Near surface shear wave velocity in Bucharest, Romania

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 <i>Vs</i> structure and to improve the available microzonations we conducted seismic refraction measurements in two parks of the city. There the shallow <i>Vs</i> structure is determined along five profiles, and the compressional-wave velocity (<i>Vp</i>) 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 <i>Vp</i> model along profile 1 can be correlated with the known near surface sedimentary layers. <i>Vp</i> increases from 320 m/s near the surface to 1280 m/s above 55–65 m depth. The <i>Vs</i> models along all five profiles are characterized by low <i>Vs</i> (&lt;350 m/s) in the upper 60 m depth and a maximum <i>Vs</i> of about 1000 m/s below this depth. In the upper 30 m the average <i>Vs</i>³⁰ varies from 210 m/s to 290 m/s. The <i>Vp-Vs</i> 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.