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Probabilistic seismic hazard is usually assessed by means of computer codes using seismogenic sources, parametric catalogues, seismicity rates and attenuation relationships. All these ingredients are conditioned by expert judgement that influences the final results. Even the attenuation relationships, though strictly based on experimental data, are considered a weak point due to the difficulty of modelling the interaction between seismic energy radiation and site response, and because earthquakes do not usually repeat themselves according to one theoretical model. Recently, methods making wide use of site intensity data have been developed in regions such as Italy, where the observed seismic history at selected sites is quite exhaustive. We analyzed these observations to assess seismic hazard at about 600 sites. We used a probabilistic counting technique, integrating the observations (when necessary), with computed shakings obtained from a logistic-type attenuation model. The results were then compared with the estimates provided by the recent seismic hazard map of Italy, compiled according to the traditional probabilistic seismic hazard approach. The match shows significant differences for some sites. A tentative explanation which seems to point to three alternatives is provided: (1) The mismatch between the two methodologies might appear because the stationary assumption has a poor fit with reality (at least in certain areas); (2) Some sites show a response that is systematically different from the average values predicted using attenuation relationships; (3) The definition of seismogenic zones leads to a bias in the seismicity rate estimate.

The sensitivity of different parameters used in probabilistic seismic hazard calculation is investigated by different logic tree runs with alternative magnitude sets, source zone models and attenuation relations, and with different sets of values for the seismicity parameters and the σ-value. Also the influence from the different parameters on the hazard uncertainty, represented by fractiles, is investigated. The calculations are made for peak ground acceleration at a site near Aachen in the Lower Rhine Embayment. The model where the site is located in a larger source zone gives lower hazard values. This is typical for the case where the seismicity near the site is high relative to its surrounding. The hazard curves for the different attenuation functions are similar, an effect of the similarity of the functions themselves. However, a large sensitivity of this parameter is indicated for small mean return periods. An increased σ-value implies a moderate increase of the hazard at long mean return periods. The hazard is increasing for decreased focal depth, decreased β-value and increased maximum expected magnitude, respectively. However, the effects are noteworthy only at low hazard levels for variations in the focal depth and to some extent in the maximum expected magnitude. Finally, decreasing the minimum magnitude thought to be of engineering relevance causes a drastic increase of the hazard at small mean return periods.

Deterministic and probabilistic seismic hazard assessment are frequently represented as irreconcilably different approaches to the problem of calculating earthquake ground motions for design, each method fervently defended by its proponents. This situation often gives the impression that the selection of either a deterministic or a probabilistic approach is the most fundamental choice in performing a seismic hazard assessment. The dichotomy between the two approaches is not as pronounced as often implied and there are many examples of hazard assessments combining elements of both methods. Insistence on the fundamental division between the deterministic and probabilistic approaches is an obstacle to the development of the most appropriate method of assessment in a particular case. It is neither possible nor useful to establish an approach to seismic hazard assessment that will be the ideal tool for all situations. The approach in each study should be chosen according to the nature of the project and also be calibrated to the seismicity of the region under study, including the quantity and quality of the data available to characterise the seismicity. Seismic hazard assessment should continue to evolve, unfettered by almost ideological allegiance to particular approaches, with the understanding of earthquake processes.

This paper presents response spectral attenuation laws used in the new French Safety Rule, which is the reference for nuclear safety studies in France. Attenuation laws were derived from 965 horizontal and 485 vertical components from a two-step inversion method and accounts for geometrical spreading, anelastic attenuation and geological site condition. The datasets are mainly constituted of European strong motion records (83%) recently collected and homogeneously processed. In order to complete the distribution data beyond magnitude 6, a few American records were added, representing 17% of the datasets. The magnitude type and source-to-site distance definitions chosen to derive the laws are tested with respect to other definitions. These parametric tests induce a conservative law, for some magnitude and distance ranges of interest. The residual values between observed and predicted spectral accelerations are studied and do not exhibit any bias. The inferred laws are in good agreement with classical strong motion attenuation laws.

This paper presents the seismic hazard assessment and seismic zoning of the United Arab Emirates (UAE) and its surroundings based on the probabilistic approach. The area that has been studied lies between 50°E–60°E and 20°N–30°N and spans several Gulf countries. First, the tectonics of the area and its surroundings is reviewed. An updated catalogue, containing both historical and instrumental events is used. Seismic source regions are modelled and relationships between earthquake magnitude and earthquake frequency is established. A modified attenuation relation for Zagros region is adopted. Seismic hazard assessment is then carried out for 20 km interval grid points. Seismic hazard maps of the studied area based on probable Peak Ground Acceleration (PGA) for 10% probability of exceedance for time-spans of 50, 100 and 200 years are shown. A seismic zone map is also shown for a 475-year return period. Although the results of the seismic hazard assessment indicated that UAE has moderate to low seismic hazard levels, nevertheless high seismic activities in the northern part of UAE warrant attention. The northern Emirates region is the most seismically active part of UAE. The PGA on bedrock in this region ranges between 0.22 g for a return period of 475 years to 0.38 g for a return period of 1900 years. This magnitude of PGA, together with amplification from local site effect, can cause structural damage to key structures and lifeline systems.

The Eurocode 8 (EC8) currently proposes two standard shapes for the design response spectra. Type 1 spectra are enriched in long period and are suggested for high seismicity regions. Conversely, Type 2 spectra are proposed for low to moderate seismicity areas (like France), and exhibit both a larger amplification at short period, and a much smaller long period contents, with respect to Type 1 spectra. These propositions, however, were constrained using few events mostly recorded on analogical instruments. In the present study, we use the Japanese high quality digital K-net array in order to evaluate the proposed EC8 response spectra. Furthermore, all K-net stations have geotechnical characterisation. We first constructed a database of shallow events, depth less than 25 km, to avoid subduction related records. The database spans six years of seismicity from 1996 until 2003. Thus, 591 events were selected with moment magnitude between 4 and 7.3, recorded at 691 stations, giving a total of 6812 two horizontal components accelerograms. Using these records, we computed spectral ground-motion prediction equations and we used them to review the shape of the proposed EC8 spectra. In particular, we studied the plateau-PGA ratio level, the period interval where this plateau is constant, and site amplification effects. The results show surprisingly that the Type 2 rock better envelope the Japanese data. Another interesting observation is that the K-net data corresponding to all soil classes are rich in short periods around 0.1 s. This characteristic has not been observed in other worldwide databases. Normalised empirical predictions show a widening of the plateau as the soil conditions degrade. This suggests that the Type 2 EC8 spectra do not cover enough the long periods for EC8-soil classes C, D and E. Finally, the computed ground-motion prediction equations show that the peak ground acceleration (PGA) is nearly invariant to the soil conditions. Soil effects are mainly seen in the shape and plateau level.

A study aimed at evaluating earthquake damage scenarios and seismic hazard of Messina using historical data, is presented. The analysis of coeval reports allowed us to reconstruct the seismic history of the city and to obtain a homogeneous earthquake site catalogue based on intensity assessed by the European Macroseismic Scale 1998. In the last 1200 years Messina was destroyed once (1908, intensity X-XI EMS) and suffered effects estimated between intensities VII and IX EMS many times (e.g. 853, 1169, 1494, 1509, 1599, 1693, 1783, 1894, 1909). Destruction or severe damage which affected the city are mainly related to earthquakes occurring in the Messina Straits and Southern Calabria, while slighter, moderate effects are usually due to shocks taking place in the seismogenic sources of SE Sicily, Gulf of Patti and Northern Calabria. The damage scenarios of the most relevant events, delineated using coeval urban plans of the city, showed that damage distribution is strongly conditioned by the different soil response. A probabilistic seismic hazard assessment was obtained by using site observed intensities: The expected intensity in a time span of 50 years (i.e. maximum intensity characterised by at least 10% exceedance probability in 50 years) is IX EMS; the expected intensity in a time span of 300 years (10% exceedance probability in 300 years) is X EMS.

A road-network reliability analysis for a scenario seismic event is performed for a region of southern Italy characterised by a large number of small to medium municipalities quite close to each other and served by a dense network of roads. Among the many functions of the road network, whose links may fail after an earthquake due to the collapse of the bridges within them, the one selected for the present study is that of allowing rescue operations to be carried out at the sites of collapsed schools. For this to be possible, connection must be maintained between schools that survived, rescue centres and hospitals. Required elements for the study are the fragility curves of the bridges, the schools, the hospitals and the rescue centres. Output of the study is the expected value of the fraction of the total population in the area that is in need of assistance and cannot be hospitalised due to either failure of the network or other vulnerable components.

New aspects of the frequency-dependent attenuation of the seismic waves travelling from Vrancea subcrustal sources toward NW (Transylvanian Basin) and SE (Romanian Plain) are evidenced by the recent experimental data made available by the CALIXTO'99 tomography experiment. The observations validate the previous theoretical computations performed for the assessment, by means of a deterministic approach, of the seismic hazard in Romania. They reveal an essential aspect of the seismic ground motion attenuation that has important implications on the probabilistic assessment of seismic hazard from Vrancea intermediate-depth earthquakes. The attenuation toward NW is shown to be a much stronger frequency-dependent effect than the attenuation toward SE and the seismic hazard computed by the deterministic approach fits satisfactorily well the observed ground motion distribution in the low-frequency band (<1 Hz). The apparent contradiction with the historically-based intensity maps arises mainly from a systematic difference in the eigenperiods (type and size) of the buildings in the intra- and extra-Carpathians regions, thus the existing macroseismic data, based on buildings of small dimensions, i.e. with high eigenfrequency (5–10 Hz), can hardly be representative of the real hazard for new and large dimension, tall buildings, with eigenfrequency above 1 Hz.

Requirements for the siting and design of critical facilities have, in the past, pushed the horizons of engineering seismology and earthquake engineering. The nuclear industry, in particular, contributed greatly to the development of these subjects in the 1960s and 1970s. Public concern and consequently regulatory scrutiny were the major factors for the criteria set for the siting and design of nuclear facilities. At present, the siting and design of new nuclear facilities have slowed down considerably. Attention is increasingly focused on the safety of operating plants and the issues involved in decommissioning and waste disposal. This paper discusses some of these new issues and suggests areas of scientific development which may assist in their resolution.

A method is demonstrated using Monte Carlo simulation (stochastic modelling) techniques that allows the extraction of information about design earthquakes that constitute the most likely combinations of earthquake magnitude/epicentral distance that would actually generate the computed hazard ground motion at a site of interest, taking into account the log-normal scatter in the attenuation relationship. A worked example, at three different return periods, is shown for a realistic case in Greece. The results demonstrate the range of events that may contribute to the hazard, from which median or modal values can be derived. This method should be very useful in cases where it is desired to select earthquake time series representing the hazard for engineering analyses. In the example shown, the most probable magnitude/distance pairs are those that would produce predicted deterministic acceleration values of about half the design ground motion.

A seismic hazard analysis of Florence city was performed in the frame of a project concerning the dynamic behaviour of cable-stayed bridges. Both a probabilistic approach and a methodology based on the use of a local macroseismic catalogue were applied. A local catalogue was expressly compiled for this purpose, to collect the macroseismic intensities actually observed at the site as a result of past earthquakes. This sort of catalogue is an independent tool to verify the assumptions of the probabilistic approach (seismic zoning, earthquake recurrence relation, attenuation model), though it can supply results in terms of macroseismic intensity only and reflects the effective seismic history at the site, without taking into account any variability. The Cornell's methodology was used to assess probabilistic hazard in terms of macroseismic intensity, peak ground acceleration, peak ground velocity, and pseudovelocity uniform response spectra. The local catalogue points out level VII of the Mercalli–Cancani–Sieberg scale (MCS) as the maximum intensity historically observed in Florence. The probabilistic approach leads to the consideration of intensity VIII MCS as the maximum credible for the city. The probabilistic analysis in terms of ground motion was performed using attenuation relations estimated for alluvium sites, since the geology of Florence area is represented by fluvial and lacustrine deposits of various thickness. Peak ground acceleration values with 90% non exceedence probability in 50 and 500 years are respectively 145 and 219 cm/s*s for a shallow alluvium site, and 95 and 157 cm/s*s for a deep alluvium site; the corresponding peak ground velocity values for sites located on alluvium are 6.41 and 11.76 cm/s. Uniform response spectra are provided for shallow and deep alluvium sites, according to frequency-dependent attenuation relations estimated from strong Italian earthquakes.

An important component of hazard mitigation is to estimate the future hazard for design calculations. In the present study, a deterministic seismic hazard assessment of Andaman and Nicobar region is carried out, which is one of the most seismically active regions of India. The study area is divided into seven seismogenic source zones based on seismicity and tectonic setting. For ground motion estimation at Andaman and Nicobar, for each seismogenic zone different attenuation relationship is used as per tectonic setting of that seismogenic zone. In order to generate the site specific design spectrum, final results are calculated in the form of peak ground acceleration (PGA) and 5%-damped pseudo-spectral acceleration (PSA) for 0.2 s and 1 s. Calculated results are compared with some earlier works for the studied area and the probable reasons for variations are discussed.

This paper presents the selected case studies of seismic microzonation and the lessons learned from the microzonation studies in India. India has experienced major damages and loss of life due to earthquakes. Macrozonation map in Indian seismic code BIS-1893 is frequently revised soon after a major earthquake in the country. The latest revision, which was published in 2002 after Bhuj earthquake in 2001, contains four macro zones. These zones are delineated based on geology and past seismic activity and without considering geotechnical aspects such as site effects and liquefaction. The government of India has initiated microzonation work of 63 cities in India to evaluate the earthquake vulnerability of major urban centers and prepare new zonation map. Most of these microzonation studies are under progress and a few of them have been completed. This paper presents a brief overview of some of these studies. Most of the microzonation studies done in India have not given due consideration to the geotechnical aspects in microzonation studies. The geotechnical aspects were fully incorporated in the recently completed Microzonation work of Bangalore and the ongoing microzonation study of Chennai. A detailed of description of the microzonation study of Bangalore urban centre is included in this paper.