Source scaling of intermediate-depth Vrancea earthquakes

Source scaling properties are studied for the intermediate-depth seismic nest in the Vrancea region, Romania, which has been the source of many destructive earthquakes. We investigate spectral and time-domain scaling properties using wide-band digital records from 16 earthquakes (3.7 less than or equal to M-W less than or equal to 7.4). All processing variants (P or S waves, spectral or time domain, etc.) produce consistent results. The observed corner-frequency versus M-W trend generally follows the constant-stress-drop model, with typical stress-drop values of 1-10 MPa. This kind of scaling, seen over the entire magnitude range analysed, is similar to that observed for shallow events. However, this trend seems to be violated for the largest earthquakes (M-W > 6.5). They show a clear tendency for higher static stress drops than shallow events, and for magnitudes above 7, average stress drops exceeding 10 MPa may be expected. These results are of particular significance for seismic hazard studies, and specifically for the estimation of future strong motions.