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R. M. Russo
Assistant Professor
223 Williamson Hall
phone: 2-6766
rrusso@ufl.edu

Office hours
MWF: 11:40-12:40
or by appointment

    

Seismic Attenuation and Anisotropy in the Carpathians Region

Does the upper mantle seismicity of the Romanian Vrancea Zone represent the last gasp of subduction of an embayment of the Tethys Ocean?  Or is it actually a unique example of active continental lithosphere delamination?  With Co-Principle Investigators Victor Mocanu, Laurentiu Munteanu, Mircea Radulian, Mihaela Popa, Klaus Bonjer, graduate student Mirela Dardac, and former UF undergraduate student Teresa Garcia, we are seeking to use the Vrancea Zone seismicity to characterize the upper mantle thermal state and current flow pattern in the Carpathian Arc region.  The frequent seismicity from depths of 70-200 km beneath Earth's surface are well recorded at seismic stations in the Transylvanian Basin, the Eastern and Southern Carpathians, Dobrogea, the Moesian Platlform, and the Moldavian and Scythian portions of the East European cratonic platform.  Seismic attenuation and anisotropy can help us resolve the question of the origin of the Vrancea seismicity, and the late Cenozoic geodynamics of the Transylvanian Basin, the Carpathian Arc, and the adjacent stable Platforms.

The following figures are from our publication:

Russo, R.M., V. Mocanu, M. Radulian, M. Popa, and K. Bonjer, Seismic Attenuation in the Carpathian Bend Zone and Surroundings, Earth and Planetary Science Letters, vol. 237, pp. 695-709, 2005.

Map of Carpathians study region

Figure 1, above, shows the geography and topography of the Romanian Carpathians region.  The concentration of red stars are the locations of intermediate depth Vrancea zone earthquakes.  The blue-outlined fields are outcrop areas of mostly andesitic volcanic rocks erupted during the late Cenozoic, persumably as terranes in the Transylvanian Basin converged with the stable East Europen and Moesian Platforms.  The heavy red line and dashed red line show geologists' best estimates of the suture line between Transylvania terranes and Platforms, and an extremal estimation (dashed line) of the suture.  Note that the intermediate depth earthquakes - normally associated with subduction of lithosphere - lie SE of even the extremal potential suture.

Vrancea Zone seismicity

Figure 2.  Map of major regional structures, including active or recently active surface faults.  Seismic stations of the German-Romanian K-2 Network, operated by the University of Karlsruhe and the Romanian National Institute of Earth Physics (NIEP) are shown as purple triangles.  In the first phase of our study, we use recordings of Vrancea earthquakes at these stations to determine seismic attenuation and anisotropy in the study region.  Note, however, that only one station (OZU) lies within the Transylvanian Basin.


                 3-D image of Vrancea seismicity


Figure 3.  Vrancea zone seismicity recorded and located by NIEP, plotted in a 3-D simulating perspective.  View is from the NW looking SE.  Topography plotted on box bottom as an aid to geographic recognition.  Note that the Vrancea zone earthquakes define an oddly-shaped body with steep or nearly vertical plunge.  "Shadows" of the seismicity are projected as black circles onto the eastern and southern sides of the box to clarify the body's shape.

                 


             Station VRI seismogram


Figure 4.  Seismograms of Vrancea zone earthquake recorded at station VRI, almost immediately above the Vrancea zone (see Fig. 2).  P and S waves are clear the window about each phase used in the iterative QS procedure shown as gray shaded areas about P and S.  Also shown is pre-signal noise window used to estimate noise spectra.


              Q measurement 1

Figure 5.  P and S spectra for event shown in Figure 4.  P spectrum measured from the vertical component, S spectrum from the transverse component.  Note peaks of P and S energy do not coincide, and in order to avoid positive slopes, we limit the frequency band of calculation to the cross-over frequency.


Method 1

Figure 6.  Seismograms of Vrancea zone earthquake shown in Figure 4, detailing the two portions of each phase used in the iterative QS routine.  P and S windows are each divided into two parts.  The first portion of each window is always included in the spectral amplitude calculations.  The second portion of the window is divided into 20 equal portions and sequentially added to the time series for P and S, respectively, before taking the Fourier transform.  For each of the 400 (20x20) time series thus formed, the S-to-P spectral ratio is calculated and QS is determined.  The mean QS and standard deviation are thus determined.


Method 2
 
Figure 7.  Each of the 400 spectra determined as described in Figure 6 is retained and summed to all its predecessors.  This provides a composite or 'stacked' spectrum for both P (top left) and S (top right), filling holes in the spectra and suppressing noise (uncorrelated) as the time windows are lengthened.  Noise spectra are shown as dotted lines.  The spectral ratio of the normalized composite spectra is then determined and QS is calculated for the composite spectral ratio (bottom).
                                                                                                   
Results:  East European stations

Figure 8.  Ray paths from Vrancea events to stations on the East European, Scythian, and eastern Moesian Platforms, color coded according to observed QS for each event-station pair.  Color code:  whiteS blocked;  red:  QS = 100-200; orange:  QS = 200-250; yellow:  QS = 250-300; green:  QS = 300-350; cyan:  QS = 350-400; blue:  QS = 400-1000.  Stations are magenta triangles and events are red stars.  Topography, stations, and events shown projected on bottom of block.


Results:  Low Q region


Figure 9.  Ray paths from Vrancea events to stations in the Carpathians and Transylvanian Basin, color coded according to observed QS for each event-station pair.  Color code:  whiteS blocked;  red:  QS = 100-200; orange:  QS = 200-250; yellow:  QS = 250-300; green:  QS = 300-350; cyan:  QS = 350-400; blue:  QS = 400-1000.  Stations are magenta triangles and events are red stars.  Topography, stations, and events shown projected on bottom of block.



Results:  LUC and environs


Figure 10.  Ray paths from Vrancea events to station LUC, in the eastern MoesianPlatform, color coded according to observed  QS for each event-station pair.  Color code:  whiteS blocked;  red:  QS = 100-200; orange:  QS = 200-250; yellow:  QS = 250-300; green:  QS = 300-350; cyan:  QS = 350-400; blue:  QS = 400-1000.  Stations are magenta triangles and events are red stars.  Topography, stations, and events shown projected on bottom of block.


Transylvania Basin - EEP contrast 1

Figure 11.  Comparison of QS observed at three stations lying along a line extending from the back-arc (OZU) across the Carpathians to the presumed forearc and foreland.  Color-coded ray paths projected to surface.  Note QS to OZU, in the Transylvanian Basin is almost all low (red and orange).  At station GRE, results are strongly mixed, with apparently lower QS values from events in the southwestern portion of the Vrancea zone.  At LUC, QS shows a pattern similar to that at GRE but less pronounced in QS variability.  Heavy black line:  NW-SE line of section for Figure 12.  Color code:  whiteS blocked;  red:  QS = 100-200; orange:  QS = 200-250; yellow:  QS = 250-300; green:  QS = 300-350; cyan:  QS = 350-400; blue:  QS = 400-1000.  Stations are magenta triangles and events are red stars.


Schematic interpretive section 1

Figure 12a.  Color-coded ray paths to three stations shown in Figure 11 projected onto NW-SE cross section.  Tectonic units adapted from [Girbacea and Frisch, Geology, vol. 26, pp. 611-614, 1998], assuming a delamination horizon (heavy black-white dashed line) at 70 km, consistent with mantle xenolith compoisition in Persani basalts.  High-Q paths to LUC and GRE cross preumably low-attenuation continental mantle lithosphere and crust.  Not all the low QS rays to station OZU can be explained by this model, since many do not travel through highly attenuating asthenosphere.  Either attenuation occurs at shallow depths beneath OZU, or the model should be modified.

Schematic interpretive section 2

Figure 12b.  One possible modification to delamination model that would make it consistent with our results:  raise the delamination horizon (heavy black-white dashed line) to a shallower depth so low-Q paths to OZU cross a significant thickness of asthenosphere.


Visit the UF-UB Carpathians Project Web Site

   http://seismology.geology.ufl.edu/romania

The University of Florida-University of Bucharest Project is sponsored by the Geophysics Program and International Programs of the U.S. National Science Foundation, and the University of Bucharest.  We also gratefully acknowledge support from IRIS PASSCAL.


   
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