Seismotectonics of the west of Golestan province, the east of south Caspian region

Golestan province is located in one of the seismically active zones of Iran. The seismicity maps of Golestan province show that density of earthquakes in the west is more than the east. In this research, the parameters including b-value, recurrence time and seismic moment were studied to investigate the seismicity of the west of Golestan province. The focal mechanism of earthuqakes and field data were used to determine the stress orientations in the study area. The estimated b-value is obtained as 1.24±0.2 which is comparable with the Alborz. Since the northern and southern parts of the study area have different geomorphological and structural characteristics, some of these parameters were calculated for two subdivisions; Dasht-e-Gorgan in the north and foothill in the south. Results of this study show that the earthquakes in the Dasht-e-Gorgan compared to the foothill are smaller with shorter recurrence times. For the foothills, the calculated P-axes using the stress tensor inversion method is found to be subhorizontal with trend N-NNE. For this region, at least two trends, N and NW, is calculated using field data. This result show the change of stress directions during the structural evolution of this area. The calculated p-axes trend for the Dasht-e-Gorgan is NE. In both studied areas, the focal mechanisms of greater earthquakes are consistent with the E-W to NE-SW trend of main faults especially the Khazar fault. But, the smaller events in the Gorgan Plain show a different trend and mechanism. Comparison of these two subdivisions shows that the northern Alborz foothill is more dangerous than the plain.