ROMANIAN EARTHQUAKE OF MARCH 4, 1977 .2. AFTERSHOCKS AND MIGRATION OF SEISMIC ACTIVITY

The seismic activity following the March 4, 1977 earthquake in Romania was observed by fixed and supplementary mobile seismic stations. Epicenter and hypocenter determinations in March and April provided new information on the structure of the seismic zone of Vrancea, with intermediate foci at depths between 7o and 13o km, a seismic gap between 4o and 7o km, and shallow crustal foci. Seismic activity migrated into the southwestern foredeep, into the Carpathian mountains and towards Bucharest at different velocities of km/minute, km/hour, and km/day, respectively. The viscosity of the asthenosphere under the Carpathian arc is estimated from this migration to be 1017 to l018 Poise, a value encountered only at island arcs. A new model for the mechanism of Continent-continent collision at the Eastern Carpathians is proposed, which tries to explain the isolated occurrence of the intermediate seismic zone in Vrancea, at the edge of a paleoseismic zone beneath the Eastern Carpathian mountains. +)Paper of the Romanian-German Seismological Working Group which was formed after the earthquake of March 4 by the above mentioned institutes. Contribution No. 158, Geophysical Institute, University of Karlsruhe. © 1979.