Franco Vaccari

Seismic wave propagation in laterally heterogeneous anelastic media: Theory and applications to seismic zonation

Publisher Summary The ability to estimate accurately seismic hazard at very low probability of exceedance may be important in protecting special objects in the built environment against rare earthquakes. The deterministic approach, based upon the assumption that several earthquakes can occur within a predefined seismic zone, represents a conservative definition of seismic hazard for preevent localized planning for disaster mitigation, over a broad range of periods. Computation of realistic synthetic seismograms, using methods that take into account source, propagation, and site effects, and utilizing the huge amount of available geological, geophysical, and geotechnical data provides a powerful and economically valid scientific tool for seismic zonation and microzanation. First-order zonations can be made at regional scale, considering average structural models and a set of sources with damaging potential distributed within the identified seismogenic areas. Seismic microzonations of urban areas can be performed even more accurately when the required geotechnical data are available, so that local site effects can be effectively modeled.

Seismogenic potential and earthquake hazard assessment in the Tell Atlas of Algeria

Seismotectonic zonation studies in the Tell Atlas of Algeria, a branch of the Africa-Eurasia plate boundary, provide a valuable input for deterministic seismic hazard calculations. We delineate a number of seismogenic zones from causal relationships established between geological structures and earthquakes and compile a working seismic catalogue mainly from readily available sources. To this catalogue, for a most rational and best-justified hazard analysis, we add estimates of earthquake size translated from active faulting characteristics. We assess the regional seismic hazard using a deterministic procedure based on the computation of complete synthetic seismograms (up to 1 Hz) by the modal summation technique. As a result, we generate seismic hazard maps of maximum velocity, maximum displacement, and design ground acceleration that blend information from geology, historical seismicity and observational seismology, leading to better estimates of the earthquake hazard throughout northern Algeria. Our analysis and the resulting maps illustrate how different the estimate of seismic hazard is based primarily on combined geologic and seismological data with respect to the one for which only information from earthquake catalogues has been used.

Zoning of the Italian territory in terms of expected peak ground acceleration derived from complete synthetic seismograms

Abstract An automatic procedure for the seismic zonation of a territory is presented. The results consist of deterministic computation of acceleration time series distributed on a regular grid over the territory. For the estimation of the accelerations, complete synthetic seismograms are computed by the modal summation technique. A first rough zonation can be accomplished by considering a map showing the distribution of peak ground acceleration. In this work the new procedure has been applied to the Italian territory. The structural and source models necessary to compute the synthetic signals have been fixed after an extensive bibliographic research. Seismogenic areas have been defined in the framework of the GNDT (Gruppo Nazionale per la Difesa dai Terremoti of the Consiglio Nazionale delle Ricerche, Rome) research activities dedicated to the definition of the kinematic model of Italy. Information on historical and recent seismicity has been taken from the most updated Italian earthquake catalogues. The estimated peak ground accelerations have been found to be compatible with available data, both in terms of intensity (historical earthquakes) and accelerations (recent earthquakes).

Seismic input modelling for zoning and microzoning

The strong influence of lateral heterogeneities and of source properties on the spatial distribution of ground motion indicates that the traditional methods require an alternative when earthquake records are not available. The computation of broadband synthetic seismograms makes it possible, as required by a realistic modelling, to take source and propagation effects into account, fully utilizing the large amount of geological, geophysical and geotechnical data, already available. For recent earthquakes, where strong motion observations are available, it is possible to validate the modelling by comparing the synthetic seismograms with the experimental records. The realistic modelling of the seismic input has been applied to a first-order seismic zoning of the whole territory of several countries. Even though it falls in the domain of the deterministic approaches, the method is suitable to be used in new integrated procedures which combine probabilistic and deterministic approaches and allow us to minimize the present drawbacks which characterise them when they are considered separately. Detailed modelling of the ground motion for realistic heterogeneous media (up to 10 Hz) can be immediately used in the design of new seismo-resistant constructions and in the reinforcement of existing buildings, without having to wait for a strong earthquake to occur. The discrepancies between the ground responses computed with standard methods and the results of our detailed modelling cannot be ignored when formulating building codes and retrofitting the built environment.

DETERMINISTIC SEISMIC HAZARD ASSESSMENT

Using the available information about regional structural models, past seismicity, and the seismotectonic regime in Italy, we have generated a set of synthetic seismograms covering the whole Italian territory on a 0.2° × 0.2° grid. Peak values of ground motion (displacement, DMAX, and velocity, VMAX) and Design Ground Acceleration (DGA) based on Eurocode 8 (EC8, 1993), extracted from the synthetic signals, have been compared with the macroseismic intensities felt in Italy. The correlation relations that we have obtained are in a good agreement with empirical relationships given by other authors and compare quite well with the few observations available in the Italian territory.

Seismic Zoning of Slovenia Based on Deterministic Hazard Computations

Abstract. Seismic hazard of the territory of Slovenia is estimated using a deterministic approach based on the computation of complete synthetic seismograms. The input data are the catalogues of earthquakes and fault plane solutions for Slovenia and surrounding regions. Structural models are defined based on available seismological and geophysical information, but are mainly constrained by surface-wave dispersion and 3-D tomographic modelling of the upper crust. Seismogenic zones are delineated considering geotectonic characteristics, fault plane solutions and distribution of earthquake hypocentres. Outside Slovenia seismogenic zones are extended up to distances from which they can considerably influence seismic hazard estimates.¶Synthetic seismograms are computed using the “receiver” structure along the entire path by normal mode summation (up to 1 Hz) for receiver sites on a 0.2 × 0.2 degrees grid and scaled to the magnitude of the earthquake allowing for spectral falloff. At each site the maximum value of horizontal velocity, horizontal displacement and design ground acceleration are considered as hazard parameter. The highest values are obtained for western Slovenia where the hazard is controlled by the strongest earthquake in the catalogue, the “Idrija” event of March 26, 1511.

Neo-deterministic seismic hazard assessment in North Africa

North Africa is one of the most earthquake-prone areas of the Mediterranean. Many devastating earthquakes, some of them tsunami-triggering, inflicted heavy loss of life and considerable economic damage to the region. In order to mitigate the destructive impact of the earthquakes, the regional seismic hazard in North Africa is assessed using the neo-deterministic, multi-scenario methodology (NDSHA) based on the computation of synthetic seismograms, using the modal summation technique, at a regular grid of 0.2 × 0.2°. This is the first study aimed at producing NDSHA maps of North Africa including five countries: Morocco, Algeria, Tunisia, Libya, and Egypt. The key input data for the NDSHA algorithm are earthquake sources, seismotectonic zonation, and structural models. In the preparation of the input data, it has been really important to go beyond the national borders and to adopt a coherent strategy all over the area. Thanks to the collaborative efforts of the teams involved, it has been possible to properly merge the earthquake catalogues available for each country to define with homogeneous criteria the seismogenic zones, the characteristic focal mechanism associated with each of them, and the structural models used to model wave propagation from the sources to the sites. As a result, reliable seismic hazard maps are produced in terms of maximum displacement (Dmax), maximum velocity (Vmax), and design ground acceleration.

A Contribution to Seismic Hazard Assessment in Croatia from Deterministic Modeling

Abstract—Some of the elements of regional seismic hazard in Croatia are assessed by computing synthetic accelerograms at a predetermined set of sites. The input dataset consists of structural models, parameters of seismic sources, and an updated earthquake catalog. Synthetic strong-motion time series for frequencies below 1 Hz are computed on a grid of sites using the modal summation technique. The long-period hazard is described by the distribution of estimated peak values of ground displacement, velocity and acceleration, while the short-period hazard is represented by the map of design ground acceleration values (DGA). The highest values of DGA exceeding 0.35 g on the base-rock level are found in the southeastern coastal part of the country, in the greater Dubrovnik area.

Amplification of strong ground motion in the city of Zagreb, Croatia, estimated by computation of synthetic seismograms

A hybrid technique consisting of modal summation and subsequent finite differences modelling is applied for the computation of synthetic accelerograms along a profile crossing the city of Zagreb, the capital of Croatia. Assuming the source geometry is known, the amplification properties of the underlying soil may be determined by comparison of synthetics and their response spectra computed for a bedrock model with the ones obtained under the assumption of a realistic laterally varying local model. The peak ground acceleration is larger by a factor of up to 3.5 than the value obtained for the bedrock model. The amplification of the response spectra is most prominent for frequencies below 2 Hz, and increases sharply to the SW from the mapped fault running through the centre of the city.

Seismic zonation in Albania using a deterministic approach

Abstract The modal summation method to compute synthetic seismograms is applied to carry out a study of seismic zonation in Albania. From a seismotectonic synthesis and the available earthquake data, eight seismogenic zones are defined and for each of them, a characteristic focal mechanism is selected. A new seismic velocity model is used in the computations and is constructed on the basis of previous studies. Correlation relations between the maximum intensity values observed and the ground motion parameters obtained from this procedure are derived. The distribution of the calculated values of displacement, velocity and acceleration, will contribute to the quality of anti-seismic design practice in Albania and to the mitigation of earthquake consequences.