Extension and direction of ruptures from teleseismic P-wave patterns for the earthquakes of Gazli 1976, April 8 and Vrancea 1977, March 4

For the earthquakes of Gazli (April 8, 1976) and Vrancea (March 4, 1977) seismogram interpretation was carried out by means of a new technique taking into account the finite dimensions of the source. This interpretation is based on theoretical and experimental considerations of wave patterns generated by a moving rupture. The amplitude of P-waves up to the maximum phases, P_max, were taken into account. The arrival time of P_max on the seismogram was considered as corresponding to the time of the maximum energy release. Source extension and direction of rupture were estimated by means of the azimuthal travel-time curve of the P_max-wave.For the Vrancea earthquake the process at the focus occurred in the form of bi-directional rupturing at azimuths of 180° and 290° from the initial hypocenter. For the Gazli earthquake rupturing developed in several branching directions with azimuths 180°, 240° and 300°. A northwestern direction was prevailing in this event. Records along the rupture and in the opposite direction are shown for both earthquakes.