Dedicated to the memory of three pioneers, İhsan Ketin, Sırrı Erinç and Melih Tokay, and a recent student, Aykut Barka, who burnt himself out in pursuit of the mysteries of the North Anatolian Fault. ▪ Abstract  The North Anatolian Fault (NAF) is a 1200-km-long dextral strike-slip fault zone that formed by progressive strain localization in a generally westerly widening right-lateral keirogen in northern Turkey mostly along an interface juxtaposing subduction-accretion material to its south and older and stiffer continental basements to its north. The NAF formed approximately 13 to 11 Ma ago in the east and propagated westward. It reached the Sea of Marmara no earlier than 200 ka ago, although shear-related deformation in a broad zone there had already commenced in the late Miocene. The fault zone has a very distinct morphological expression and is seismically active. Since the seventeenth century, it has shown cyclical seismic behavior, with century-long cycles beginning in the east and progressing westward. For earlier times, the record is less clear but does indicate a lively seismicity. The twentieth century record has been successfully interpreted in terms of a Coulomb failure model, whereby every earthquake concentrates the shear stress at the western tips of the broken segments leading to westward migration of large earthquakes. The August 17 and November 12, 1999, events have loaded the Marmara segment of the fault, mapped since the 1999 earthquakes, and a major, M ≤ 7.6 event is expected in the next half century with an approximately 50% probability on this segment. Currently, the strain in the Sea of Marmara region is highly asymmetric, with greater strain to the south of the Northern Strand. This is conditioned by the geology, and it is believed that this is generally the case for the entire North Anatolian Fault Zone. What is now needed is a more detailed geological mapping base with detailed paleontology and magnetic stratigraphy in the shear-related basins and more paleomagnetic observations to establish shear-related rotations.

In this study, seismological techniques are combined with surface observations to investigate the faulting associated with three large earthquakes in western Turkey. All involved normal faulting that nucleated at 6-10 km depth with dips in the range 30-50". The two largest earthquakes, at Alajehir (1969.3.28) and Gediz (1970.3.28), were clearly multiple events and their seismograms indicate that at least two discrete subevents were involved in producing the observed surface faulting. In addition, their seismograms contain later, longer-period signals that are likely to represent source, not structure or propagation, complexities. These later signals can be modelled by subevents with long time functions on almost flat detachment-type faults.As a result of these observations, we propose a model for the deformation of the lower crust, in which brittle failure of the top part occurs when high strain rates are imposed during an earthquake that ruptures right through the upper, brittle crust. Under these special circumstances, seismic motion occurs on discrete faults in the lower crust, which otherwise normally deforms by distributed creep. In the case of the normal faults studied here, motion in the uppermost lower crust takes place on shallow dipping faults that are downward continuations of the steeper faults that break to the surface. The faults thus have an overall listric geometry, flattening into a weak zone below the brittle layer at a depth that is probably dependent on the termperature gradient. This interpretation explains why detachment-type mechanisms are not seen in first motion fault plane solutions of normal faulting earthquakes, and suggests an origin for the Metamorphic Core Complexes seen in the Basin and Range Province, which probably represent flat lower crustal faults, analogous to those postulated at Alajehir and Gediz, that have been uplifted to the surface.

SUMMARY The East Anatolian Fault Zone accommodates most of the motion between the Arabian plate and the apparently little‐deforming interior of central Turkey. The direction of overall slip across this zone is crucial to the determination of the slip rate on the North Anatolian Fault. We use long‐period P‐ and SH‐waveforms to determine the source parameters of the four largest earthquakes that occurred in, or near, the East Anatolian Fault Zone in the last 35 years. Only one of these actually involved left‐lateral strike–slip motion on a NE–SW fault. But the other three, and the nearby 1975 Lice earthquake, all had steeply dipping nodal planes with a NNW strike: if these were the auxiliary planes then all the earthquakes had a slip vector direction within about 10° of 063°. If this direction represents the Arabia–Turkey motion, then the slip rate on the North Anatolian Fault must be in the range 31 to 48 mm yr−1, with a probable value of 38 mm yr−1, and the overall slip rate across the East Anatolian Fault Zone must be about 29 mm yr−1 with a range of 25–35 mm yr−1.

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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 1939and 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 (4 = 122"; 6 = 63"; A= -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 O F than in the 1784 earthquake gap Q 1997 RAS 1 2 Fuenzalida et al.