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USING EARLY INSTRUMENTAL DATA TO DETERMINE THE SOURCE PARAMETERS OF THE STRONGEST HISTORICAL EARTHQUAKES OCCURRED IN WEST ROMANIA (1900 - 1980)

Eugen Oros

Abstract

We present a study of the strongest historical earthquakes (Mw > 5.0) occurred in West Romania between 1900 and 1980 to review and homogenize the Romanian Earthquakes and Focal Mechanisms Catalogues on instrumental data basis. New hypocenters relocation, moment magnitudes, Mw and focal mechanisms solutions are obtained on historical instrumental data basis (seismograms, bulletins, catalogues). The vectorized seismograms were analysed to pick up the arrival times of the body and surface waves after careful identification of the phases as a function of the velocity model, first P wave polarity, amplitudes and corresponding periods. We located the hypocenters of 15 events by a grid search algorithm often using S-P differences instead of absolute times. The magnitude was estimated from displacement spectra by the scalar seismic moment method. The focal mechanisms for 9 events were obtained by polarities and sometimes by full-wave inversions depending on the data quality. The solutions based on polarities data are high quality being constrained through the comparative analysis of the observed and computed waveforms. On average, the P axes are NE oriented (N67OE) like the regional trend of the horizontal component of the stress field and the general orientation of the nodal planes

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