THE ERZINCAN (TURKEY) EARTHQUAKE (M(S)6.8) OF MARCH 13, 1992 AND ITS AFTERSHOCK SEQUENCE

The Erzincan strike-slip earthquake of March 13, 1992 ruptured a secti on of the North Anatolian fault (NAF) at the northern margin of the Er zincan basin. The focal depth of about IO km was less than given by IS C and NEIC. Erzincan and the surrounding villages were considerably da maged. In the Erzincan basin and in the neighbouring mountains a seism ic network of ten stations was installed. It was operating continuousl y from March 21 through June 16, 1992. More than 3,000 aftershocks wer e recorded of which 505 could be located. The spectral parameters of 3 94 and the fault-plane solutions of 53 aftershocks were determined. Fo r the given region the frequency dependent coda Q was derived as Q(c) = 122 f(0.68). The aftershock area increased with time, reflecting the process of stress redistribution. Some events clustered in the immedi ate vicinity of the town of Erzincan close to the epicentre of the mai n event and seem to trace the NAF. Their source mechanism is similar t o that of the main event (strike slip). About 150 aftershocks clustere d in the southeastern part of the Erzincan basin where a concentration of the events in a small volume of 5 x 5 x 3 km(3) was observed. The majority of fault-plane solutions available for these aftershocks show ed a normal faulting mechanism with an east-west directed extension. M ost of the aftershocks southeast of the basin clustered between two li neaments that were mapped by satellite images. The P-wave velocity bel ow the Erzincan basin, derived from travel-time residual analysis, is lower compared to areas NE and SW of the basin. Three-dimensional stre ss modelling of the Erzincan region qualitatively explains the occurre nce of the aftershocks southeast of the basin. The calculated displace ment distribution which exhibits the north-westward motion of the basi n and tension at its southeastern margin, caused by the Erzincan earth quake, is in agreement with derived fault-plane solutions.