Analysis of strong ground motions to evaluate regional attenuation relationships

Italian attenuation relationships at regional scale have been refined using a data set of 322 horizontal components of strong ground motions recorded mainly during the 1997-1998 Umbria-Marche, Central Italy, earthquake sequence. The data set includes records generated by events with local magnitude (M L ) ranging between 4.5 and 5.9, recorded at rock or soil sites and epicentral distance smaller than 100 km. Through a multiple step regression analysis, we calculated empirical equations for the peak ground acceleration and velocity, the Arias Intensity and for the horizontal components of the 5% damped velocity pseudo response spectra, corresponding to 14 frequencies ranging from 0.25 to 25 Hz. We compared our results with well known predictive equations, widely used on the national territory for Probabilistic Seismic Hazard Analysis. The results obtained in this study show smaller values for all the analyzed ground motion indicators compared to other predictive equations

Modeling the 1980 Irpinia earthquake by stochastic simulation. Comparison of seismic scenarios using finite-fault approaches

To define more accurately the near field and the directivity effect, different methodologies of finite-fault modelling have been used to describe the behaviour of ground shaking based on deterministic, stochastic and hybrid stochastic-deterministic approaches as in the framework of the ongoing European project “LESSLOSS – Risk Mitigation for Earthquakes and Landslides”. In this study, we simulate and compare seismic scenarios obtained from the complex source characteristic of the 1980 Irpinia earthquake, M 6.9, Southern Italy, using models based on the source models hypothesized in Bernard and Zollo (1989) and in Valensise et al. (1990). Furthermore, two finite-fault numerical approaches are used: 1. The approach RSSIM [Carvalho et al., 2004] that is a non-stationary stochastic simulation method that synthesizes the ground motion due to an extended source; 2. The approach EXSIM [Motazedian and Atkinson, 2005] that is a new version of FINSIM [Beresnev and Atkinson, 1998] introducing a new variation based on a “dynamic corner frequency”. The shaking scenarios are computed in terms of Response Acceleration Spectra (PSA), time series, peak ground acceleration (PGA) at bedrock level. Source and path propagation parameters taken from other studies were tested and the computed shaking scenarios are compared to acceleration records to eight different stations. Preliminary results are here presented in terms of PGA maps for the Campania region (Southern Italy)

Bayesian analysisof a probability distribution for local intensity attenuation

Intensity attenuation and its variation as a function of the distance and earthquake size is still a critical issue in evaluating seismic hazard. We present a method that allows us to incorporate additional information from the historical earthquake felt reports in the probability estimation of local intensity attenuation. The approach is based on two ideas: a) standard intensity versus epicentral distance relationships constitute an unnecessary filter between observations and estimates; and b) the intensity decay process is affected by many, scarcely known elements (the physical parameters of the source, propagation path effects, building vulnerability, the semi-qualitative character of macroseismic scales, etc.). Hence intensity decay should be treated as a random variable as is the macroseismic intensity. We assume here that decay, defined on the set {0,1, ..., I0}, follows a binomial distribution with parameters (I0, p); p depends on the distance from the epicenter and is related to the probability of null decay at that distance. According to the Bayesian approach this p parameter is, in turn, a Beta random variable. The observations related to earthquakes with their epicenter outside the area concerned, but belonging to homogeneous zones, are used as prior knowledge of the phenomenon, while the data points of events inside the area are used to update the estimates through the posterior means of the quantities involved. Our methodology is described in detail in an application to the Umbria-Marche area in Central Italy. The data sets examined are the macroseismic intensity database DOM4.1 and the zonation ZS.4, both compiled by the Italian Group for Defence against Earthquakes (GNDT). The method is validated by comparing the observed and the estimated intensity data points of the Camerino (28/07/1799) and of the Colfiorito (26/09/1997) earthquakes

Bayesian analysis of the local intensity attenuation

We present a method that allows us to incorporate additional information from the historical earthquake felt reports in the probability estimation of local intensity attenuation. The approach is based on two ideas: a) standard intensity versus epicentral distance relationships constitute an unnecessary lter between observations and estimates; and b) the intensity decay process is a ected by many, scarcely known elements; hence intensity decay should be treated as a random variable as is the macroseismic intensity. The observations related to earthquakes with their epicenter outside the area concerned, but belonging to homogeneous zones, are used as prior knowledge of the phenomenon, while the data points of events inside the area are used to update the estimates through the posterior means of the quantities involved

Mining Macroseismic Fields to Estimate the Probability Distribution of the Intensity at Site

The analysis of the seismic attenuation is a prominent and problematic component of hazard assessment. Over the last decade it has become increasingly clear that the intrinsic uncertainty of the decay process must be expressed in probabilistic terms. This implies estimating the probability distribution of the intensity at a site Is as the combination of the distribution of the decay DI and of the distribution of the intensity I0 found for the area surrounding that site. We focus here on the estimation of the distribution of DI. Previous studies presented in the literature show that the intensity decay in Italian territory varies greatly from one region to another, and depends on many factors, some of them not easily measurable. Assuming that the decay shows a similar behavior in function of the epicenter-site distance when the same geophysical conditions and building vulnerability characterize different macroseismic fields, we have classified some macroseismic fields drawn from the Italian felt report database by applying a clustering algorithm. Earthquakes in the same class constitute the input of a two-step procedure for the Bayesian estimation of the probability distribution of I at any distance from the epicenter, conditioned on I0, where DI is considered an integer, random variable, following a binomial distribution. The scenario generated by a future earthquake is forecast either by the predictive distribution in each distance bin, or by a binomial distribution whose parameter is a continuous function of the distance. The estimated distributions have been applied to forecast the scenario actually produced by the Colfiorito earthquake on 1997/09/26; for both options the expected and observed intensities have been compared on the basis of some validation criteria. The same procedure has been repeated using the probability distribution of DI estimated on the basis of each class of macroseismic fields identified by the clustering algorithm

Stochastic Finite-Fault Ground Motion Simulation in a Wave Field Diffusive Regime: Case Study of the Mt. Vesuvius Volcanic Area

The main aim of the present work consists in the validation of stochastic method for simulating weak ground motion in a diffusive regime due to low-to-moderate magnitude earthquakes, and in particular in its application to a volcanic area. We simulated the peak ground acceleration and the response acceleration spectra caused by two earthquakes scenarios (MD = 4.3 and MD = 5.4) at Mt. Vesuvius volcanic area by using the stochastic finite-fault simulation method. We validated the stochastic methodology by combining source, path and site parameters of the investigated area considering the time duration parameter, Trms, calculated on the study seismograms. The values of time durations are confirmed by calculating the same parameter, Trms, on the seismogram energy envelope described by multiple scattering models, in terms of scattering and the intrinsic dissipation coefficient. Initially, the simulations were evaluated for 10 local earthquakes (1.7 ≤ MD ≤ 3.6) that occurred at Mt Vesuvius in 1999 and are then compared with the observed data. The comparison between simulated and observed seismograms has been used to calibrate the stochastic procedure, and has been considered as the starting point for simulating ground motion for the scenario earthquake (MD > 3.6) that could occur in the study area. The scenario earthquake and the relative fault features were chosen on the base of statistical, tectonic, structural and historical studies of the study area. We simulated ground motions for a maximum magnitude value, Mmax, of 4.3, determined from examination of the Gutenberg-Richter law for the study area, and also for an Mmax = 5.4, a magnitude that is associated with the earthquakes that struck the ancient town of Pompei 17 years before the eruption of Mt Vesuvius that occurred in 79 AD. The largest values of Amax for the MD = 4.3 seismic event are in the range of 0.140 g to 0.029 g. In the case of MD = 5.4, we obtain PGA values in the range between 0.17 and 0.55 g

The intensity attenuation of Colfiorito and other strong earthquakes: the viewpoint of forecasters and data gatherers

This article has been originated by thoughts on previous analyses related to the proba- bilistic treatment of the macroseismic attenuation, from which it turns out that in Italian territory the intensity decay I varies greatly from one region to another, depending on many factors, some of them not easily measurable. By applying a clustering algorithm we classified some macroseismic fields drawn from the Italian felt report database in three classes. Earthquakes into the same class constituted the input of a two-step procedure for the Bayesian estimation of the probability distribution of I at any distance from the epicenter, conditioned on I0, where I is considered an integer, random variable, following a binomial distribution. The estimated distributions were validated by forecasting the macroseismic field of the Colfiorito earthquake. In this article we deal with the issues left open by those statistical analyses by following two ways: on one hand we test the procedure by forecasting the macroseismic field of other strong earthquakes recorded in Italy during the last century and, on the other hand, we ask the reasons of peculiarities in the results to experts in other fields. The article is hence an introductory work, an example of the possibility and of the need of exchange of knowledge

Dissemination of information on hazards and risks: the Icelandic experience

This paper deals with on-going activities related to the Task Group H activities on "Disaster prevention strategies based on an education information system", carried out within the project framework of UPStrat-MAFA "Urban prevention strategies using macro-seismic and fault sources". The emphasis is on the Icelandic experience and related work. Civil Defence/Civil Protection in Iceland has distributed information to the public about disasters since the seventies, first about nuclear war and later about natural disasters. For the last decade the Earthquake Engineering Research Centre of the University of Iceland has used these materials, along with their own material and technical capacity, to educate school children, aged 10 to 12, and provided information for local exhibitions. This article presents an overview of these efforts and the factors guiding the development of information packages to the public, school staff and children, along with a discussion of the challenges and success of these efforts

The Bridge Over the Adda River in Brivio: History, Full-Scale Testing and FE Modelling

Selected results from the investigation program of a historic R.C. bridge are presented in the paper. The bridge, crossing the Adda river between the Municipalities of Brivio and Cisano Bergamasco, consists of three tied arch spans, about 44.0 m length each, and represents a crucial node for the vehicular traffic of the region. After a maintenance intervention performed on one pier, the local Authorities committed to Politecnico di Milano an extensive investigation including: (i) documentary research and review of the bridge history; (ii) visual inspection and geometric survey of each span; (iii) full-scale load tests and ambient vibration tests; (iv) FE modelling of each span and validation of the numerical models (through comparison with the available experimental results) and (v) structural assessment of the bridge in its present condition. After a concise review of the bridge history, the paper summarizes the complete results of the experimental tests performed on one span (visual inspection, geometric survey, full-scale load tests and dynamic tests) as well as the development of numerical models

Probabilistic procedure to estimate the macroseismic intensity attenuation in the Italian volcanic districts

In this work we apply a probabilistic procedure to estimate the macroseismic intensity attenuation in the volcanic areas of Italy which allows to exploit additional information on historical earthquakes following the Bayesianapproach. The method starts from the intensity data points of the selected earthquakes and arrives at theassessment of the probability distribution for the intensity at a site given the epicentral intensity and thesite-epicenter distance. The CMTE local earthquake catalogue has been used for the Etna region while for theother Italian volcanic districts (Aeolian Islands, Ischia, Vesuvius and Albani Hills) the CPTI04 Italian seismic catalogue and the DBMI04 associated database have been considered. For the analysis, subsets of earthquakeswith epicentral intensity I0 ≥ VII MCS and I0 ≥ VI MCS were used for the Etna region and for the other Italian volcanic districts, respectively. Only earthquakes with more than 10 felt observations have been considered. The results show a specific attenuation trend for the Etna region compared with the other Italian volcanic areas