Asymmetric distribution of seismic motion across south-eastern Carpathians and its implications

One of the most striking features of the seismic motion distribution radiated by the Vrancea intermediate-depth earthquakes is a strong lateral variation when passing from the fore-arc region to the back-arc region. The analysis of the seismograms recorded during the CALIXTO'99 tomography experiment for small- and moderate-magnitude earthquakes shows that the amplitudes are severely diminished by a factor of 10 to 100 in the back-side part (Transylvanian Basin) relative to the fore-side part (Romanian Plain). At the same time the predominant frequencies are shifted toward lower values for the stations located in the back-side area. Such effects could be explained in principle by source radiation pattern, attenuation and local site response. Since systematic lateral variations are obtained regardless of the fault geometry, we reject the hypothesis of the source effect. From other point of view, the information available regarding the structure of the uppermost crust shows no systematic difference between the 'fore-arc' and 'back-arc' areas. Consequently, we conclude that the difference observed in the frequency content and amplitude level comes from the deeper part of the structure and reflects a high inhomogeneous structure of the seismic attenuation, with high Q values in the colder seismogenic (subducting) slab and low Q values in the mantle wedge. This interpretation is consistent with the tomography image (Martin et al., 2001) and the heat-flow measurements (Demetrescu and Andreescu, 1994). Similar patterns are reported in other regions of the world (e.g., Japan, Tonga-Fiji, Vanuatu, North America, Brazil) characterized by typical active subduction systems (e.g., Satake and Hashida, 1989; Tsumura et al., 2000). We assume that the presence of a hot asthenospheric body just behind the Carpathian arc could explain the highly anomalous ground motion attenuation towards the Transylvanian basin, as revealed by macroseismic and instrumental data. (authors)