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Abstract:
The Vrancea region is one of the most active intracontinental seismic areas in Europe, and main source of seismic hazard in the extra-Carpathian zone of Romania.
The frequency content of the earthquake ground motion is an important characteristic of earthquake loading for the seismic response of engineering systems.
Our study is focused on two frequency content – based parameters: the ground motion dominant frequency and the mean period Tm.
The study data consist of horizontal acceleration records of 19 earthquakes with moment magnitude ≥ 4.5: four strong earthquakes (Mw between 6.4 and 7.4) recorded with analogue instruments in the period 1977-1990, and 15 moderate-size events (Mw in the range 4.5-6.0) digitally recorded in the period 2004-2022.
To point out the relative contribution of the travel path and seismic source to the formation of the frequency content indicators, we examine separately the effects of hypocentral distance, focal depth and earthquake magnitude on the two parameters.
The analysis does not evidence a significant effect of attenuation, up to hypocentral distances of about 350 km. Likewise, no effect of source depth (85 < h < 150 km) is detected. The influence of earthquake magnitude is considerably stronger than the influence of distance, while the effect of site conditions is prevailing on the dominant frequency and mean period.
Finally, we develop simple empirical predictive relationships for the investigated parameters. The proposed models predict the frequency content indicators as function of magnitude, focal distance and local site conditions.
