New Approaches in the Investigation of the Characteristics of Romanian Vrancea Earthquakes I.G. Craifaleanu URBAN-INCERC, INCERC Bucharest Branch, Romania Technical University of Civil Engineering Bucharest, TUCEB, i.craifaleanu@gmail.com I.S. Borcia URBAN-INCERC, INCERC Bucharest Branch, Romania, isborcia@incerc2004.ro ABSTRACT: The paper focuses on the evaluation and mapping of the frequency-dependent spectrum based seismic intensity, also called instrumental intensity. This index, calculated basically from the integration of the square values of spectral acceleration ordinates, has proved very promising in estimating the destructive potential of earthquakes. The values of the instrumental intensity are calibrated to match the values of the EMS-98 intensity scale. Previous studies have shown that the proposed index could be used as a basis for the development of a new intensity scale, which would rely as well (possibly mainly) on instrumental data. The paper presents the first maps describing the spatial distribution of the instrumental intensity ordinates, for the three strongest Vrancea earthquakes for which accelerographic data was recorded at multiple stations, i.e. the August 30, 1986, May 30, 1990 and May 31, 1990 events. Maps are generated for separate significant frequency bands, in order to reveal the destructiveness of the earthquakes for different classes of buildings. Results are compared and correlated with previous studies on Vrancea earthquakes and with information provided by damage reports from the considered earthquakes. Keywords: Spectrum based seismic intensity, Vrancea earthquakes, EMS-98 intensity scale, seismic intensity maps 1. INTRODUCTION The intensity based on destructiveness spectrum, ( ) ϕ d i is defined by the following expression (Sandi, 1987, Sandi et al., 1998) ( ) ( ) ( ) 75 . 5 , , log 2 4 + ξ ϕ = ϕ  dt t w i a d (1.1) where ( ) ξ ϕ, , t w a is the (absolute) acceleration (m/s 2 ) for a pendulum of natural frequency ϕ (Hz) and % 5 = ξ is the damping ratio. The values of the above instrumental intensity are calibrated to match the values of the EMS-98 intensity scale. In order to assess the destructiveness on separate frequency bands, the intensity in equation (1.1) was averaged upon spectral bands, (ϕ', ϕ"), the averaging rule being described by the following equation: ( ) ( ) ( ) ( ) [ ] { } 75 . 5 , , ln 1 log , 2 4 * + ϕ ϕ ξ ϕ ϕ′ ϕ′ ′ = ϕ′ ′ ϕ′  d dt t w i a d (1.2) The analysis was related in principle to the 36 dB frequency band (0.25 Hz…16.0 Hz), adopted as a reference, divided into three 12 dB subintervals, as shown in table 1.1. The Id3 intensity values calculated for these 3 subintervals were denoted, in order, by Id31, Id32 and Id33. The intensity calculated for the entire frequency band was denoted by Id1. Similar intensities were calculated by