Introduction
The studied area is located in edge of southern Central Alborz between important faults of the north Tehran, Mosha, Taleghan, Ipak and Eshtehard and includes significant earthquakes. In this research, the tectonic activity rate of the region has been evaluated using geomorphological indicators.

Materials and methods

In order to assess the relative tectonic activity through the study area, sub-basins and stream network were extracted by using Arc Hydro Tools software (an extension of Arc GIS software, ESRI) based on the DEM and in turn, 23 sub-basins have been resulted. The active tectonic 5 geomorphologic indices were used as follow: Stream length–gradient index (SL), Asymmetric factor (Af), hierarchical anomaly index (Δa), concavity index and normalized steepness index.
 
Stream Length–Gradient Index (SL): The SL index indicates an equation between erosive processing as streams and rivers flow and active tectonics. The SL is defined by Eq. (1) 
SL= (∆H/∆Lr) Lsc.    (1)
 Where ΔH is change in altitude, ΔLr is the length of a reach, and Lsc is the horizontal length from the watershed divide to midpoint of the reach. The SL index can be used to evaluate relative tectonic activity.  The quantities of the SL index were computed along the streams for all sub-basins.
 
 Asymmetric Factor (Af): The asymmetric factor (Af) is a way to evaluate the existence of tectonic tilting at the scale of a drainage basin. The method may be applied over a relatively large area. Af is defined by Equation (2).
Af= 100(Ar/At)        (2)
Where Ar is the area of the basin to the right (facing downstream) of the trunk stream and at is the total area of the drainage basin. If the value of this factor is close to 50, the basin has a stable condition with little or tilting; while values above or below 50 may result from basin tilting, resulting from tectonic activity or other geological conditions such as lithological structure.
 
Hierarchical anomaly index (Δa): The hierarchical anomalies index is calculated based on the number of hierarchical anomalies expressed by Equation (3).
 
Hai→j= 2^(j-2)−2^(i-2)              (3)
Where i is the primary stream, j is end stream and Hai → j the number of hierarchical anomalies of each stream. The number of hierarchical anomalies has been calculated in Equation (4).
 
Ha_t= Σ (Ha_i →j × Ns_i →j)              (4)
 
Where (Ns_i → j) is the total number of streams entering the high-level streams. The index Δa is expressed by the following relationship.
 
Δa=Ha_t/ N₁                                     (5)
 
Where Ha_t is the number of the hierarchical anomaly and N₁ is the number of first-order segments of the streams.
 
Normalized steepness index and Concavity index: Flint’s empirical power-law defines the river profile in a steady-state: S=K_SA^-θ       (6)
Where S, Ks, A, and θ indicate the slope, the steepness index, the drainage area, and stream concavity, respectively. In addition, Ks and θ are directly computed by the regression analysis of the slope-area data. Further, a steady-state landscape demonstrates that erosion, incision, and uplift rates are equal and stable over time. Different empirical studies indicated a direct relationship between the values of the steepness index (Ksn) and the bedrock erosion rate or rock uplift rate in the steady-state of river systems.
 
K_S= (E/K)^{( 1/n)}                           (7)
 
Where E denotes the uplift of the bedrock and K indicates the erosion coefficient which relies on the climatic and morphotectonics conditions of the area. Finally, n represents a positive exponent which is associated with the predominant erosion process of the area.  In the present study, Ksn was normalized to a reference concavity θref = 0.45 these parameters were implemented to fit a steady-state stream power solution to individual river longitudinal profiles in Matlab (Topotoolbox). The best profile was obtained from the matching of the uplift rate (U = E) or erosion (K) and the predicted rate. Furthermore, normalized steepness and concavity indices were determined using the longitudinal profile of the river.
 

Results and discussion

In this research, 5 morphological indicators related to the river channel and drainage basin were calculated for each basin, and for each of the 23 sub-basins, the rate of tectonic activity was defined for each of the indicators. The indices represent a quantitative approach to differential geomorphic analysis related to erosion and depositional processes which include the river channel and valley morphology as well as tectonically derived features, such as fault scarps. We also evaluated the outputs of the morphometric analyses based on field-based geomorphological observations. Thus, these results are proved to be extremely beneficial to evaluate relative rates of active tectonics of this region. The values of morphological indicators show that the basins with high tectonic activity have a good match with the main faults of the region, such as North Tehran, Mosha, Taleghan and Ipak faults and show high correlation with observed landforms during the field investigations such as the fault gorges, Strath terraces, multiple elevated alluvial terraces and knickpoints.
 

Conclusion

 
In this study, geomorphic indicators were used to investigate the tectonic activity of the region and it was found that the studied region has high tectonic activity along the North Tehran, Mosha, Taleghan, Emamzadeh Davoud and Ipak fault zones.