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Constraints on North Anatolian Fault Zone Width in the Crust and Upper Mantle From S Wave Teleseismic Tomography

We present high‐resolution S wave teleseismic tomography images of the western segment of the North Anatolian Fault Zone (NAFZ) in Turkey using teleseismic data recorded during the deployment period of the Dense Array for Northern Anatolia array. The array comprised 66 stations with a nominal station spacing of 7 km, thus permitting a horizontal and vertical resolution of approximately 15 km. We use the current S wave results with previously published P wave teleseismic tomography to produce maps of relative VP/VS anomalies, which we use to highlight the difference in overall composition of the three terranes separated by the northern (NNAF) and southern branches of the NAFZ. Our results show a narrow S wave low‐velocity anomaly beneath the northern branch of the NAFZ extending from the upper crust, where it has a width of ∼10 km, to the lower crust, where it widens to ∼30 km. This low‐velocity zone most likely extends into the upper mantle, where we constrain its width to be ≤ 50 km and interpret it as indicative of localized shear beneath the NNAF; this structure is similar to what has been observed for the NAFZ west of 32°, and therefore, we propose that the structure of the NNAF is similar to that of the NAFZ in the east. The southern branch of the NAFZ does not show a very strong signature in our images, and we conclude that it is most likely rooted in the crust, possibly accommodating deformation related to rotation of the Armutlu/Almacik Blocks situated between the two NAFZ branches.