Mechanism of the 1992 Erzincan earthquake and its aftershocks, tectonics of the Erzincan Basin and decoupling on the North Anatolian Fault

Abstract

SUMMARY The 1992 March 13 Erzincan earthquake (Ms= 6.8) affected the Erzincan Basin on the eastern part of the North Anatolian Fault (NAF). This event occurred on the eastern tip of a westward-propagating series of major North Anatolian earthquakes between 1939 and 1967 (Ambraseys 1970). It is located within a key region of eastern Anatolia, which displays both expulsion of the Anatolian block towards the west and regional N-S convergence due to the Arabian-Eurasian collision (Philip et al. 1989). A reinterpretation of the tectonics of the Erzincan Basin is made from field information and the analysis of satellite images. A model explaining the development of the basin is proposed. It combines a pull-apart on the NAF with the effect of two compressional wedges. One is formed by the Northeast Anatolian fault (NEAF) and the NAF; the other, around Pulumur, is limited by the Ovacik Fault (OF) and the NAF. Both E-W extension and N-S compression occur according to this model. Normal faulting on the southwestern border and extension-related Quaternary rhyolitic volcanic domes along the northeastern boundary of the basin support the proposed E-W extension. The block limiting the southern side of the basin is uplifted and displaced left-laterally along the OF, suggesting southward growth of the basin. A kinematic model of the 1992 Erzincan earthquake combines aftershock field information, neotectonic observations, satellite image interpretation and bodywaveform inversion. The complex earthquake rupture starts on the NAF and propagates bilaterally. Westward rupture propagation progresses along the northern border of the basin. Eastward propagation is shifted on a segment of the NAF south of the Euphrates, and ends within the Pulumur wedge. Both branches are connected by a system of normal faults. The main focal mechanism is strike-slip with a slight normal component (o= 122°; δ= 63°Λ=−164°). The strongest aftershock mechanism is compatible with N-S compression within the Pulumur wedge. Aftershock mechanisms and stress-tensor analyses give detailed insight into the earthquake sequence. The 1992 Erzincan earthquake is located between two segments of the NAF that suffered large destructive earthquakes in the past. The 1939 earthquake (Ms= 8.2) ruptured 370 km west of Erzincan. The last major earthquake east of the Erzincan Basin occurred in 1784, rupturing a segment of at least 75 km length. This area has displayed a seismic gap since then. The mechanism of the main shock and the absence of aftershock activity east of Tanyeri indicate decoupling between the 1992 event and the 1784 gap. Aftershocks within the Pulumur wedge suggest that the probability of the occurrence of a future earthquake is higher on the OF than in the 1784 earthquake gap