Geodetic evidence for aseismic fault movement on the eastern segment of the Gediz Graben system (western Anatolia extensional province, Turkey) and its significance for settlements

Abstract

Aseismic dip-slip normal fault displacement related to numerous fast-slipping active faults was recently observed in several localities in the western Anatolia extensional province. Still, the characteristics of displacements along with the behavior of individual fault segments are poorly known. Here we analyze an aseismically active normal fault affecting the settlement area of the Sarigol district, Turkey, at the surface rupture area of the 1969 magnitude 6.5 Alasehir earthquake. A precise leveling method was implemented in this area between July 2017 and 2020, to determine the vertical movements of the hanging wall relative to the footwall of the Sarigol fault. The yearly vertical movement on the surface along Profile 1 was - 7.0, - 7.3, and - 7.0 cm, respectively, for the three years starting in July 2017, and on Profile 2 it was - 7.7, - 8.7, and - 7.8 cm for the same time period. This persistent deformation, especially in the summer and fall seasons, suggests that may be related to groundwater level changes. Intensive agriculture is conducted in the region and a high level of irrigation activity in the summer period causes a decrease in groundwater levels. In addition, the continued deformation together with intensive precipitation in winter and spring despite high groundwater levels leads to the idea that tectonic creep movement could be a second reason for the deformation in the area. In the current study, the most important result is that the aseismic deformation starting after the 1969 Mw6.5 Alasehir earthquake still continues rapidly today with a velocity of 70-80 mm/year down-dip. This indicates that the damage zone of the Sarigol fault is not appropriate for settlements in the Sarigol district due to continuous high amounts of vertical displacement, and that appropriate building policy and awareness campaigns are needed.