Present-day strain distribution across a segment of the central bend of the North Anatolian Fault Zone from a Persistent-Scatterers InSAR analysis of the ERS and Envisat archives

Abstract

The North Anatolian Fault Zone (NAFZ) is the major transform system that accommodates thewestward movement of the relatively rigid Anatolian block with respect to Eurasia. Mitigating the hazard associated with devastating earthquakes requires understanding how the NAFZ accumulates and releases the potential energy of elastic deformation both in space and in time. In this study, we focus on the central bend of the NAFZ where the strike of the North Anatolian Fault (NAF) changes from N75◦to N105◦within less than 100 km, and where a secondary fault system veers southwards within the interior of Anatolia. We present interseismic velocity fields obtained from a Persistent-Scatterers (PS) Interferometric radar analysis of ERS and Envisat radar archives. Despite the high vegetation cover, the spatial density of measurementsis high (∼10 PS/km2 in average). Interseismic velocities presented below indicate a velocitychange of∼6–8 mm/yr along the satellite line-of-sight (LOS) mainly centred on the NAF surface trace, and are in good agreement with the GPS velocity field published previously. The observed deformation is accommodated within a zone of ∼20 to 30 km width, in this area where no surface creep has been reported, contrary to the Ismetpasa segment located ∼30 km to the west of this study zone. Although less conspicuous, ∼2–3 mm/yr (∼1 mm/yra long the LOS) of the total deformation seems to be localized along the Lacin Fault. Theoverall agreement with horizontal GPS measurements suggests that the vertical component of the ground deformation is minor. This is confirmed, over the western part of our study zone, by the 3-D estimation of the ground deformation from the combination of the GPS-and the PS-mean velocity fields. However, a specific pattern of the PS velocity fields suggests that an area, enclosed between two faults with roughly south–north orientation, experiences uplift. The PS analyses of radar time-series both prior and posterior to the Izmit and Duzce earthquakes indicate that these events did not induce detectable velocity changes in this central bend. The only temporal changes we identify are due to a moderate local earthquake (Mw5.7,1996 August 14) whose precise location and coseismic deformation are determined here. Finally, we propose a model of slip-rate distribution at depth along the NAF from the joint inversion of the GPS and PS mean velocity fields. This model suggests a long-term slip-rate of ∼20 mm/yr for a rather uniform locking depth in the range of 15–20 km. However, thelocking depth increases to ∼25–30 km in the section comprised between longitudes E34◦20’and E34◦50’. This lateral evolution is in general agreement with the earthquake distributionat depth from three different catalogues.