied to earthquake recordings. The results were validated by applying a one-dimensional simulation code. The computed spectral ratios point out three different typologies of site effects: the southern sector of Ripabottoni is characterized by an absence of local amplification phenomena; the central sector of the village shows a local amplification phenomena with a fundamental frequency of 4–6 Hz; and the northeastern side of the village shows a site response at a fundamental frequency between 2–3 Hz.

Influence of Soil Modeling Uncertainties on Site Response

The scope of this work is to examine the influence of the uncertainty in soil modeling on numerical ground response estimates through a comprehensive sensitivity analysis. This allows identification of those parameters with the largest effect on both soil amplification (quantified here by a frequency-independent factor, Fa) and fundamental frequency, f0. Although extensively examined in previous articles, the effect of the input motion is also analyzed. The uncertainty affecting the shear wave velocity (VS) profile was found to be the factor contributing most to the uncertainty of both Fa and f0. The soil thickness was also found to play a key role, particularly in those cases where bedrock depth is unknown or largely uncertain. Although of secondary importance compared to the effect of VS on Fa and f0, the influence of the input motion cannot be neglected; rather it becomes predominant with regard to the uncertainty affecting frequency-dependent shaking parameters.

Site-Amplification Effects Based on Teleseismic Wave Analysis: The Case of the Pellice Valley, Piedmont, Italy

The investigation of local amplification phenomena by seismic signal analysis is a fundamental step in carefully defining the seismic response of an area. In this study we investigate the use of teleseismic recordings in assessing seismic- wave amplification in the Pellice Valley (northwestern Alps, Italy). Assuming that teleseismic P waves are sensitive to the deep structure of a basin, we deal with the computation of horizontal-to-vertical spectral ratios (hvsrs) and with the estimate of teleseismic P -wave arrival time delays and P -wave amplifications with respect to a reference site. The reliability of the hvsr results obtained by considering teleseismic signals is confirmed by the agreement with the results coming from both the hvsr of noise and hvsr of S wave of local events methods. Strong correlation between the P -wave arrival time delays and the relative P -wave amplifications with respect to thickness of the low-velocity layers and the geometry of the bedrock is found.

Long-range dependence in earthquake-moment release and implications for earthquake occurrence probability

Since the beginning of the 1980s, when Mandelbrot observed that earthquakes occur on ‘fractal’ self-similar sets, many studies have investigated the dynamical mechanisms that lead to self-similarities in the earthquake process. Interpreting seismicity as a self-similar process is undoubtedly convenient to bypass the physical complexities related to the actual process. Self-similar processes are indeed invariant under suitable scaling of space and time. In this study, we show that long-range dependence is an inherent feature of the seismic process, and is universal. Examination of series of cumulative seismic moment both in Italy and worldwide through Hurst’s rescaled range analysis shows that seismicity is a memory process with a Hurst exponent H ≈ 0.87. We observe that H is substantially space- and time-invariant, except in cases of catalog incompleteness. This has implications for earthquake forecasting. Hence, we have developed a probability model for earthquake occurrence that allows for long-range dependence in the seismic process. Unlike the Poisson model, dependent events are allowed. This model can be easily transferred to other disciplines that deal with self-similar processes.

Analysis of Site Amplification Effects at Four Installation Sites of the National Accelerometric Network (RAN) in Italy

The purpose of this work is to study seismic amplification effects at four sites hosting recording stations of the National Accelerometric Network (RAN) managed by the national Department of Civil Protection. At all sites, which are located in the Piedmont region (Northwestern Italy), both active and passive seismic prospecting methods are used, allowing identification of local amplification effects at three out of the four sites analysed. A ground type is then assigned to each site according to the national antiseismic code. Results obtained here will be fundamental to reduce the ground motion variability related to ground motion prediction equations and, consequently, to produce finer site-specific ground shaking hazard maps.

Calibration of a VS,30 Map for Shaking-map Production in Piedmont (Italy)

In this work, with the aim of producing shaking-maps which take into account the local amplification phenomena related to litho-stratigrafical condition, a raster map of Vs,30 values at a regional scale is derived for Piedmont, Italy. After the collection and the validation of all data available within the regional databases, a logic procedure in GIS is applied to assign a Vs,30 value to each of the zone with homogeneous seismic behavior already identified in a previous work. The result is a new Vs,30 map for the Piedmont region that allows a better and more reliable reconstruction of ground shaking including a more detailed evaluation of site effects.