The seismic velocity and fault structure of the Erzincan basin, Turkey, using local earthquake tomography

We present a detailed 3-D seismic velocity model for the upper crust, beneath the Erzincan Basin using tomographic inversion applied to the bodywave arrivals of the 1992 March 13 Erzincan earthquake (M-l = 6.9) aftershock sequence. High quality digital data collected by various institutions is used, consisting of 1240 hypocentres with rms errors less than 0.10 s, recorded by 25 stations on average. This provides a total of approximately 21 000 P- and 10 000 S-arrival times. The final solution is well constrained and reveals sharp boundaries bordering the basin. The shape of the basin is found to be deeper on the north and narrower at the eastern border. The velocity contrast related to the basin extends down to a depth of 3 km, but is hardly perceptible at 6 km and completely disappears at 9 km. The thickness of low-velocity unconsolidated soft sediments (V-p < 2000 m s(-1)) is estimated to be shallower than 3 km. The hypocentres were relocated using the inferred 3-D model and correlated with the fault segments that were observed in field studies. In addition to the main segment of the North Anatolian Fault (NAF), on the north of the basin, we identified a secondary faulted zone located along the southern border of the basin and merging with the Pulumur Fault Zone. We observed that this southern zone has a limited extension towards east (similar to 20 km) and therefore played only a minor role in the evolution of the basin. Despite the complex interaction between the North Anatolian Fault, the Ovaci k Fault and the Pulumur Fault Zone, it is clear that the Erzincan Basin has developed predominantly by the pull-apart mechanism. We conclude that the main continuation of NAF to the east is the branch on the north following the Euphrates valley, and constitutes, locally, the primary seismic hazard factor.