Determination of block movements of the middle segment of the North Anatolian Fault Zone (NAFZ) by GNSS

The region focused in this thesis is located in the middle part of Northern Anatolia and is bounded by Tokat-Niksar in the east, Kastamonu-Ilgaz in the west, Sinop in the north and Yozgat in the south. In addition to the main branch of the North Anatolian Fault Zone (NAFZ), there are also splay faults (faults diverging from the main fault zone) extending to the south and north. One of these, the Merzifon-Esençay Fault, is a right-lateral strike-slip fault and extends in the east-west direction parallel to the NAFZ until İskilip (Çorum) and consists of Esençay, Amasya, Suluova, Diphacı, Laçin and İskilip Segments. Another one is the Sungurlu Fault, which is known to start from Niksar (Tokat) and extend to the Çankırı Basin. In addition, the Ekinveren Fault, which is located in the northern part of the Kastamonu Basin, extends in the east-west direction and the Eldivan Fault, which became active in the Late Pliocene period due to the northwest-southeast compression between the NAFZ and the Kırıkale-Erbaa Fault Zone. In this study, a GNSS network consisting of 118 sites was designed to determine the current deformation caused by splay faults in the main branch and middle part of the NAFZ. The previous years' data of the GNSS sites in the network were provided and campaign-type GNSS measurements were carried out in 2023-2024. All data were evaluated with GAMIT/GLOBK software and the current velocity field of the region was calculated with an uncertainty of <2 mm/year. In order to determine the current kinematic characteristics of the region by using the velocities, models with 2, 3, 4, 5 and 6 blocks (final model) was designed and rotations in three components of the blocks and slip rates of the faults representing the block boundaries were calculated with TDEFNODE and GeodSuit softwares by inverse solution method and three-dimensional elastic quasi-spatial displacement model. The results obtained from this study show that the splay faults in the middle part of the NAFZ are active, indicating that deformations occur not only on the main branch but also along the splay faults in the region. The block model results revealed that the strike-slip rates along the main branch of the NAFZ increase from east to west and there are also significant slip rates on the splay faults. In particular, Sungurlu Fault has a right-lateral fault structure with reverse component, Merzifon-Esençay Fault has a strike-slip fault structure with lateral component and Ekinveren Fault has a strike-slip fault structure. These results indicate that the strain accumulation in the region occurs not only in the main branch of the NAFZ but also along the splay faults and that earthquake hazard analyses should be re-evaluated by taking these results into account.