- © 2005 by the Seismological Society of America
Bucharest, the capital of Romania with 2.5 million inhabitants, provides an ideal target to study the urban seismic wave field, which is composed of natural and man-made signal components. The city and surrounding towns in southeast Romania are endangered by strong intermediate-depth earthquakes of the Vrancea source zone (Figure 1). The bigger shocks reach moment magnitudes larger than 7 and occur about three times per century, causing potentially significant death tolls and damage (for a summary see Wenzel et al., 1999). A magnitude 4.5 event from the Vrancea slab is recorded about once a month, and there is now a well developed infrastructure to conduct seismological research. We use this environment to develop a combined structural and hazard analysis for Bucharest as well as to conduct basic research on the urban seismic wave field.
Typically, seismological stations are installed at remote and low-noise sites to achieve a high signal-to-noise ratio for seismic phases. Thus high-quality seismological data from major cities with vigorous human activity are mostly not available. Generally only triggered strong-motion equipment is used for seismic monitoring. Relatively few data are available to study the characteristics of urban seismic recordings, which may be further strongly influenced by the effects of the shallow geologic subsurface structure and the buildings themselves (Wirgin and Bard, 1996; Boutin and Roussillon, 2004).
We ran a broadband seismic network in the metropolitan area of Bucharest to record the natural and cultural microseismicity as well as earthquake- and explosion-related seismic waves. These data allow us to study a wide range of issues, such as the nature of urban noise (e.g., as a time-dependent function), the local elastic structure, and the structure of distant regions using array techniques. Exploring horizontal to vertical (H/V) spectra and spectral ratios with respect to a reference site …