Abstract
Abstract. Bucharest, the capital of Romania with nearly 2 1/2 million inhabitants, is endangered by the strong earthquakes in the Vrancea seismic zone. To obtain information on the near surface shear-wave velocity Vs structure and to improve the available microzonations we conducted seismic refraction measurements in two parks of the city. There the shallow Vs structure is determined along five profiles, and the compressional-wave velocity (Vp) structure is obtained along one profile. Although the amount of data collected is limited, they offer a reasonable idea about the seismic velocity distribution in these two locations. This knowledge is useful for a city like Bucharest where seismic velocity information so far is sparse and poorly documented. Using sledge-hammer blows on a steel plate and a 24-channel recording unit, we observe clear shear-wave arrivals in a very noisy environment up to a distance of 300 m from the source. The Vp model along profile 1 can be correlated with the known near surface sedimentary layers. Vp increases from 320 m/s near the surface to 1280 m/s above 55–65 m depth. The Vs models along all five profiles are characterized by low Vs (<350 m/s) in the upper 60 m depth and a maximum Vs of about 1000 m/s below this depth. In the upper 30 m the average Vs30 varies from 210 m/s to 290 m/s. The Vp-Vs relations lead to a high Poisson's ratio of 0.45–0.49 in the upper ~60 m depth, which is an indication for water-saturated clayey sediments. Such ground conditions may severely influence the ground motion during strong Vrancea earthquakes.
Highlights
The shallow seismic velocity structure of the uppermost subsurface is crucial in estimating the amplification of earthquake-induced ground motion
There the shallow Vs structure is determined along five profiles, and the compressional-wave velocity (Vp) structure is obtained along one profile
It is well known that variations of the elastic properties at shallow depths can map directly into the earthquake damage pattern, due to local site effects (e.g. Borcherdt, 1970; Meremonte et al, 1996)
Summary
The shallow seismic velocity structure of the uppermost subsurface is crucial in estimating the amplification of earthquake-induced ground motion. Of special importance is the knowledge of the Vs structure, because of its correlation with high ground motions and earthquake damage Information about these parameters is especially valuable in cities which are situated in the vicinity of major earthquake focal regions. Within the uppermost Quaternary layers there are unconsolidated fluvial deposits (Ciugudean-Toma and Stefanescu, 2006) This special situation causes large ground motion amplitudes in Bucharest, which is situated about 140–160 km southwest from Vrancea (Fig. 1), and in several other towns in this region (for a summary see Wenzel et al, 1999). To understand and mitigate the earthquake risk for Bucharest, a major effort was made over the last 12 years by the Collaborative Research Centre 461 at the Universitat Karlsruhe (TH), Germany and the Romanian Group for Strong Earthquakes (Wenzel, 1997) Within this framework we chose two locations in the city of Bucharest to measure Vs along five seismic refraction profiles.
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