Physical and dynamic structure of seismic-active areas from Moesian platform and Dobrogea

The research project no 35/2001 proposes a complex study of seismic-active areas from main geological unit of Moesian Platform and northern Dobrogea. In the first half of 2002 the research work was carried out in the western part of platform, west of 26 angle meridian. In the North, the study extended up to 45 angle N parallel, in southern sector of Carpathian foredeep. Western part of the Moesian Platform is characterized by a moderate seismic activity, of crustal pattern. During 1893-2001, 88 earthquakes occurred, 8 of them having magnitudes greater than 4.0. Temporal analysis of seismic shocks produced in the XX century evidenced a small activity between 1900 and 1943, no activity between 1944 and 1973, and an intensified seismic activity in 1974-2000. A maximum appears in 1991 by a strong earthquake of 5.6 degrees magnitude. Spatial analysis of quakes showed a migration of seismic activity, from east to west, from a shock of 4.5 magnitude in 1912 (west Ramnicu Valcea) to earthquake of 5.2 magnitude in 1943 (North Tg. Jiu) and the seismic event in 1991 with 5.6 magnitude (North-West Turnu Severin). Local mechanism analysis has been possible for 22 earthquakes. This processing utilized the P-waves recorded by the Romanian stations (conventional and telemetered) and information of World Data Center for Seismology (Denver, USA). Fault plane azimuths indicated two distinct groups, characterized by the following average directions: I- N 10 angle W and II- N 70 angle E. These represent directions of the principal faulting systems from platform (I- N-S faults, perpendicular on Carpathian orogen direction; II- E-W faults, parallel with orogenic area). Fault plane dips have great angles, between values of 37 angle and 89 angle (with an average value of 63 angle). Compression (P) and tension (T) axes show different directions indicating two faulting systems. For the N-S faults (I system) the compression forces (P) are oriented N 70 angle W(WNW-ESE) and tension forces N 15 angle E(NNE-SSW). For the II fault system (E-W oriented) compression forces have N 20 angle W and N 55 angle E directions and tension forces are oriented in N 20 angle E and N 75 angle W directions. These results include all earthquake data (including seismic shocks occurred North-West Turnu Severin). Seismic and acoustic log data inform us on seismic waves velocity in connection with physical and lithological constitution of sedimentary deposits. General consideration on seismic characteristics of the Moesian Platform was evidenced in the paper. In the frame of our project we obtained sketches with velocities at four depth levels (500 m, 1000 m, 2000 m, 3000 m). On these maps we located earthquake epicenters. This information were correlated with heat flow values (measured at surface) and temperatures observed in boreholes at 3000 m depth. This tomography image of sedimentary crust evidenced some characteristics connected by lithology and depth of sediment. High and low velocity zones are not correlated with epicenter position. Epicenter distribution pointed out in the platform some active areas as: Alexandria-Chiriacu, Turnu Magurele-Caracal and Calafat-Strehaia. In North, in the transition zone to Carpathian foredeep we remarked some areas with seismic moderate potential; in West Tg. Jiu and North-West Turnu Severin, and Ludesti-Cobia and Tartasesti-Butimanu in eastern part. In framework of project we described crustal structure of these areas, fault plane solutions (if exist) and other geophysical data (seismic in principal). Analyzed sector of the Moesian Platform evidenced a relative low level of the seismicity. The fact indicates a stable behavior of this structural unit in comparison with Dobrogea or external zone of the Southern Carpathians (Mehadia area). Probable cause of earthquakes is isostatic reposition of crustal blocks resulted by faulting process. Recent paleostress studies identified some tectonic episode in external zone of Southern Carpathians and in Moesian Platform. (authors)