H/V Spectral Ratios Technique Application in the City of Bucharest: Can We Get Rid of Source Effect?

The main issue of this paper is to show that contrary to many examples of monitored strong earthquakes in different urban areas, the intensity and spectral characteristics of the strong ground motion induced in Bucharest area, by Vrancea intermediate-depth earthquakes, is controlled by the coupled source-site properties rather than by the local site conditions alone. Our results have important implications on the strategy to follow when assessing the seismic microzoning for Bucharest city: we recommend the application of deterministic approaches rather than empirical techniques, like H/V spectral ratios. However, when applied to noise data, the H/V spectral technique succeeds to reproduce the predominant frequency response characteristic for the sedimentary cover beneath the city and the relatively uniform distribution of this structure over the city area. The same technique is clearly inadequate when small earthquakes are considered and our results strongly disagree with any strategy of extrapolation from small and moderate earthquakes to strong earth- quakes for microzoning purposes

Seismic Hazard of Romania: Deterministic Approach

The seismic hazard of Romania is estimated in terms of peak-ground motion values\u2014 displacement, velocity, design ground acceleration (DGA)\u2014computing complete synthetic seismograms, which are considered to be representative of the different seismogenic and structural zones of the country. The deterministic method addresses issues largely neglected in probabilistic hazard analysis, e.g., how crustal properties affect attenuation, since the ground motion parameters are not derived from overly simplified attenuation \u2018\u2018functions,\u2019\u2019 but rather from synthetic time histories. The synthesis of the hazard is divided into two parts, one that of shallow-focus earthquakes, and the other, that of intermediate-focus events of the Vrancea region. The previous hazard maps of Romania completely ignore the seismic activity in the southeastern part of the country (due to the seismic source of Shabla zone). For the Vrancea intermediate-depth earthquakes, which control the seismic hazard level over most of the territory, the comparison of the numerical results with the historically-based intensity map show significant differences. They could be due to possible structural or source properties not captured by our modeling, or to differences in the distribution of damageable buildings over the territory (meaning that future earthquakes can be more spectacularly damaging in regions other than those regions experiencing damage in the past). Since the deterministic modeling is highly sensitive to the source and path effects, it can be used to improve the seismological parameters of the historical events

Seismic hazard in Romania associated to Vrancea subcrustal source: Deterministic evaluation

Our study presents an application of the deterministic approach to the particular case of Vrancea intermediate-depth earthquakes to show how efficient the numerical synthesis is in predicting realistic ground motion, and how some striking peculiarities of the observed intensity maps are properly reproduced. The deterministic approach proposed by Costa et al. (1993) is particularly useful to compute seismic hazard in Romania, where the most destructive effects are caused by the intermediate-depth earthquakes generated in the Vrancea region. Vrancea is unique among the seismic sources of the World because of its striking peculiarities: the extreme concentration of seismicity with a remarkable invariance of the foci distribution, the unusually high rate of strong shocks (an average frequency of 3 events with magnitude greater than 7 per century) inside an exceptionally narrow focal volume, the predominance of a reverse faulting mechanism with the T-axis almost vertical and the P-axis almost horizontal and the more efficient high-frequency radiation, especially in the case of large earthquakes, in comparison with shallow earthquakes of similar size. The seismic hazard is computed in terms of peak ground motion values characterizing the complete synthetic seismograms generated by the modal summation technique on a grid covering the Romanian territory. Two representative scenario earthquakes are considered in the computation, corresponding to the largest instrumentally recorded earthquakes, one located in the upper part of the slab (M sub w = 7.4; h = 90 km), the other located in the lower part of the slab (M sub w = 7.7; h = 150 km). The seismic hazard distribution, expressed in terms of Design Ground Acceleration values, is very sensitive to magnitude, focal depth and focal mechanism. For a variation of 0.3 magnitude units the hazard level generally increases by a factor of two. The increase of the focal depth leads to stronger radiation at large epicentral dista and smaller radiation in the near-epicenter area. Certainly, the structural modeling is another essential factor in shaping the hazard distribution pattern. We consider in this paper the influence of the lateral variation of the upper mantle structure, suggested by the new tomography results in Vrancea region (Martin et al., 2002). Our results suggest that the deterministic modeling is a more sensitive way of constraining source parameters (magnitude, depth and focal mechanism) than the usual seismological methods and, that it can be used to improve the knowledge about the seismological parameters of the historical events

Seismic activity and stress field characteristics for the seismogenic zones of Romania

Biblioteca Centrale CNR / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Seismogenic zones of Romania

Biblioteca Centrale CNR / CNR - Consiglio Nazionale delle RichercheSIGLEITItal

Seismic hazard in Romania associated to Vrancea subcrustal source: deterministic evaluation

Consiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome / CNR - Consiglio Nazionale delle RichercheSIGLEITItal