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

Hybrid MS-BIEM for seismic site response phenomena: case study of Sofia

The study presents and solves the 2-D elastodynamic model for seismic in-plane wave propagation in laterally inhomogeneous geological profiles imbedded in a vertically inhomogeneous half-space in which an earthquake source is buried. To this end, an efficient hybrid modal summation- boundary integral equation method (MSM-BIEM) is developed and applied. The MSM is used as a tool for simulating wave propagation from the source position to the multilayered laterally inhomogeneous geological profile where the BIEM is applied. The proposed model and the hybrid tool are used to investigate the phenomena of site effects. In fact, such a methodology has the potential to investigate the combined effects of different physical phenomena like surface topography, lateral inhomogeneity and the existence of water saturation in soils on the estimation of site effects. The model and hybrid computational tool developed are applied to contribute to the seismic risk analysis of the Bulgarian capital Sofia

Deterministic modelling for microzonation of Sofia - An expected earthquake scenario

The capital of Bulgaria, Sofia is a growing city with population around 1.22 million. The city is exposed to a high seismic risk since it is placed in the centre of Sofia seismic area. Over the centuries in the town of Sofia the macroseismic intensities have been larger than IX (MSK). A study of the site effects and the microzonation of a part of metropolitan Sofia, based on a  modelling of seismic ground motion along three cross sections are presented. Realistic synthetic strong motion waveforms have been computed for an expected scenario earthquake (M=7) applying a hybrid modelling method, based  on the modal summation technique and finite differences scheme. The site amplification is determined in terms of response spectra ratio (RSR). A set of time histories and quantities of earthquake engineering interest are supplied, that allow the definition of six zones characterized by specific response spectra. The results from this study constitute a “database” that describes the ground shaking of the urban area.  The synthetic velocigrams are employed to calculate the distribution of the horizontal strain factor Log10e using a simplified relation between particle velocity and velocity of shear waves in the surface layer. It is shown  that it is possible to estimate liquefaction susceptibility in terms of standard penetration tests (SPT), Nvalues and initial over burden stress. Using the data for maximum particle velocity and empirical relationships developed from the Northridge earthquake, 1994 the distributions of the expected pipe breaks and red-tagged buildings for Sofia city are shown

Deterministic earthquake scenarios for the city of Sofia

The 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 at Sofia, originate either beneath the city or are caused by seismic sources located within a radius of 40km. 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 makes 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, based on 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