Scalar Frequency Content Parameters of Ground Motion from Vrancea (Romania) Intermediate-Depth Earthquakes

We investigate the frequency content of ground motions caused by the intermediate-depth earthquakes of Vrancea region – one of the most active intracontinental seismic areas in Europe, and the main source of seismic hazard in the extra-Carpathian area of Romania. The study is focused on two scalar parameters derived from the Fourier amplitude spectrum of acceleration time history: the dominant frequency and the mean period. The data are strong motion records from 19 earthquakes (four events with moment magnitudes between 6.4 and 7.4, which occurred in the period 1977–1990, and 15 events with magnitudes in the range 4.5–6.0, which occurred in the interval 2004–2022), collected by 167 stations of the Romanian seismic network. We examine the influence of source, path and local factors on the two frequency content indicators, and, taking into consideration the observed trends, we develop empirical models relating the dominant frequency and mean period to earthquake magnitude, hypocentral distance, site effects. These models are the first prediction relations proposed for the two parameters of the ground motions generated by the subcrustal events of Vrancea. To reduce the uncertainties in our models, we introduce empirical site terms, defined as the average residuals – by site, over the recorded earthquakes – between the observed and predicted spectral indicators. The site corrections, estimated for the stations that provided at least 5 records to the study dataset, lead to a considerable decrease in the standard deviation of the residuals of the proposed relations.