HR: 1330h
AN: S22B-0448    [PDF]
TI: Seismic Attenuation in the Carpathian Bend Zone and Surroundings
AU: * Russo, R M
EM: ray@earth.northwestern.edu
AF: Dept. of Geological Sciences, Northwestern University, 1850 Campus Dr., Evanston, IL 60208 United States
AU: Mocanu, V I
EM: mocanu@gg.unibuc.ro
AF: Faculty of Geology & Geophysics, University of Bucharest, 6, Traian Vuia Str., Bucharest, 70139 Romania
AU: Radulian, M
EM: mircea@infp.ro
AF: National Inst. of Earth Physics, P.O. Box MG-2, Bucharest-Magurele, 76900 Romania
AU: Popa, M
EM: mihaela@infp.ro
AF: National Inst. of Earth Physics, P.O. Box MG-2, Bucharest-Magurele, 76900 Romania
AU: Bonjer, K
AF: Geophysikalisches Institut, University of Karlsruhe, Hertzstrasse 16, Karlsruhe, 76187 Germany
AB: We present measurements of crustal and upper mantle seismic attenuation in the southeastern Carpathian Arc and surroundings to test mantle responses to Tethys closure in this region. The unusual intermediate depth seismicity, volcanism, and surface deformation at and around the most tightly curved portion of the Carpathian arc, the Vrancea bend zone ($45.5^o$ N, $25.5^o$ E), has been the subject of many studies recently, culminating in a debate about the nature of the processes involved: one possible tectonic scenario to explain the Vrancea seismicity and surrounding structures invokes subduction of Tethys oceanic lithosphere, a remnant of which is presumed to be just detaching from unsubductible continental lithosphere of the East European and Moesian platforms. A second model holds that oceanic lithosphere subduction ended some time in the late Miocene, and that since then a portion of East European or Moesian platform continental mantle lithosphere has been delaminating along a horizontal mid-lithospheric interface and dripping down into the upper mantle. The active seismicity at Vrancea can be exploited to delimit both slab and asthenosphere distributions via measurements of seismic attenuation at stations distributed above the Vrancea zone. We measure attenuation via an iterative spectral ratio method which compares P and S spectra for an evolving time window of both arrivals, yielding 400 individual estimates of Q and one composite estimate derived from the stacked spectra of the idividual measurements. This procedure allows us to identify and exclude subtle multipathed phases with attenuation different from that of the direct arrivals and also yields a robust estimate of the measurement uncertainty. Measurements are retained for interpretation if the mean of the 400 individual Q estimates and the composite spectra Q estimate agree to within the (generally small) standard deviation of the 400 measurements. Results for all data recorded at the Romanian-German K2 accelerometer network during 1999 fall into clear groups: Attenuation is low (high Q) at stations east and north of the Vrancea zone on the East European platform and in the Dobrogea region. Attenuation at stations on the Moesian Platform, including those in and around Bucharest, is highly path-dependent in a complex way that probably reflects structure of the Vrancea body itself. Attenuation at stations above and near the Vrancea zone, and at stations in the Transylvanian Basin is high (low Q), most likely due to the presence of hot asthenosphere in these areas. These preliminary results may ultimately yield a strong test of the competing hypotheses put forth to explain the unusual seismicity and volcanism of the Carpathian arc. Finally, improved understanding of seismic wave propagation and attenuation in the study region will directly enhance our understanding of seismic hazard in this populous area.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
SC: Seismology [S]
MN: 2003 Fall Meeting