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Bulletin of the Seismological Society of America; April 1979; v. 69; no. 2; p. 513-530
© 1979 Seismological Society of America
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Analysis of the Bucharest strong ground motion record for the March 4, 1977 Romanian earthquake

STEPHEN HARTZELL

INSTITUTE OF GEOPHYSICS AND PLANETARY PHYSICS SCRIPPS INSTITUTE OF OCEANOGRAPHY UNIVERSITY OF CALIFORNIA,, SAN DIEGO, LA JOLLA, CALIFORNIA 92093

Abstract

The major features of the Bucharest strong ground motion record for the March 4, 1977 Romanian earthquake can be explained with a source at a depth of 100 km which propagates to the southwest, toward Bucharest, with a rupture velocity near the shear-wave velocity. Rayleigh-wave amplitudes from SRO, HGLP, and WWSSN stations yield a moment estimate in the range 1.0 to 2.0 x 1027 dyne-cm, after correction for a moving source and averaging over 13 stations. The Bucharest strong ground motion record shows a peak acceleration of 221 cm/sec2 (in the period range from 1 to 2 sec) and a peak displacement of 27 cm on the NS component at a hypocentral distance of 190 km. Both line source models in a homogeneous half-space and single and multiple event source models in a layered half-space are used to analyze the Bucharest record. The relatively simple form and high amplitudes of the Bucharest accelerogram are attributed to the small epicentral distance to source depth ratio and the low rigidity flysch sediments in the Bucharest area. The relative amplitudes on the three components of the Bucharest record suggest that the rupture plane steepened after the initial break from a dip of 70° to an approximately vertical orientation. However, other factors may also be important, such as near source or receiver heterogeneity or the bending of rays by the subducting lithospheric plate under the Carpathian arc. The moment estimate from the Bucharest accelerogram is 2.0 x 1027 dyne-cm. Using the above moment estimate and assuming a circular fault plane with a radius of 25 km, the average offset is estimated to be 1.5 meters with a stress drop of 57 bars.




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