Active Faulting and City Sprawl Hazard in Limites of Eartquake Fault (Case study: North Tabriz Basins)

The city of Tabriz is one of the four large city of Iran and the largest inhabitation center in northwest of Iran. This city is located on a large and active tectonic fault which enlongate from north Anatolian fault system-located in Turkey- to the Alborz mountains in Iran. The regions of northwest Iran, eastern Turkey and Caucasus are one of the most complex regions of the Arabia – Eurasia collision. The seismicity of this region is due Arabia-Eurasia plates convergence. The results of convergence indicated by moderate to large magnitude earthquakes. So that, the cities of this region (as Tabriz) destroyed repeatedly by large magnitude earthquakes. Right lateral strike-slip motions along the North Tabriz fault have given rise to strong earthquakes, which threaten the city of Tabriz with its population of more than 1.5 millon.This research try while review of seismicity and seimotectonic situation, evaluate active tectonic by geomorphic indexs in Varkesh-chai, Gomanab-chai and Nahand-chai and remember city extension in limites of earthquake fault. Area of the study region is 1258 Km2 and is located northeast of Ormieh lake and north of Tabriz city. North Tabriz Fault (NTF) pass from north of Tabriz city. 2-

In this research, seismotectonic characteristics the study area analyzed by historical and seismography data from Institute of Geophysics Tehran University. We analyzed six geomorphic indices for evaluating of active tectonic in the study area. Indices used include: the stream- gradient index (SL), drainage basin asymmetry (Af), Ratio of valley – floor with to vally height (Vf), drainage basin shape ratio (Bs), topographic Symmetry Factor (T) and stream sinusity(S). We the computed a single index (Iat) from the six indices to characterize relative active tectonics. Iat is cbtained by the average of the different classes of geomorphic indices (S/n) and divided into four classes from relatively low to highest tectonic activity. 3-

Studies of region seismicity indicated that North Tabriz Fault (NTF) is a NW-SE trending strike – slip fault. Rate of right lateral slip determined 7±1 mm/yr for the North Tabriz Fault.This value is consistent with earthquakes of magnitude 7-7.3 and recurrence times of 250-300 yr. This rates conform to historical seismicity of the North Tabriz Fault in 858, 1024, 1273,1304,1641,1721,1780 and 1786. This conditions indicating the potential for a significant earthquake on the NTF in the next 50 yr.Map of earthquake focal distribution present relation earthquakes locations with active faults. This map shows that dense clusters are located on both ends of the North Tabriz Fault. There is another cluster near the city of Tabriz. The depth of earthquakes from the surface is almost 20 Km in the seismotectonics.The results of geomorphic indices show active tectonic faulting in the study drainage basins (Varkesh-chai, Gomanab-chai and Nahand-chai). One of the indices is Asymmetic factor (AF). This indexcan be used to evaluate tectonic tilting at the scale of drainage basin. Asymmetic factor shows that Nahand-chai is classified in class 1(active tectonic), Varkesh-chai and Gomanab-chai are located in class 2 (moderate). Topographic Symmetry Factor (T) indicates asymmetry of Nahand-chai is more than another rivers.Because index values are near 1, Therefore drainage basins of the study area are active, based up on values topographic Symmetry Factor. Another index sensitive to tectonic uplift is the valley floor width to vally height ratio. Calculation of ratio of valley floor width to vally height (Vf) is done at a distance 1 km upstream from mountain front. This index shows valleys with a wide fioor relative to the height of valley walls (U shape) in the study area. Therefore the study drainage basins are classified in class 3(low active).An other index for studing active tectonic is basin shape index. Relatively, young drainage badins in tectonically active area tend to be elongated in shape.Based upon index of drainage basin shape (Bs), Nahand-chai has longer shape than north Tabriz drainage basins. Varkesh-chai and Gomanab-chai are located in class3. The results of stream sinusity shows that the study drainage basins don’t reach to equilibrium. They are active based upon stream sinusity.We calculated SL index along the study rivers using a digital elevation model and GIS, and computed its average value for each basin. Values of stream length- gradient index (SL) differentiantes along the study rivers. So that, average Value SL index is high in the nahand-chai, but in Varkesh-chai and Gomanab-chai are moderate. Iat indicates high tectonic activity (Class 2) in the study drainage basins. Index of relative active tectonic (Iat) obtained in the study area, based values of stream- gradient index (SL), drainage basin asymmetry (Af), Ratio of valley – floor with to vally height (Vf), drainage basin shape ratio (Bs), topographic Symmetry Factor (T) and stream sinusity (S). 4-Conclutions: Seismcity and seismotectonic conditions and analyses of geomorphic indices in the north Tabriz drainage basins are indicating that Tabriz city is located in hazardous region. Clusring of strong earthquakes along active faults is an additional factor of seismic hazard characteristic of the considered area. Historical evidence indicate that there were several earthquake on the North Tabriz Fault. The results of geomorphic indices show that the study area is classified in class 2 (high active tectonic).This problem must to pay attent in urban planning and development.Now that, Tabriz city sprawl is enlongating toward slopes of Onebneali heights (location of fault). In recently decades increase buildings and build towers is disturbed stability of this region.This conditions increased instability and earthquake hazards. Besides there are buildings at city margins where located along of Onebneali heights (location of fault)and build by undesirable material.These houses can´t resist against moderate magnitude earthquakes.This conditions increase earthquake hazards intensely in this region.