HR: 1340h
AN: S23B-0248    [Abstracts]
TI: Intraplate Seismicity in the Vrancea Region and In-Slab Intermediate Depth Earthquakes in the Plate Tectonic Frame
AU: * Purcaru, G
EM: purcaru@geophysik.uni-frankfurt.de
AF: Institute of Meteorology & Geophysics, Univ. Frankfurt, Feldbergstr. 47, Frankfurt/Main, 60323 Germany
AB: All shallow and intermediate-depth earthquakes in Vrancea-Romania and surroundings occur within the Eurasian plate. The in-slab intermediate-depth (I-D), h>60-70 km, generated possible largest (Mw=7-7.7) earthquakes. The shallow in-slab events are significantly of smaller magnitude; the same is true for those occurring in the broad area around the bend of the Carpathian Arc. The observed depth of Wadati-Beniof zone, situated at the contact between the Moesia and East European continental platforms, is about 220~km, dipping to NW at high angle, (>80°), as shown by reliable relocations. The (I-D) events are in the hard, cold oceanic part of the subducted slab; the subduction ceased in late Miocene. From some tomographic studies the slab may be present in the mantle down to 350~km, although using different data and quality, methods, boundary conditions, etc. Based on new data analysis and other recent results, we complement the model of Purcaru and Spence (1989) as:
1) the slab has five segments: the subducted crustal continental together with about 17~km thick sediments, down to 40--45~km; the weak one at 40--60~km, with low density with only rare, small earthquake activity; the highly seismic oceanic down to 175~km; the seismically weak oceanic to 220~km where a small earthquake (M~4) was located; the aseismic oceanic, that probably penetrated the mantle to 350~km or so.
2) the seismicity is distributed almost everywhere in the seismic slab,
3) the slab is continuous and was subducted nearly vertically,
4) no large shallow events occur at the interface of the continental lithospheres.
5) there exist no microplates or subplates in the region, which would be related to the occurrence of Vrancea earthquakes, as proposed earlier.
6) the tensional large events are consistent with the dominance of gavitational pull (slab-pull), as its major driving force whose effect is to extend and make thus the slab thiner.
7) the slab appears to be heated from below, loosing thus its coldness which can lead, on geological scale, to its assimilation by the surrounding hot mantle.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: Fall Meeting 2005