A 2-D Velocity Model of the Vrancea Region In Romania: Prediction of Teleseismic Waveforms

Abstract

SUMMARY In previous studies, anomalies in arrival time and amplitudes of depth phases pP and sP recorded at teleseismic distances and produced by large intermediate-depth events of the Vrancea region were pointed out (Perrot et al. 1994). The modelling of major recent events has shown that the presence of a dipping interface and thick low-velocity sediment layer in the upper crust above the hypocentral area can explain such anomalies. Simulating broad-band records of the 1990 May 31 earthquake, it is shown that the 2-D crustal velocity model derived by Perrot et al. (1994) can also be used to explain observed waveforms in an extended azimuthal range from 105 to 243 and is valid for earthquakes having different hypocentral locations in the deep Vrancea seismic zone. Using records in the azimuthal range where a classical spherical model produces a good fit to the observed waveforms, the source is best modelled by two point sources at 90 and 93 km depth releasing the same amount of seismic moment, and separated by 1.5 s in time. Synthetics calculated for this source time history and the 2-D velocity model above the source give a much improved fit of the observed waveforms over what could be achieved with a 1-D spherical model, The results suggest that teleseismic waveforms could be reasonably well predicted for any event in the region with the 2-D velocity model, which accounts for the main structural features in the upper crust of the Vrancea region.