Neotectonic stresses in the Sangchal earthquake (1957) area – Mazandaran

The Sangchal earthquake occurred on July 2, 1957, in the central part of the North Alborz fault zone. The North Alborz fault is an arched shape fault that continued from Lahijan to Gonbad-e Kavos. This fault, along with Khazar, Lalehband, Kandovan and Mosha faults affected by the shortening and rising of the Alborz Mountains. This shortening and uplift accompanied by large earthquake events that continue today. With knowledge of the geometry of faults and how faults are displaced and the use of computational methods for fault analysis and also, the Focal mechanism, direction of maximum tension governing the part of north Alborz fault has been determined. The two methods show acceptable results. In addition, the comparison of the findings with the results of geodetic studies of the scope, accuracy of the findings confirmed. The results of the study show that during the North Alborz Fault, the dominant vector is compressive, and the left-lateral slip is observed. From the central part of the Alborz to the east, the left lateral displacements increases and cause shear mechanism in the eastern parts of the Alborz. This movement has caused compressive earthquakes in the central Alborz and compressive-shear earthquakes in the eastern parts. Therefore, the magnitude of future earthquakes in the eastern part is slightly smaller than the central Alborz earthquakes.