mahnaz nedaei

Structural and petrological evidence suggests an asymmetric and two-sided subduction of the Sistan oceanic lithosphere under the Lut and Afghan blocks. The subduction under the Lut block was characterized by a steeper dip angle and a higher subduction rate, leading to the retreat of the plate boundary towards the ocean and subsequent back-arc extension. Studies on the geochemistry and possible origin of the mafic-intermediate igneous rocks of the Lut block corroborate the occurrence of crustal thinning and back-arc extension, highlighting the significant role of subduction in their formation. In this research, we employed laboratory or analog modeling with scale lengths to simulate the characteristics of the Sistan oceanic lithosphere subduction and the geodynamic processes in eastern Iran. We investigated the parameters controlling the style of subduction by examining the sinking of a thin viscoelastic silicone putty sheet with varying geometric and rheological properties in a glucose syrup-filled reservoir. Our findings revealed that an increase in length and thickness, coupled with a low viscosity ratio of the lithosphere and asthenosphere, accelerates the subduction rate. We also identified a significant correlation between the dip angle and the subduction rate. In summary, the deformation of the back-arc region was analyzed through kinematic investigation in the performed experiments. The experimental outcomes, which confirm the high rate and dip angle of subduction, and also the back-arc extension, can be identified as characteristics of the Sistan Ocean’s subduction zone under the Lut block.

One of the significant negative factors involved in exploiting granite stones as ornamental stones is the presence of heterogeneous fractures within the rock mass. Joints can either be destructive or beneficial in the production granite piles and building stone mines depending on their characteristics. This work focuses on evaluating the joints in the Divchal mine area of Kelardasht, north Iran. To get to that point, the main faults are surveyed from the aerial photograph, geological and tectonic maps, and field observations. According to this implementation, a density map of faults is provided for the entire studied area. The characteristics of the main joints including the length, slope, number, and orientation are collected in the mine area. The volumetric percentage of joints ( ) and joint set spacing ( ) parameters are computed at specific stations to identify suitable locations for granite extraction. The findings of this work suggest that the lower the value of  (  < 10), the larger the blocks can be extracted. On the other hand, at the high  values, the width of the extraction block increases. These conditions are typically found in locations far from the main faults where the density of joints is low, and as a result, the distance between joints is higher. The values  > 60 indicate a crushed rock mass, and are typically observed in clay-free shear zones. It is recommended that the opening of the working face be avoided in situations near the main faults due to the fragmentation of rocks and denser joint spacing.

The 11th August 2012 Ahar-Varzaghan earthquake doublet Mw 6.4 and 6.2 occurred near the city of Ahar, northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics and the state of pre and post-seismic stress distribution of the source region, we applied a combination of Coulomb stress change, b-value mapping, and the Fry method. Inferred Coulomb stress field reveals the E–W-striking (dextral) fault responsible for the first event and the NNE–SSW-striking (sinistral reverse) fault for the second event. The high slip stress-released regions in the obtained b-value map and the dominant anisotropies of aftershocks on regional stress-parallel cross-sections achieved by the Fry method, together with the distribution of aftershocks mechanisms, merely highlight the particular wedge-shaped structures namely the rhombic structures. The clockwise block rotation about the vertical axis under the right-lateral regional shear between the Kura basin to the north and the Central Iranian Block to the south and NW-oriented coeval shortening leads to the formation of rhombic structures. The results of this study improve our understanding of the kinematics of active deformation in NW Iran and have important implications for seismic hazard assessment of the region and potential future failure area.

The April 5th 2017 Mw 6.15 Sefidsang earthquake occurred east-northeast of the town of Fariman near the Cimmerian arc-related Fariman complex. The stress distribution was estimated by seismic parameters and Coulomb stress distribution in the source region. Inferred Coulomb stress field and general pattern of aftershocks distribution revealed that the Sefidsang earthquake occurred on a northeast-dipping listric fault with dextral reverse movement. Proximity of the Sefidsang sequence to Fariman complex proposes the reactivation of the pre-existing Cimmerian arc-related faults in the present-day stress field. The crustal-penetrating low-angle inverted faults coupled with Mesozoic mafic–ultramafic magmatism can shed light on the structural aspects of the region. The kinematics was also investigated by GPS velocity fields and morphotectonic features. The counterclockwise block rotation under the left-lateral regional shear between the Doruneh fault system and the North Kopeh Dagh fault system and NE-oriented coeval shortening led to the formation of rhombic structures. Cimmerian-related basement faults in NE Iran confined the expansion of rhombic cells and sense of block rotation. The results of this study improved our understanding about kinematics of active deformation in NE Iran and had important implications for seismic hazard assessment of the region and potential future failure area.

On 11 August 2012, an earthquake doublet (Mw 6.4 and 6.2) occurred near the city of Ahar and Varzaghan, northwest Iran. Both events were only 6 km and 11 minutes apart, producing a surface rupture of about 12 km in length. It occurred on a so far unknown structure having sparse historical and modern seismicity in the area. According to the earthquake interaction phenomenon and stress transfer during the earthquake, the Coulomb stress changes was resolved on four different binary combination of nodal planes of the first and second mainshocks to specify “optimally oriented” nodal planes for defined orientation of the principal axes of the regional stress field and an assumed friction coefficient. Those binary nodal planes were chosen having the best fit to the most positive Coulomb failure stress changes and good correlation with the spatial distribution of the aftershocks greater than 3 for 3 months after the earthquake doublet. The results led to the values of 82°, 89°, and 164° respectively for strike, dip, and rake angles of the first earthquake source fault with right-lateral strike slip mechanism and the values of 7°, 49°, and 31° respectively for strike, dip, and rake angles of the second earthquake source fault with oblique reverse mechanism. There is also an excellent correlation between the spatial variation of b-value before the Ahar-Varzaghan earthquake and the stress increased regions along the optimally oriented source faults. The b-value parameter is actually the slope of the Gutenberg-Richter law and depends on tectonics and stress distribution in the region. Based on the Coulomb stress changes calculation, the spatial changes of the b-value before the Ahar-Varzaghan earthquake, the manifestation of the surface rupture caused by the earthquake, field observations, and the dominant trend of afterschocks revealing structural dependency of two source faults, we proposed a restraining bend structure and a complex style of deformation causing the earthquake. The proposed structural model can help to estimate the seismic hazard risk of future potential failure areas.

One of the most common parameter in seismic studies is b-value that is proportional to the relative earthquake-size distribution. This parameter is actually the slope of the Gutenberg-Richter law and depends on tectonics and stress distribution in the region. In this study, we investigated spatial and temporal changes of b-value and its confirmity with GPS data before and after the Ahar-Varzaghan earthquake in the northwest seismotectonic region of Iran. The spatial and temporal changes of b-value were mapped using the Zmap software. The map of spatial changes of b-value show doublet events occurred in low b-value area which is probably structurally different from adjucent areas. The temporal changes of b-value also represent intense and abrupt decrease of b-value a few days before the earthquake. The time series of the GPS stations at low b-value area showed dramatic change for a while before the main shocks. Seismic and GPS data, together, could have been considered as earthquake precursors if they had been monitored precisely and continuously. Based on this calculation, the time series of GPS data and field observations, we proposed a restraining bend structure causing the earthquake. The proposed structural model can help to estimation seismic hazard risk of potential future failure area.