Florina Chitea

Active Fault Systems and Their Significance for Urban Planning in Bucharest, Romania

In urban planning activities, besides a detailed seismic zonation, knowledge of the areal distribution of active faults, and particularly of faults crossing the city in heavily built districts, may be valuable in numerous cases. This chapter aims to identify active faults or fractures in Bucharest, a large city developed over thick layers of Quaternary unconsolidated sediments. To achieve this goal, the research is based on an integrated analysis of all existing relevant data coming from geology, geomorphology, geophysics and seismology. Another purpose is to understand the role of faults in the spatial variability of seismic damaging effects on buildings in view of a better future urban planning. The N–S fault system seems to be involved in neotectonics, affecting the geomorphology between Bucharest and Ploiesti cities. It could represent an active transcrustal fault, the deepest seismic event recorded within Bucharest at subcrustal depth being located on its southern prolongation. Within Bucharest the presence of faults affecting Pleistocene sediments was previously illustrated using borehole data, shallower vertical displacements of Quaternary formations along faults being interpreted in this study. The existing seismic reflection data recorded within Bucharest was used to interpret shallow faults considered as active or capable faults crossing Quaternary sedimentary deposits.

Scenarios for Local Seismic Effects of Tulcea (Romania) Crustal Earthquakes—Preliminary Approach of the Seismic Risk Characterization for Tulcea City

This paper is a multidisciplinary presentation of the seismogenic area situated in the North-Dobrogea Orogen (Tulcea). This zone is characterized by significant crustal seismic activity, with crustal earthquakes of magnitude Ms ≥ 5.0 on the Richter scale. Geological and geophysical data for the area are presented. The seismicity of the region is presented by making use of the latest catalogues, exemplified with maps and a 3D figure. Focal mechanisms with their parameters for several earthquakes are analyzed together with the observation data (provided by different seismological stations). A brief presentation of the main geological features, which are characteristic of the tectonic units that build up the North-Dobrogean Orogene, outlines the diverse petrographic constitution of the various structural levels. In order to discuss the local seismic site effects two scenarios are considered, both of which take into account the characteristics of the seismogenic area. The first one considers the city exposed to an earthquake (superficial) from the E Vrancea zone and the second one considers the city exposed to a seismic event with magnitude Mw = 5.1 from Sf. Gheorghe fault. The earthquake epicentres are located in very active seismic areas. The important features taken into account are the nonlinear behavior of the upper soil strata, the effect of the bedrock elasticity, and different shear modulus and damping of the linear-equivalent-type system. Additionally, several local amplification functions are presented.

The Space-Time Distribution of Moderate- and Large-Magnitude Vrancea Earthquakes Fits Numerically-Predicted Stress Patterns

Each of the three major earthquakes (M w ≥ 6.9) recorded within the Vrancea seismogenic body in the years 1977, 1986 and 1990 may have been the result of long-range interactions. The latter seemed to be initiated in coincidence with a moderate (4.7 ≤ m b ≤ 4.9) shock that systematically occurred in the 160–175 km depth-range, 3–4 years in advance of the major earthquake. In addition, each corresponding pair of moderate/major events systematically exhibited, in terms of focal mechanisms, a particular pattern: the latter complied with predictions of a numerical model that had addressed along-strike break-off experienced by a near-vertical slab, which—at the same time—was strongly coupled with its overriding plate along a steeply dipping contact extending on a significant vertical length. It was thus suggested that the Vrancea moderate events of thrust-fault type occurred in the 160–175 km depth-range could correspond to the along-strike propagation of a detachment horizon tip; while the major earthquakes subsequently occurred at shallower depths (85–135 km), were possibly caused by the shearing forces that acted, at the upper-plate/underlying-plate interface, in response to the increased downward pull experienced by the Vrancea slab as the break-off was propagating laterally.