Assesment of site effects in downtown Bucharest by recordings of ambient noise moderate and large intermediate depth earthquakes from the Vrancea focal zone

The amplification of the soil in the Romanian capital Bucharest is studied using reference and non-reference site methods. Besides the traditional H/V-techniques, simultaneous inversions for source and site spectra of multiple recorded events from the Vrancea focal zone were performed. This area, located in the SE Carpathians, is characterized by a high rate of occurrence of large earthquakes in a narrow focal volume. The epicentral region is confined to about 60 x 20 km2 and the seismic activity ranges prevailingly within an almost vertical stripe in depths between 70 and 180 km. Four large events occurred during the last century in this small seismogenic zone: on November 10, 1940 with magnitude Mw = 7.7, March 4, 1977 with Mw = 7.5, on August 30, 1986 with Mw = 7.2, and on May 30, 1990 with Mw = 6.9. For the 1986 and 1990 events, strong-motion recordings of analog SMA-1 systems are available from about 12 freefield stations in the Romanian capital, which is about 150 to 200 km apart from the epicenter area. These data and the recordings of moderate events by the digital K2-network are compared to ambient noise measurements, which were collected in downtown Bucharest during campaigns in 1997 and 2002 at about 16 and 20 sites respectively. All three datasets show a broad but dominant resonance of the soil with periods between 1 and 2 seconds for the whole urban area of Bucharest. The implications for the building stock in Bucharest will be discussed. (authors)