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Abstract

The seismic source parameters (seismic moment, corner frequency, source radius, static stress drop) of the moderate-size earthquakes which occurred in the Vrancea intermediate-depth seismogenic region (Romania) during the last decade are determined using the spectral analysis of S waves. The events have moment magnitudes between 4.5 and 5.7, and depths in the range from 90 to 150 km.

The observed data are high-quality horizontal acceleration records. The source parameters are estimated from the displacement amplitude spectrum of the transverse component of motion. The duration of the time windows selected to compute the Fourier spectra is dependent on the earthquake magnitude and amplitude behaviour of the wave trains.

The relations between the retrieved source parameters have also been examined. The trend of increasing fault radius with increasing seismic moment is visible, however the slope of the regression line is clearly smaller than the theoretical value predicted by the theory of self-similarity. The tendency of decreasing stress drop with decreasing seismic moment can also be noticed, which might indicate a deviation from the constant stress drop model.