First International Conference on MOLDAVIAN RISKS – FROM ...

<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.
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