Assessment of the activity in the gap zone of the Rudbar Fault, based on the morphotectonic indices of the Sefidrud River, Gilan Province

In this study the morphotectonic indices for a specified length of the Sefidrud River, which is located in the meizoseismal area of the 1990 Rudbar earthquake, have been investigated to constrain the activity of a gap between the Rudbar Fault segments. The Digital Elevation Model (DEM) with 30 m resolution was used to obtain the elevation data. Fifteen km of the Sefidrud River also was divided into 100 -meter segments to calculate the morphotectonic indices. The morphotectonic indices include the sinuosity (SI), stream-length gradient (SL) and river long profile. The sinuosity in this region, where the trend of the Rudbar fault cut across the river, is approximately 1-1.5 and has a lower value relative to the other segments of the river. This indicates that the uplift possibly affected this region and then indicates the high tectonic activity. The stream-length gradient in the target region is approximately >3000 and has a higher value in the region which may be related to the lithological and active tectonic effects. The high value of SL can be used for determining the regions with high potential for the landslides along the strike of the Sefidrud River. The Sefidrud long profile, in aforementioned area, has a bulge or convexity. Such convexity together with the high value of stream-length gradient can indicate that the tectonic activities are significant in this region. By comparing the morphotectonic analysis with the seismotectonic studies such as the Coulomb stress changes due to the 1990 Rudbar earthquake, in which the high stress zones were detected in this region, it can be cluded that the studied region has high tectonic activities and consequently its monitoring to prevent the seismic hazard is essential.