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.