Source-side shear wave splitting and upper mantle flow in the Romanian Carpathians and surroundings

Abstract

We present shear wave splitting measurements from 5 earthquakes that occurred in the Vrancea seismic zone of the Carpathian Arc. S waves from these events, all with magnitudes > 5.4 M w and deeper than 88 km, were recorded at broadband stations of the Global Seismic Network, and the Geoscope and Geofon Networks, and used by us to measure shear wave splitting corrected for sub-station splitting and anisotropy. In order to carry out these corrections we used published shear wave splitting parameters, thus isolating contributions to observed splitting from the Vrancea source region and upper mantle surrounding the Carpathian Arc. The resulting 32 good observations of source-side shear wave splitting, along with 54 null splitting observations (which yield two potential splitting directions) clearly show that upper mantle anisotropy is strongly variable in the region of the tightly curved Carpathian Arc: shear waves taking off from Vrancea along paths that sample the East and Southern Carpathians have fast anisotropy axes parallel to these ranges, whereas those leaving the source region to traverse the upper mantle beneath the Transylvanian Basin (i.e., mantle wedge side) trend NE–SW. Shear waves sampling the East European and Scythian Platforms are separable into two groups, one characterized by fast shear trends to the NE–SW, and a second, deeper group, with trends to NW–SE; also, the majority of null splits occur along paths leaving Vrancea in these NE–E azimuths. We interpret these results to indicate the presence of at least three distinct upper mantle volumes in the Carpathians region: the upper mantle beneath the Carpathian Arc is strongly anisotropic with fabrics parallel to the local arc strike; the Transylvanian Basin upper mantle fabrics trend NE–SW; and the anisotropy beneath the westernmost East European Platform may be characterized by a shallow NW–SE trending fabric concentrated in the cratonic lithosphere of the East European Platform, and a second, deeper fabric with E–W trend marking asthenospheric flow beneath the craton's base. This more complex anisotropy beneath the western edge of the East European Platform would account for both the variability of observed splitting of waves that sample this volume, and also the strong prevalence of nulls observed along eastward-departing azimuths.