Empirical site-specific response-spectra correction factors for the Gubbio basin (central Italy)

Providing quantitative microzonation results that can be taken into account in urban land-use plans is a challenging task that requires collaborative efforts between the seismological and engineering communities. In this study, starting from the results obtained by extensive geophysical and seismological investigations, we propose and apply an approach to the Gubbio basin (Italy) that can be easily implemented for cases of moderate-to-low ground motion and that takes into account not only simple 1D, but also more complicated 3D effects. With this method, the sites inside the basin are classified by their fundamental resonance frequencies, estimated from the horizontal-to-vertical spectral ratio applied to noise recordings (HVNSR). The correspondence between estimates of the fundamental frequency from this method and those derived from earthquake recordings was verified at several calibration sites. The amplification factors used to correct the response spectra are computed by the ratio between the response spectra at sites within the basin and the response spectra at a hard-rock site using data from two seismic transects. Empirical amplification functions are then assigned to the fundamental frequencies after applying an interpolation technique. The suitability of the estimated site-specific correction factors for response spectra was verified by computing synthetic response spectra for stations within the basin, starting from the synthetic recording at a nearby rock station, and comparing them with observed ones

Task 2 - Deliverable 4: Software for strong-motion data display and processing

The deliverable 4 is composed by a set of routines created to read and process the time series stored in the data base format, that is an ASCII file with a 43 row header, with the name codified as specified in the Deliverable 1. The files can also be read in SAC format

Deterministic ground-motion scenarios for engineering applications: the case of thessaloniki, Greece.

In this paper we present a deterministic study to estimate seismic ground motions expected in urban areas located near active faults. The purpose was to generate bedrock synthetic time series to be used as seismic input into site effects evaluation analysis and loss estimates for the urban area and infrastructures of Thessaloniki (Northern Greece). Two simulation techniques (a full wave method to generate low frequency,~< 1Hz, and a hybrid deterministic-stochastic technique to simulate high-frequency seismograms, ~> 1 Hz) were used to compute time series associated with four different reference earthquakes having magnitude from 5.9 to 6.5 and located within 30 km of Thessaloniki. The propagation medium and different source parameters were tested through the modeling of the 1978 Thessaloniki earthquake (M 6.5). Moreover two different nucleation points were considered for each fault in order to introduce additional variability in the ground motion estimates. Between the two cases, the quasi-unilateral rupture propagation toward the city produces both higher median PGA and PGV values and higher variability than bilateral ones. Conversely, the low-frequency ground motion (PGD) is slightly influenced by the position of the nucleation point and its variability is related to the final slip distribution on the faults of the reference earthquakes and to the location of the sites with respect to the nodal planes of the radiation pattern. To validate our deterministic shaking scenarios we verified that the synthetic peak ground motions (PGA, PGV) and spectral ordinates are within one standard deviation of several ground-motion prediction equations valid for the region. At specific sites we combined the low- and high-frequency synthetics to obtain broadband time series that cover all the frequency band of engineering interest (0-25 Hz). The use of synthetic seismograms instead of empirical equations in the hazard estimates provides a complete evaluation of the expected ground motions both in frequency and time domains, including predictions at short distances from the fault (0 – 10 km) and at periods larger than 2 – 3 seconds

Uncertainties in strong ground-motion prediction with finite-fault synthetic seismograms: an application to the 1984 M 5.7 Gubbio, central Italy, earthquake.

This study investigates the engineering applicability of two conceptually different finite-fault simulation techniques. We focus our attention on two important aspects: first to quantify the capability of the methods to reproduce the observed ground-motion parameters (peaks and integral quantities); second to quantify the dependence of the strong-motion parameters on the variability in the large-scale kinematic definition of the source (i.e. position of nucleation point, value of the rupture velocity and distribution of the final slip on the fault). We applied an approximated simulation technique, the Deterministic-Stochastic Method DSM, and a broadband technique, the Hybrid-Integral-Composite method HIC, to model the 1984 Mw 5.7 Gubbio, central Italy, earthquake, at 5 accelerometric stations. We first optimize the position of nucleation point and the value of rupture velocity for three different final slip distributions on the fault by minimizing an error function in terms of acceleration response spectra in the frequency band from 1 to 9 Hz. We found that the best model is given by a rupture propagating at about 2.65 km/s from a hypocenter located approximately at the center of the fault. In the second part of the paper we calculate more than 2400 scenarios varying the kinematic source parameters. At the five sites we compute the residuals distributions for the various strong-motion parameters and show that their standard deviations depend on the source-parameterization adopted by the two techniques. Furthermore, we show that, Arias Intensity and significant duration are characterized by the largest and smallest standard deviation, respectively. Housner Intensity results better modeled and less affected by uncertainties in the source kinematic parameters than Arias Intensity. The fact that the uncertainties in the kinematic model affects the variability of different ground-motion parameters in different ways has to be taken into account when performing hazard assessment and earthquake engineering studies for future events

ITACA: Italian Accelerometric Archive

ITACA is the Italian strong motion database. It contains more than 2000 three component waveforms generated by about 1000 earthquakes from 1972 to 2009. Strong motion data come mainly from National Accelerometric Network, operated by Dipartimento della Protezione Civile - DPC. Corrected and uncorrected time-series as well as spectral data can be downloaded in ASCII format. ITACA interfaces allow to set parameters of interest and retrieve specific events, stations, waveforms and their metadata

Ground‐Motion Simulations for the M 6.9 Irpinia 1980 Earthquake (Southern Italy) and Scenario Events

In this paper, we adopt three ground‐motion simulation techniques (EXSIM, Motazedian and Atkinson, 2005, DSM, Pacor et al., 2005 and HIC, Gallovič and Brokešová, 2007), with the aim of investigating the different performances in near‐fault strong‐motion modeling and prediction from past and future events. The test case is the 1980, M 6.9, Irpinia earthquake, the strongest event recorded in Italy. First, we simulate the recorded strong‐motion data and validate the model parameters by computing spectral acceleration and peak amplitudes residual distributions. The validated model is then used to investigate the influence of site effects and to compute synthetic ground motions around the fault. Afterward, we simulate the expected ground motions from scenario events on the Irpinia fault, varying the hypocenters, the rupture velocities and the slip distributions. We compare the median ground motions and related standard deviations from all scenario events with empirical ground motion prediction equations (GMPEs). The synthetic median values are included in the median ± one standard deviation of the considered GMPEs. Synthetic peak ground accelerations show median values smaller and with a faster decay with distance than the empirical ones. The synthetics total standard deviation is of the same order or smaller than the empirical one and it shows considerable differences from one simulation technique to another. We decomposed the total standard deviation into its between‐scenario and within‐scenario components. The larger contribution to the total sigma comes from the latter while the former is found to be smaller and in good agreement with empirical inter‐event variability

Toward validation of simulated accelerograms via prediction equations for nonlinear SDOF response

Seismic structural risk analysis of critical facilities may require nonlinear dynamic analysis for which record selection is one of the key issues. Notwithstanding the increasing availability of database of strong-motion records, it may be hard to find accelerograms that fit a specific scenario (e.g., in terms of magnitude and distance) resulting from hazard assessment at the site of interest. A possible, alternative, approach can be the use of artificial and/or simulated ground motion in lieu of real records. Their employment requires systematic engineering validation in terms of structural response and/or seismic risk. Prediction equations for peak and cyclic inelastic single degree of freedom systems’ response, based on Italian accelerometric data, are discussed in this study as a possible benchmark, alongside real record counterparts, for the validation of synthetic records. Even if multiple events would be in principle required, an extremely preliminary validation is carried out considering only four simulated records of the 1980 Irpinia (southern Italy) M w 6.9 earthquake. Simulated records are obtained through a broadband hybrid integral-composite technique. Results show how this simulation method may lead to generally acceptable results. It is also emphasized how this kind of validation may provide additional results with respect to classical signal-to-signal comparison of real and simulated records

Identification of accelerometric stations in ITACA with distinctive features in their seismic response

In this work, we analyze the records of the Italian strong motion database (ITACA, http://itaca.mi.ingv.it) with the aim of identifying stations affected by site effects that are not captured by standard seismic classification schemes. In particular, we consider four different site classifications, two of them based on geological/geophysical characteristics and two driven by data. For each classification we develop a ground motion prediction equation using a random effect approach to isolate the between-station and within-station distribution of errors. The site coefficients obtained for the different classes confirm that site amplification effects are significant for both the horizontal and vertical components. The between-station error normalized to the standard deviation of the between-station error distribution is then used to identify stations characterized by large errors, attributable to site effects not accounted for by the classification schemes. The results show that large errors can affect the predictions when the site effects are not uniquely related to the reduction of the seismic impedance in the uppermost layers. For example, amplifications of ground motion over the long period range are observed for stations installed within alluvial closed-shape basins, as consequence of locally generated surface waves. For these stations, classifications based on the horizontal to vertical response spectra ratio are not reliable, since amplifications are also affecting the vertical component. Another interesting feature which emerges from the analysis is the significant de-amplification of short period spectral ordinates that seems to be related to stations typically set in at the foundation level of massive structures. To increase the usefulness of the data set, the most important distinctive features of the strong motion stations are documented in the ITACA database reports containing the instrument information and the available geological-geotechnical data

Modelling directivity effects of the October 21, 2002 (Mw = 5.7), Molise, Southern Italy, earthquake

Acceleration time series recorded by the Italian Strong Motion Network (RAN) during the October 31, 2002 (Mw=5.8), Molise earthquake, are employed in order to investigate source effects on the ground motion in the epicentral area. We consider two different seismogenic sources: a fault model inferred from inversion of teleseismic, regional and local seismic signals [Vallée and Di Luccio, 2005], and a fault model based on seismotectonic data [Basili and Vannoli, 2005]. Both source studies suggest a deep location of the earthquake fault plane (ranging from 6.0 to 20.1 km and from 12.0 to 19.9 km, respectively), however, with considerably different fault lengths (5.2 and 10.5 km, respectively), and widths (14.2 and 8 km, respectively). Due to these differences, only the second model allows for effective horizontal unilateral rupture propagation. Finite fault effects are modelled by the Deterministic-Stochastic-Method (DSM) [Pacor et al., 2005], and the Hybrid Integral-Composite source model (HIC) [Gallovic and Brokesova, 2006]. In both methods k-square slip distributions on the faults are considered. We simulate the October 31, 2002 earthquake considering: 1) Vallée and Di Luccio [2005] faultwith a bilateral rupture propagation, and 2) Basili and Vannoli [2005] fault with unilateral directions of the rupture propagation. The spectral attenuation is modelled using a regional estimate of the quality factor [Castro et al., 2004] and k values estimated from acceleration records. Comparison between synthetic and recorded data at nearby stations (hypocentral distances < 60 km) performed in terms of frequency content and peak ground motion, favours the model with unilateral propagation of the rupture. Assuming the source model with unilateral rupture propagation, we utilize both asymptotic and full wave field methods in order to simulate ground shaking scenarios for an area extending up to 150 km epicentral distance. These results are then subjected to comparison with peak ground accelerations recorded in the far field

The Mw 6.3, 2009 L’Aquila earthquake: source, path and site effects from spectral analysis of strong motion data

The strong motion data of 2009 April 6 L’Aquila (Central Italy) earthquake (Mw = 6.3) and of 12 aftershocks (4.1 ≤ Mw ≤ 5.6) recorded by 56 stations of the Italian strong motion network are spectrally analysed to estimate the source parameters, the seismic attenuation, and the site amplification effects. The obtained source spectra for S wave have stress drop values ranging from 2.4 to 16.8 MPa, being the stress drop of the main shock equal to 9.2 MPa. The spectral curves describing the attenuation with distance show the presence of shoulders and bumps, mainly around 50 and 150 km, as consequence of significant reflected and refracted arrivals from crustal interfaces. The attenuation in the first 50 km is well described by a quality factor equal to Q( f ) = 59 f 0.56 obtained by fixing the geometrical spreading exponent to 1. Finally, the horizontal-to-vertical spectral ratio provides unreliable estimates of local site effects for those stations showing large amplifications over the vertical component of motion