1,720,996 research outputs found
Preface to the Pageoph Topical Volume “Seismic ground motion in large urban areas”
No full textThis volume summarizes the main results achieved in the framework of the UNESCO-IUGS-IGCP project 414 ‘‘Realistic Modelling of Seismic Input for Megacities and Large Urban Areas’’. It addresses the problems of pre-disaster orientation like hazard prediction, risk assessment, and hazard mapping, in connection with seismic activity. The unequal distribution of damage and casualties from earthquakes, which often affects urban areas, gives rise to problems that are still
a challenge that has fostered numerous studies on the estimation of the seismic ground motion before the occurrence of a damaging earthquake. This task requires the detailed knowledge of both the subsurface structure within the city and of the probable location and characteristics of seismic sources around it. On the other hand one must use theoretical methods and related computer codes that allow us the realistic and reliable simulation of the expected seismic ground motion. These numerical simulations play an important role, especially in areas of complex geology, and they can provide realistic synthetic waveforms at places where no recordings are available. Synthetics are compared with observations wherever instrumental data are available. During recent years, several methods have been
proposed for the theoretical estimation of the seismic response at a specific site.
This volume contains results obtained using tools which enable us to estimate realistic amplification effects in complex structures, exploiting the available knowledge relative to geotechnical, lithological and geophysical parameters, topography of the medium, tectonic, historical data, and seismotectonic models.
The ground motion modeling technique applied in most of the papers proves that it is possible to investigate the local effects even at large epicentral distances, too often neglected event, taking into account both the seismic source and the propagation path effects.
This collection gathers original studies which offer quantitative information required for the design, construction and retrofitting of the built environment. In particular, these studies have a long-lasting impact on the reduction of the environmental hazard associated with the seismically active regions, and contribute to the definition of the source and response spectra to be used in the mosaic of the
studied world seismic regions.
The accurate knowledge of subsoil structures and probable complex source mechanisms is used to study the local site effects in large urban areas and Megacities like: Algiers, Alexandria, Beijing, Bucharest, Cairo, Delhi, Napoli, Santiago de Cuba, Sofia, Thessaloniki and Zagreb. Alongside, a comparison with traditional methods for seismic microzoning is made for selected regions and cities
Deterministic Earthquake Scenarios for the City of Sofia
No full textThe city of Sofia is exposed to a high seismic risk. Macroseismic intensities in the range of VIII – X (MSK) can be expected in the city. The earthquakes that can influence the hazard in Sofia originate either beneath the city or are caused by seismic sources located within a radius of 40 km. The city of Sofia is also prone to the remote Vrancea seismic zone in Romania, and particularly vulnerable are the long-period elements of the built environment. The high seismic risk and the lack of instrumental recordings of the regional seismicity make the use of appropriate credible earthquake scenarios and ground-motion modelling approaches for defining the seismic input for the city of Sofia necessary. Complete synthetic seismic signals, due to several earthquake scenarios, were computed along chosen geological profiles crossing the city, applying a hybrid technique, which combines the modal summation technique and finite differences. The modelling takes into account simultaneously the geotechnical properties of the site, the position and geometry of the seismic source and the mechanical properties of the propagation medium. Acceleration, velocity and displacement time histories and related quantities of earthquake engineering interest (e.g., response spectra, ground-motion amplification along the profiles) have been supplied. The approach applied in this study allows us to obtain the definition of the seismic input at low cost, exploiting large quantities of existing data (e.g. geotechnical, geological, seismological). It may be efficiently used to estimate the ground motion for the purposes of microzonation, urban planning, retrofitting or insurance of the built environment, etc
Deterministic ground motion modelling at Russe, NE Bulgaria, associated to the Vrancea intermediate-depth earthquakes
No full textAn analytical deterministic technique, based on the detailed knowledge of the seismic source process and of the propagation of seismic waves, has been applied to generate synthetic seismic signals at Russe, NE Bulgaria, associated to the strongest intermediate-depth Vrancea earthquakes, which occurred during the last century (1940, 1977, 1986 and 1990). The obtained results show that all ground motion components contribute significantly to the seismic loading and that the seismic source parameters influence the shape and the amplitude of the seismic signal. The approach we used proves that realistic seismic input (also at remote distances) can be constructed via waveform modelling, considering all the possible factors influencing the ground motion
Application of the neo-deterministic seismic microzonation procedure in Bulgaria and validation of the seismic input against Eurocode 8.
No full textAn approach of microzonation of the Sofia city
No full textThe city of Sofia has the densest population of Bulgaria – around 1.27 million people. The capital is exposed to high seismic risk. According to the Bulgarian seis- mic code, Sofia has been included in a seismic category characterized by intensity IX (MSK), which corresponds to a horizontal acceleration of 0.27 g for the anchoring of the elastic response spectrum. Because of the lack of instrumental seismic data, real- istic synthetic strong motion waveforms for two opposite positions of seismic sources, have been generated for an expected earthquake scenarios (M = 7) along three ge- ological profiles. A hybrid modeling method has been used for the modeling, which is based on the modal summation technique and finite difference scheme. The calcu- lation has been done using an extended source model. The site effect is represented in terms of response spectra ratio (RSR), with respect to a bedrock model. The three components synthetic seismograms, computed in the domain of displacement, velocity and acceleration have been processed to extract some parameters very useful for engineering applications
Site dependent estimation of the seismic strong motion – case study for Sofia region.
No full textThe problem of site dependent estimation of the seismic strong motion for Sofia region is discussed. Deterministic modelling for micro-zonation of Sofia region is implemented for receiving an estimation of local site conditions as result of displacements provoked by seismic events in Sofia region with stochastic and neural modelling. An approach for seismic waves classification on the base of principal axis transformation, long-range dependence time series analysis and neural modelling is suggested. With the help of scene oriented model are determined the boundaries of destructive phase of strong motion acceleration. For selected diapason of destructive phase is implemented two-dimensional vector quantization for transformed accelerograms with maximal and middle values. With self-organizing map are determined weight centres of selected classes of destructive phase. Learned and trained self-organizing map optimize selected target classes and determine probability density function. Suggested model helps to provide side dependent estimation of the seismic strong motion for Sofia region
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