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<strong>First</strong> <strong>International</strong> <strong>Conference</strong> on MOLDAVIAN RISKS - FROM GLOBAL TO LOCAL SCALE 16-19 May 2012, Bacau, Romania 14
<strong>First</strong> <strong>International</strong> <strong>Conference</strong> on MOLDAVIAN RISKS - FROM GLOBAL TO LOCAL SCALE 16-19 May 2012, Bacau, Romania INSTABILITY OF THE MANTLE LITHOSPHERE DRIVES INTERMEDIATE-DEPTH SEISMICITY BENEATH THE VRANCEA REGION OF ROMANIA Gregory A. Houseman, Piroska Lorinczi, Yong Ren, Graham W. Stuart 1 School of Earth and Environment, University of Leeds, LS2 9JT, Leeds, UK Corresponding author: Greg Houseman, G.A.Houseman@leeds.ac.uk Abstract: The South Carpathian Project, a major seismological experiment carried out during 2009-2011 by the University of Leeds, the National Institute of Earth Physics in Bucharest, the Eötvös Loránd Geophysical Institute in Budapest, and the Seismological Survey of Serbia in Belgrade, has resulted in the most detailed tomographic images yet obtained of the upper mantle structure beneath the Pannonian – Carpathian region (Ren et al., 2012). These images illuminate the upper mantle over a wide region, but they specifically shed new light on the unique geological structure which is responsible for the damaging earthquakes that occur in the upper mantle beneath the Vrancea Zone of the South-east Carpathians. Since 1940 there have been four large earthquakes (M > 6.9) in this structure. These earthquakes create a major threat to life and infrastructure in Bucharest and other Romanian cities. The earthquakes occur at the NE end of an asymmetric high velocity structure that extends upward to the SW, oblique to the southern edge of the South Carpathians. This sub-vertical high-velocity body is bounded by slow anomalies to the NW and SE, which extend down to the top of the Mantle Transition Zone. With increasing depth, the fast region becomes more circular in cross-section until about 400 km where the fast anomaly fades out. The main mass of fast (presumably dense) material is located directly beneath the seismic activity. The earthquakes are all characterised by near-vertical T-axes, which means they are caused by vertical stretching. The seismic moment release rate can be used to estimate vertical strain rates; these strainrates imply that the mantle at 200 km is moving downward at about 20 mm/yr relative to the surface. The depth distribution of seismic-moment release rate follows a characteristic pattern that is most easily explained if this high velocity structure is produced by a Rayleigh-Taylor instability acting on an unstable stratification of mantle lithosphere above asthenosphere. Three-dimensional numerical experiments of viscous flow confirm that the drip-like structure that we image may be a natural consequence of a Rayleigh-Taylor instability triggered by recent convergence of Adria and Europe acting on a relatively weak Transylvanian lithosphere. The shallow (above 200 km) asymmetry of the fast structure, which connects the unstable mass to the relatively rigid Moesian platform, may play a significant role in enabling the efficient propagation of seismic energy toward the SW (toward Bucharest). One property of Rayleigh-Taylor instability is that rates of deformation increase with time during the peak development phase of the instability. The present high rate of tectonic activity in this region therefore is probably short-lived on a geological scale (< 1 Myr) but is likely to increase in energy before it is abates. Key words: South-East Carpathians; Upper Mantle Seismicity; Rayleigh-Taylor instability; Seismic Moment Rate. 15
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