مجله زمین ساخت
پیاپی 7 (پاییز 1397)

The Nesa Anticline located in the southern part of the central Alborz is bounded by the Taleghan Fault to the North and by the Mosha Fault to the South. Unlike the western and eastern part of the Alborz range, in Central Alborz, the Evidence of Eo-cimmerian Event has shown as extensional event. In this study, evidence on development of growth strata in the Nesa Anticline which can be considered as a structural evidnece on the performance of the Eo-cimmerian event in Central Alborz is presented. Investigations of cross sectional mapping across the Nesa Anticline indicate a decrease in the thickness of the Triassic Elika Formation in the anticline hinge area due to development of growth strata. This represent synchronous deposition of the Elika Formation and growth of the anticline. This simultaneous folding and sedimentation is analysed to the effect of the Eo-cimmerian event in the study area and the Central Alborz.

Yamchi dam was constructed in 2005 in south of Ardabil. In order to investigation of effect of reservoir impounding on seismicity rate changes of dam area (30 km radius) and proving of Reservoir Induced Seismicity, Firstly, structures were determined and surveyed and their Quaternary and Recent activities documented. Then, the earthquake catalogue was completed and historical and instrumental seismicity investigated. Finally, the completed magnitude and b-values were calculated and then, the impounding of reservoir effect on seismicity rate changes studied. The results shown that two main Ardabil and Sarein faults, with Quaternary and recent activities located in 30 km radius of dam site and many of earthquakes taken placed in cross cut of these faults. Investigation of effect of impounding of reservoir on seismicity rate, as cumulative diagram of released energy relative to reservoir volume changes, determined four stage of b-value and energy released changes. Impounding of dam reservoir, caused to rising of seismic activities as increasing b-value and diagram slope of releasing cumulative energy. This phenomenon proved that low magnitude/ high magnitude earthquake ration was increased as occurrences earthquakes with M<4 in depth <5 km.

The study area is located in central part of the Urumieh-Dokhtar magmatic arc and the southeast Ardestan area. Eocene volcanic rocks units with a combination of andesite and basaltic andesite constitute the most extensive rock units in the study area. These units are host to Ag-Cu and Au mineralization and alteration zones. Utilization of remote sensing methods especially composite pseudo color and Filters helped us to obtain valuable information on alteration zones, existing brittle structures and the role of Zafarghand fault in controlling alteration zone. The distribution pattern of the faults on processed satellite images, shows three generally orientations, changing in fault orientations, the density of faults in the north of the area and positive flower structure. Two different patterns of alteration zones are observed including propylitic alteration and zones of secondary iron oxides with large distributions around copper veins and argillic-sericitic alteration zones with limited and linear distributions around quartz veins. Comparison of the pattern of faults, mineralization and alteration zones indicate a decrease in the degree of mineralization and increase the extent of alterations along with reducing the density of faults from north to south of the region. The Zafarghand fault has an effective role in the creation and development of alteration in the study area. In this way, alteration is only seen toward the northern parts of this fault. The flower structure has created a tenstion and suitable location for mineralization and confluence of alteration zone in the south of the area.

Necessity of the human to the safe and permanent water sources is one of the important problems. In the last decades, soft rock aquifers of country mainly are used, therefore, exploration for groundwater in the hard rocks has focused Tectonics and faults one of the important factors to crush the rocks and formations and make and develop the faulted aquifers. Therefore, in this study the role of the structural geology is researched in development of the underground fractures and have used the geophysical tests to explore water sources and well locating. In this case three study area researched in country. An important property means the Lambda parameter for the first time have used in the studied areas to determine the fractures density. In this research evaluate the Lambda parameter and fracture density in three study area in Iran. where Lambda parameter used to determine and print the joints and fracture density and anisotropy countor maps. These maps show potentially for developing the ground water is high where Lambda parameter is highest. Lambda was 8 in Aghlighan-chay and Khomarlou and 1.76 in Kermanshah University. The location of the 2nd soundage in Khomarlou project after boring have had up to 40 L/S well water. Therefore this method is a new method to evalute the aquifers before borring the wells to acces the new resources of ground water and manegment the drought hazards. Key words: Crushed fault zones, Anisotropy

The Khazar fault in the northern part of the Alborz mountain range have changed the topography by making an obvious terrace in which along that the mountain drainage lines is ended where it is impossible to observe the geology structures due to the severe erosion and massive vegetation. In this study, some characters of this structure , along it from the west to the center, have presented in which indicates the high activity along this fault system. Here, in spite of some studies that deny or introduce it as an inactive fault, we believe the Khazar fault is active and can be cause serious earthquakes. The progressive activity of the Khazar fault has caused the fault segments, with strike-slip mechanisms, originating from it. These faults almost aren’t so long and are ended to some fractures in their southern range. The northern ridge of Alborz mountain range is controlled by their strike-slip displacement and the Khazar fault.

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.