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EXTENSIONAL TECTONICS IN VRANCEA ZONE (ROMANIA) INTERPRETED ON RECENT SEISMICITY, GEOPHYSICAL AND GPS DATA

Dumitru Ioane

Abstract

Due to increased precision in positioning and depth determination, recent seismicity data (January 2014 - January 2018) from ROMPLUS Earthquake Catalogue of Romania[1] have been used to illustrate active faults at crustal and lithospheric depths in Vrancea, a seismogenic sector located in the East Carpathians Bend Zone (Romania). Multiple N-S, W-E, NW-SE trending seismicity sections across the Vrancea area have been built and analyzed, as well as seismicity maps at various depths, or for depth intervals. The seismic events magnitude ranged between 1.2 Mw and 5.6 Mw, and their depth ranged from 1 km to 160 km. The recent crustal seismicity (1 ? 60 km) revealed two lineaments trending NNE-SSW, presently interpreted as the re-activated tectonic contacts by extension processes of the graben-like Permo-Triassic structure, illustrated till 45 ? 50 km by refraction seismics[2]. The lithospheric seismicity (90 ? 160 km) shows a quite compact NE-SW seismic swarm elongated on cca 50 km, located between the two crustal lineaments. Seismicity sections, built for depths between 1 km and 160 km across the Vrancea zone, illustrate two main extensional tectonics stages: a) a Permo-Triassic one, developed NNE-SSW at crustal depths, its re-activated contacts contributing to the regional crustal seismicity; b) a more recent one, possibly Quaternary in age, developed beneath and between the crustal tectonic structure. They form presently a single seismo-tectonic structure, the active faults which represent their external boundary starting from a common sector at the highest lithospheric depths and showing the maximum extension immediately beneath the Carpathians. The evident change in orientation between the crustal and lithospheric structures suggests significant modifications in time of the regional driving forces involved in tectonic processes and may be an important cause of the associated high seismicity. The regional tectonic and geodynamic background for interpreting the recent seismicity in Vrancea is offered by the Bouguer gravity map of Romania[3], which depicts at the East Carpathians Bend Zone anomalous lineaments trending NE-SW, associated with a system of normal faults crossing Vrancea and its adjacent regions[4]. Results of GPS geodynamic observations[5], showing a horizontal displacement of the East Carpathians Bend Zone foreland towards SE, favored the interpretation of regional extension processes, in an area where compressional ones have been postulated in the past by most geodynamic models.

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