Strong Intermediate-Depth Vrancea Earthquakes: Damage Capacity

Abstract The sustainable development of the society depends not only on a reasonable policy for economical growth but also on the reasonable management of natural risks. The regional earthquake danger due to the Vrancea intermediate-depth earthquakes dominates the hazard of NE Bulgaria. These quakes have particularly long-period and far-reaching effects, causing damages at large epicentral distances. Vrancea events energy attenuates considerably less rapidly than that of the wave field radiated by the seismically active zones in Bulgaria. The available strong motion records at Russe, NE Bulgaria, due to both Vrancea events - August 30, 1986 and May 30, 1990 show higher seismic response spectra amplitudes for periods up to 0.6 s for the horizontal components, compared to the values given in the Bulgarian Code and Eurocode 8. A neo-deterministic analytical procedure which models the wavefield generated by a realistic earthquake source, as it propagates through a laterally varying anelastic medium, is applied to obtain the seismic loading at Russe. After proper validation, using the few available data and parametric analyses, from the synthesized seismic signals damage capacity of selected scenario Vrancea quakes is estimated and compared with available capacity curves for some reinforced concrete and masonry structures, representative of the Balkan Region. The performed modelling has shown that the earthquake focal mechanisms control the seismic loading much more than the local geology, and that the site response should be analyzed by considering the whole thickness of sediments until the bedrock, and not only the topmost 30 m. 2 THE VRANCEA EARTHQUAKE HAZARD The Vrancea subduction seismogenic zone is a peculiar intermediate-depth source that, in case of large magnitude earthquake, strongly affects a significant part of the Bulgarian territory including major cities in NE Bulgaria, among which the biggest Bulgarian port on the Danube -the town of Russe. The impact of a major Vrancea intermediate-depth earthquake may produce strong direct damage, as well as indirect losses in other regions of the country, thus leading to a national disaster. when no seismic regulation were available). This fact, the lack of enough instrumental strong motion records and the peculiarity of the intermediate-depth Vrancea seismic source call our attention to the necessity of providing, by modelling, reliable seismic input that might be used for the purpose of retrofitting and urban planning. THE NEO-DETERMINISTIC GROUND MOTION MODELLING PROCEDURE: APPLICATION FOR THE CASE-STUDY OF RUSSE, NE BULGARIA A neo-deterministic analytical procedure has been applied to obtain the seismic input at Russe . The major advantage of the applied neo-deterministic procedure is the simultaneous treatment of the contribution of the seismic source and of the seismic wave propagation media to the strong motion at the target site/region, as required by basic physical principles. Generally, the computation model, describing the seismic wave propagation path from the seismic source to the target site consists of two structural models, bedrock, representing the travel path from the source to the site, and local model, representing the local engineering geological features of the site of interest. To model the seismic input at Russe the analytical neodeterministic approach based on mode coupling technique, is used Input data Seismic wave propagation path The structural model used in the computations consists of two horizontally layered half spaces in welded contact. The bedrock structure contains the source and the path from the Vrancea seismic sources to the target sites. The profile Vrancea-Russe passes through the Carpathians and the Moesian Platform, where Pliocene and significant Quaternary deposits are present. Details on the computation model and on the geological information used are published by Paskaleva et al. [2001]. In this study the target site of Russe is represented by three generalized local geological models corresponding to the soil classes A, B and C according to the Eurocode 8 (EC8) ground type classification. A summary of the local geological velocity models, constructed following the EC8 soil classification and used in this study, is given in Scenario earthquakes In accordance with the international experience, a reasonable choice of scenario earthquakes should take into account both historical earthquakes record and seismic hazard analysis. Simulation tests and results verification Following the models defined i

Estimates of some ground motion parameters, related to the damage potential of strong intermediate-depth Vrancea earthquakes

This study is related to the particular strong intermediate-depth Vrancea earthquakes. Its purpose is to confirm once again the significant influence of the seismic source mechanism to the seismic input at a given site through estimates of different engineering parameters, used to describe the damage potential of the ground motion due to an earthquake. Real and synthetic time-series have been used in the computations. The synthetic seismic wavelets have been computed applying the neo-deterministic seismic hazard assessment procedure and the relevant software and hardware facilities at the Department of Earth Sciences, University of Trieste, Italy. To derive the ground motion parameters of interest, all seismic signals have been uniformly processed by the Seismosignal software